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class="vector-toc-numb">3.1</span> <span>Secondary realizations by lunar observations</span> </div> </a> <ul id="toc-Secondary_realizations_by_lunar_observations-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Secondary_realizations_by_atomic_clocks" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Secondary_realizations_by_atomic_clocks"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.2</span> <span>Secondary realizations by atomic clocks</span> </div> </a> <ul id="toc-Secondary_realizations_by_atomic_clocks-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Revision_of_time_scales" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Revision_of_time_scales"> <div class="vector-toc-text"> <span class="vector-toc-numb">4</span> <span>Revision of time scales</span> </div> </a> <ul id="toc-Revision_of_time_scales-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-JPL_ephemeris_time_argument_Teph" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#JPL_ephemeris_time_argument_Teph"> <div class="vector-toc-text"> <span class="vector-toc-numb">5</span> <span>JPL ephemeris time argument T<sub>eph</sub></span> </div> </a> <ul id="toc-JPL_ephemeris_time_argument_Teph-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Use_in_official_almanacs_and_ephemerides" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Use_in_official_almanacs_and_ephemerides"> <div class="vector-toc-text"> <span class="vector-toc-numb">6</span> <span>Use in official almanacs and ephemerides</span> </div> </a> <ul id="toc-Use_in_official_almanacs_and_ephemerides-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Redefinition_of_the_second" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Redefinition_of_the_second"> <div class="vector-toc-text"> <span class="vector-toc-numb">7</span> <span>Redefinition of the second</span> </div> </a> <ul id="toc-Redefinition_of_the_second-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Notes_and_references" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Notes_and_references"> <div class="vector-toc-text"> <span class="vector-toc-numb">8</span> <span>Notes and references</span> </div> </a> <ul id="toc-Notes_and_references-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Bibliography" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Bibliography"> <div class="vector-toc-text"> <span class="vector-toc-numb">9</span> <span>Bibliography</span> </div> </a> <ul id="toc-Bibliography-sublist" class="vector-toc-list"> </ul> </li> </ul> </div> </div> </nav> </div> </div> <div class="mw-content-container"> <main id="content" class="mw-body"> <header class="mw-body-header vector-page-titlebar"> <nav aria-label="Contents" class="vector-toc-landmark"> <div id="vector-page-titlebar-toc" class="vector-dropdown vector-page-titlebar-toc vector-button-flush-left" > <input type="checkbox" id="vector-page-titlebar-toc-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-vector-page-titlebar-toc" class="vector-dropdown-checkbox " aria-label="Toggle the table of contents" > <label id="vector-page-titlebar-toc-label" for="vector-page-titlebar-toc-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--icon-only " aria-hidden="true" ><span class="vector-icon mw-ui-icon-listBullet mw-ui-icon-wikimedia-listBullet"></span> <span class="vector-dropdown-label-text">Toggle the table of contents</span> </label> <div class="vector-dropdown-content"> <div id="vector-page-titlebar-toc-unpinned-container" class="vector-unpinned-container"> </div> </div> </div> </nav> <h1 id="firstHeading" class="firstHeading mw-first-heading"><span class="mw-page-title-main">Ephemeris time</span></h1> <div id="p-lang-btn" class="vector-dropdown mw-portlet mw-portlet-lang" > <input type="checkbox" id="p-lang-btn-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-p-lang-btn" class="vector-dropdown-checkbox mw-interlanguage-selector" aria-label="Go to an article in another language. Available in 27 languages" > <label id="p-lang-btn-label" for="p-lang-btn-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--action-progressive mw-portlet-lang-heading-27" aria-hidden="true" ><span class="vector-icon mw-ui-icon-language-progressive mw-ui-icon-wikimedia-language-progressive"></span> <span class="vector-dropdown-label-text">27 languages</span> </label> <div class="vector-dropdown-content"> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li class="interlanguage-link interwiki-bg mw-list-item"><a href="https://bg.wikipedia.org/wiki/%D0%95%D1%84%D0%B5%D0%BC%D0%B5%D1%80%D0%B8%D0%B4%D0%BD%D0%BE_%D0%B2%D1%80%D0%B5%D0%BC%D0%B5" title="Ефемеридно време – Bulgarian" lang="bg" hreflang="bg" data-title="Ефемеридно време" data-language-autonym="Български" data-language-local-name="Bulgarian" class="interlanguage-link-target"><span>Български</span></a></li><li class="interlanguage-link interwiki-ca mw-list-item"><a href="https://ca.wikipedia.org/wiki/Temps_d%27efem%C3%A8rides" title="Temps d'efemèrides – Catalan" lang="ca" hreflang="ca" data-title="Temps d'efemèrides" data-language-autonym="Català" data-language-local-name="Catalan" class="interlanguage-link-target"><span>Català</span></a></li><li class="interlanguage-link interwiki-de mw-list-item"><a href="https://de.wikipedia.org/wiki/Ephemeridenzeit" title="Ephemeridenzeit – German" lang="de" hreflang="de" data-title="Ephemeridenzeit" data-language-autonym="Deutsch" data-language-local-name="German" class="interlanguage-link-target"><span>Deutsch</span></a></li><li class="interlanguage-link interwiki-et mw-list-item"><a href="https://et.wikipedia.org/wiki/Efemeriidiaeg" title="Efemeriidiaeg – Estonian" lang="et" hreflang="et" data-title="Efemeriidiaeg" data-language-autonym="Eesti" data-language-local-name="Estonian" class="interlanguage-link-target"><span>Eesti</span></a></li><li class="interlanguage-link interwiki-es mw-list-item"><a href="https://es.wikipedia.org/wiki/Tiempo_de_efem%C3%A9rides" title="Tiempo de efemérides – Spanish" lang="es" hreflang="es" data-title="Tiempo de efemérides" data-language-autonym="Español" data-language-local-name="Spanish" class="interlanguage-link-target"><span>Español</span></a></li><li class="interlanguage-link interwiki-fr mw-list-item"><a href="https://fr.wikipedia.org/wiki/Temps_des_%C3%A9ph%C3%A9m%C3%A9rides" title="Temps des éphémérides – French" lang="fr" hreflang="fr" data-title="Temps des éphémérides" data-language-autonym="Français" data-language-local-name="French" class="interlanguage-link-target"><span>Français</span></a></li><li class="interlanguage-link interwiki-ga mw-list-item"><a href="https://ga.wikipedia.org/wiki/Am_eifimirise" title="Am eifimirise – Irish" lang="ga" hreflang="ga" data-title="Am eifimirise" data-language-autonym="Gaeilge" data-language-local-name="Irish" class="interlanguage-link-target"><span>Gaeilge</span></a></li><li class="interlanguage-link interwiki-ko mw-list-item"><a href="https://ko.wikipedia.org/wiki/%EC%97%AD%ED%91%9C%EC%8B%9C" title="역표시 – Korean" lang="ko" hreflang="ko" data-title="역표시" data-language-autonym="한국어" data-language-local-name="Korean" class="interlanguage-link-target"><span>한국어</span></a></li><li class="interlanguage-link interwiki-ig mw-list-item"><a href="https://ig.wikipedia.org/wiki/Oge_Ephemeris" title="Oge Ephemeris – Igbo" lang="ig" hreflang="ig" data-title="Oge Ephemeris" data-language-autonym="Igbo" data-language-local-name="Igbo" class="interlanguage-link-target"><span>Igbo</span></a></li><li class="interlanguage-link interwiki-id mw-list-item"><a href="https://id.wikipedia.org/wiki/Waktu_efemeris" title="Waktu efemeris – Indonesian" lang="id" hreflang="id" data-title="Waktu efemeris" data-language-autonym="Bahasa Indonesia" data-language-local-name="Indonesian" class="interlanguage-link-target"><span>Bahasa Indonesia</span></a></li><li class="interlanguage-link interwiki-it mw-list-item"><a href="https://it.wikipedia.org/wiki/Tempo_effemeride" title="Tempo effemeride – Italian" lang="it" hreflang="it" data-title="Tempo effemeride" data-language-autonym="Italiano" data-language-local-name="Italian" class="interlanguage-link-target"><span>Italiano</span></a></li><li class="interlanguage-link interwiki-ka mw-list-item"><a href="https://ka.wikipedia.org/wiki/%E1%83%94%E1%83%A4%E1%83%94%E1%83%9B%E1%83%94%E1%83%A0%E1%83%98%E1%83%93%E1%83%A3%E1%83%9A%E1%83%98_%E1%83%93%E1%83%A0%E1%83%9D" title="ეფემერიდული დრო – Georgian" lang="ka" hreflang="ka" data-title="ეფემერიდული დრო" data-language-autonym="ქართული" data-language-local-name="Georgian" class="interlanguage-link-target"><span>ქართული</span></a></li><li class="interlanguage-link interwiki-kk mw-list-item"><a href="https://kk.wikipedia.org/wiki/%D0%AD%D1%84%D0%B5%D0%BC%D0%B5%D1%80%D0%B8%D0%B4%D1%82%D1%96%D0%BA_%D1%83%D0%B0%D2%9B%D1%8B%D1%82" title="Эфемеридтік уақыт – Kazakh" lang="kk" hreflang="kk" data-title="Эфемеридтік уақыт" data-language-autonym="Қазақша" data-language-local-name="Kazakh" class="interlanguage-link-target"><span>Қазақша</span></a></li><li class="interlanguage-link interwiki-hu mw-list-item"><a href="https://hu.wikipedia.org/wiki/Efemerisz_id%C5%91" title="Efemerisz idő – Hungarian" lang="hu" hreflang="hu" data-title="Efemerisz idő" data-language-autonym="Magyar" data-language-local-name="Hungarian" class="interlanguage-link-target"><span>Magyar</span></a></li><li class="interlanguage-link interwiki-ms mw-list-item"><a href="https://ms.wikipedia.org/wiki/Waktu_efemeris" title="Waktu efemeris – Malay" lang="ms" hreflang="ms" data-title="Waktu efemeris" data-language-autonym="Bahasa Melayu" data-language-local-name="Malay" class="interlanguage-link-target"><span>Bahasa Melayu</span></a></li><li class="interlanguage-link interwiki-ja mw-list-item"><a href="https://ja.wikipedia.org/wiki/%E6%9A%A6%E8%A1%A8%E6%99%82" title="暦表時 – Japanese" lang="ja" hreflang="ja" data-title="暦表時" data-language-autonym="日本語" data-language-local-name="Japanese" class="interlanguage-link-target"><span>日本語</span></a></li><li class="interlanguage-link interwiki-no mw-list-item"><a href="https://no.wikipedia.org/wiki/Efemeridetidsskalaen" title="Efemeridetidsskalaen – Norwegian Bokmål" lang="nb" hreflang="nb" data-title="Efemeridetidsskalaen" data-language-autonym="Norsk bokmål" data-language-local-name="Norwegian Bokmål" class="interlanguage-link-target"><span>Norsk bokmål</span></a></li><li class="interlanguage-link interwiki-nn mw-list-item"><a href="https://nn.wikipedia.org/wiki/Efemeridetid" title="Efemeridetid – Norwegian Nynorsk" lang="nn" hreflang="nn" data-title="Efemeridetid" data-language-autonym="Norsk nynorsk" data-language-local-name="Norwegian Nynorsk" class="interlanguage-link-target"><span>Norsk nynorsk</span></a></li><li class="interlanguage-link interwiki-uz mw-list-item"><a href="https://uz.wikipedia.org/wiki/Efemerid_vaqti" title="Efemerid vaqti – Uzbek" lang="uz" hreflang="uz" data-title="Efemerid vaqti" data-language-autonym="Oʻzbekcha / ўзбекча" data-language-local-name="Uzbek" class="interlanguage-link-target"><span>Oʻzbekcha / ўзбекча</span></a></li><li class="interlanguage-link interwiki-pl mw-list-item"><a href="https://pl.wikipedia.org/wiki/Czas_efemeryd" title="Czas efemeryd – Polish" lang="pl" hreflang="pl" data-title="Czas efemeryd" data-language-autonym="Polski" data-language-local-name="Polish" class="interlanguage-link-target"><span>Polski</span></a></li><li class="interlanguage-link interwiki-ru mw-list-item"><a href="https://ru.wikipedia.org/wiki/%D0%AD%D1%84%D0%B5%D0%BC%D0%B5%D1%80%D0%B8%D0%B4%D0%BD%D0%BE%D0%B5_%D0%B2%D1%80%D0%B5%D0%BC%D1%8F" title="Эфемеридное время – Russian" lang="ru" hreflang="ru" data-title="Эфемеридное время" data-language-autonym="Русский" data-language-local-name="Russian" class="interlanguage-link-target"><span>Русский</span></a></li><li class="interlanguage-link interwiki-sk mw-list-item"><a href="https://sk.wikipedia.org/wiki/Efemeridov%C3%BD_%C4%8Das" title="Efemeridový čas – Slovak" lang="sk" hreflang="sk" data-title="Efemeridový čas" data-language-autonym="Slovenčina" data-language-local-name="Slovak" class="interlanguage-link-target"><span>Slovenčina</span></a></li><li class="interlanguage-link interwiki-sl mw-list-item"><a href="https://sl.wikipedia.org/wiki/Efemeridni_%C4%8Das" title="Efemeridni čas – Slovenian" lang="sl" hreflang="sl" data-title="Efemeridni čas" data-language-autonym="Slovenščina" data-language-local-name="Slovenian" class="interlanguage-link-target"><span>Slovenščina</span></a></li><li class="interlanguage-link interwiki-sv mw-list-item"><a href="https://sv.wikipedia.org/wiki/Efemeridtidsskalan" title="Efemeridtidsskalan – Swedish" lang="sv" hreflang="sv" data-title="Efemeridtidsskalan" data-language-autonym="Svenska" data-language-local-name="Swedish" class="interlanguage-link-target"><span>Svenska</span></a></li><li class="interlanguage-link interwiki-tr mw-list-item"><a href="https://tr.wikipedia.org/wiki/Efemeris_zaman%C4%B1" title="Efemeris zamanı – Turkish" lang="tr" hreflang="tr" data-title="Efemeris zamanı" data-language-autonym="Türkçe" data-language-local-name="Turkish" class="interlanguage-link-target"><span>Türkçe</span></a></li><li class="interlanguage-link interwiki-uk mw-list-item"><a href="https://uk.wikipedia.org/wiki/%D0%95%D1%84%D0%B5%D0%BC%D0%B5%D1%80%D0%B8%D0%B4%D0%BD%D0%B8%D0%B9_%D1%87%D0%B0%D1%81" title="Ефемеридний час – Ukrainian" lang="uk" hreflang="uk" data-title="Ефемеридний час" data-language-autonym="Українська" data-language-local-name="Ukrainian" class="interlanguage-link-target"><span>Українська</span></a></li><li class="interlanguage-link interwiki-zh mw-list-item"><a href="https://zh.wikipedia.org/wiki/%E6%9B%86%E6%9B%B8%E6%99%82" title="曆書時 – Chinese" lang="zh" hreflang="zh" data-title="曆書時" data-language-autonym="中文" data-language-local-name="Chinese" class="interlanguage-link-target"><span>中文</span></a></li> </ul> <div class="after-portlet after-portlet-lang"><span class="wb-langlinks-edit wb-langlinks-link"><a href="https://www.wikidata.org/wiki/Special:EntityPage/Q908676#sitelinks-wikipedia" title="Edit interlanguage links" class="wbc-editpage">Edit links</a></span></div> </div> </div> </div> </header> <div class="vector-page-toolbar"> <div class="vector-page-toolbar-container"> <div id="left-navigation"> <nav aria-label="Namespaces"> <div id="p-associated-pages" class="vector-menu vector-menu-tabs mw-portlet mw-portlet-associated-pages" > <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li id="ca-nstab-main" class="selected vector-tab-noicon mw-list-item"><a href="/wiki/Ephemeris_time" title="View the content page [c]" 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id="siteSub" class="noprint">From Wikipedia, the free encyclopedia</div> </div> <div id="contentSub"><div id="mw-content-subtitle"></div></div> <div id="mw-content-text" class="mw-body-content"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none">Time standard used in astronomical ephemerides</div> <p>The term <b>ephemeris time</b> (often abbreviated <b>ET</b>) can in principle refer to time in association with any <a href="/wiki/Ephemeris" title="Ephemeris">ephemeris</a> (itinerary of the trajectory of an astronomical object). In practice it has been used more specifically to refer to: </p> <ol><li>a former standard astronomical <a href="/wiki/Time_standard" title="Time standard">time scale</a> adopted in 1952 by the <a href="/wiki/International_Astronomical_Union" title="International Astronomical Union">IAU</a>,<sup id="cite_ref-ESAE_1-0" class="reference"><a href="#cite_note-ESAE-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> and superseded during the 1970s.<sup id="cite_ref-ESAA_2-0" class="reference"><a href="#cite_note-ESAA-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> This time scale was proposed in 1948, to overcome the disadvantages of irregularly fluctuating <a href="/wiki/Mean_solar_time" class="mw-redirect" title="Mean solar time">mean solar time</a>. The intent was to define a uniform time (as far as was then feasible) based on Newtonian theory (see below: <a href="#Definition_(1952)">Definition of ephemeris time (1952)</a>). Ephemeris time was a first application of the concept of a <a href="/wiki/Dynamical_time_scale" title="Dynamical time scale">dynamical time scale</a>, in which the time and time scale are defined implicitly, inferred from the observed position of an astronomical object via the dynamical theory of its motion.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup></li> <li>a modern relativistic coordinate time scale, implemented by the <a href="/wiki/Jet_Propulsion_Laboratory" title="Jet Propulsion Laboratory">JPL</a> ephemeris time argument T<sub>eph</sub>, in a series of numerically integrated <a href="/wiki/Jet_Propulsion_Laboratory_Development_Ephemeris" title="Jet Propulsion Laboratory Development Ephemeris">Development Ephemerides</a>. Among them is the <a href="/wiki/DE405" class="mw-redirect" title="DE405">DE405</a> ephemeris in widespread current use. The time scale represented by T<sub>eph</sub> is closely related to, but distinct (by an offset and constant rate) from, the <a href="/wiki/Barycentric_Coordinate_Time" title="Barycentric Coordinate Time">TCB</a> time scale currently adopted as a standard by the <a href="/wiki/IAU" class="mw-redirect" title="IAU">IAU</a> (see below: <a href="#JPL_ephemeris_time_argument_Teph">JPL ephemeris time argument Teph</a>).<sup id="cite_ref-EMS1998_4-0" class="reference"><a href="#cite_note-EMS1998-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup></li></ol> <p>Most of the following sections relate to the ephemeris time of the 1952 standard. </p><p>An impression has sometimes arisen that ephemeris time was in use from 1900: this probably arose because ET, though proposed and adopted in the period 1948–1952, was defined in detail using formulae that made retrospective use of the <a href="/wiki/Epoch_(astronomy)" title="Epoch (astronomy)">epoch</a> date of 1900 <a href="/wiki/January_0" class="mw-redirect" title="January 0">January 0</a> and of <a href="/wiki/Simon_Newcomb" title="Simon Newcomb">Newcomb</a>'s <a href="/wiki/Newcomb%27s_Tables_of_the_Sun" title="Newcomb's Tables of the Sun"><i>Tables of the Sun</i></a>.<sup id="cite_ref-NewcSun_5-0" class="reference"><a href="#cite_note-NewcSun-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> </p><p>The ephemeris time of the 1952 standard leaves a continuing legacy, through its historical unit <b>ephemeris second</b> which became closely duplicated in the length of the current standard SI <a href="/wiki/Second" title="Second">second</a> (see below: <a href="#Redefinition_of_the_second">Redefinition of the second</a>). </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="History_(1952_standard)"><span id="History_.281952_standard.29"></span>History (1952 standard)</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Ephemeris_time&action=edit&section=1" title="Edit section: History (1952 standard)"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p><b>Ephemeris time</b> (<b>ET</b>), adopted as standard in 1952, was originally designed as an approach to a uniform time scale, to be freed from the effects of irregularity in the rotation of the Earth, "for the convenience of astronomers and other scientists", for example for use in <a href="/wiki/Ephemeris" title="Ephemeris">ephemerides</a> of the Sun (as observed from the Earth), the Moon, and the planets. It was proposed in 1948 by <a href="/wiki/Gerald_Maurice_Clemence" title="Gerald Maurice Clemence">G M Clemence</a>.<sup id="cite_ref-Clemence1948_7-0" class="reference"><a href="#cite_note-Clemence1948-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> </p><p>From the time of <a href="/wiki/John_Flamsteed" title="John Flamsteed">John Flamsteed</a> (1646–1719) it had been believed that the Earth's daily rotation was uniform. But in the later nineteenth and early twentieth centuries, with increasing precision of astronomical measurements, it began to be suspected, and was eventually established, that the rotation of the Earth (<i>i.e.</i> the length of the <a href="/wiki/Day" title="Day">day</a>) showed irregularities on short time scales, and was slowing down on longer time scales. The evidence was compiled by <a href="/wiki/Willem_de_Sitter" title="Willem de Sitter">W de Sitter</a> (1927)<sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> who wrote "If we accept this hypothesis, then the 'astronomical time', given by the Earth's rotation, and used in all practical astronomical computations, differs from the 'uniform' or 'Newtonian' time, which is defined as the independent variable of the equations of celestial mechanics". De Sitter offered a correction to be applied to the mean solar time given by the Earth's rotation to get uniform time. </p><p>Other astronomers of the period also made suggestions for obtaining uniform time, including <a href="/wiki/Andre_Danjon" class="mw-redirect" title="Andre Danjon">A Danjon</a> (1929), who suggested in effect that observed positions of the Moon, Sun and planets, when compared with their well-established gravitational ephemerides, could better and more uniformly define and determine time.<sup id="cite_ref-Clemence1971_9-0" class="reference"><a href="#cite_note-Clemence1971-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> </p><p>Thus the aim developed, to provide a new time scale for astronomical and scientific purposes, to avoid the unpredictable irregularities of the <a href="/wiki/Solar_time#Mean_solar_time" title="Solar time">mean solar time</a> scale, and to replace for these purposes <a href="/wiki/Universal_Time" title="Universal Time">Universal Time</a> (UT) and any other time scale based on the rotation of the Earth around its axis, such as <a href="/wiki/Sidereal_time" title="Sidereal time">sidereal time</a>. </p><p>The American astronomer <a href="/wiki/Gerald_Maurice_Clemence" title="Gerald Maurice Clemence">G M Clemence</a> (1948)<sup id="cite_ref-Clemence1948_7-1" class="reference"><a href="#cite_note-Clemence1948-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> made a detailed proposal of this type based on the results of the English <a href="/wiki/Astronomer_Royal" title="Astronomer Royal">Astronomer Royal</a> <a href="/wiki/Harold_Spencer_Jones" title="Harold Spencer Jones">H Spencer Jones</a> (1939).<sup id="cite_ref-HSJ1939_10-0" class="reference"><a href="#cite_note-HSJ1939-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> Clemence (1948) made it clear that his proposal was intended "for the convenience of astronomers and other scientists only" and that it was "logical to continue the use of mean solar time for civil purposes".<sup id="cite_ref-11" class="reference"><a href="#cite_note-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> </p><p>De Sitter and Clemence both referred to the proposal as 'Newtonian' or 'uniform' time. <a href="/wiki/Dirk_Brouwer" title="Dirk Brouwer">D Brouwer</a> suggested the name 'ephemeris time'.<sup id="cite_ref-ESAAp79_12-0" class="reference"><a href="#cite_note-ESAAp79-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup> </p><p>Following this, an astronomical conference held in Paris in 1950 recommended "that in all cases where the mean solar second is unsatisfactory as a unit of time by reason of its variability, the unit adopted should be the sidereal year at 1900.0, that the time reckoned in this unit be designated <i>ephemeris time</i>", and gave Clemence's formula (see <a href="#Definition_(1952)">Definition of ephemeris time (1952)</a>) for translating mean solar time to ephemeris time. </p><p>The <a href="/wiki/International_Astronomical_Union" title="International Astronomical Union">International Astronomical Union</a> approved this recommendation at its 1952 general assembly.<sup id="cite_ref-ESAAp79_12-1" class="reference"><a href="#cite_note-ESAAp79-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-13" class="reference"><a href="#cite_note-13"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup> Practical introduction took some time (see <a href="#Use_of_ephemeris_time_in_official_almanacs_and_ephemerides">Use of ephemeris time in official almanacs and ephemerides</a>); ephemeris time (ET) remained a standard until superseded in the 1970s by further time scales (see <a href="#Revision_of_time_scales">Revision</a>). </p><p>During the currency of ephemeris time as a standard, the details were revised a little. The unit was redefined in terms of the tropical year at 1900.0 instead of the sidereal year;<sup id="cite_ref-ESAAp79_12-2" class="reference"><a href="#cite_note-ESAAp79-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup> and the standard second was defined first as 1/31556925.975 of the tropical year at 1900.0,<sup id="cite_ref-ESAAp79_12-3" class="reference"><a href="#cite_note-ESAAp79-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-14" class="reference"><a href="#cite_note-14"><span class="cite-bracket">[</span>14<span class="cite-bracket">]</span></a></sup> and then as the slightly modified fraction 1/31556925.9747 instead,<sup id="cite_ref-15" class="reference"><a href="#cite_note-15"><span class="cite-bracket">[</span>15<span class="cite-bracket">]</span></a></sup> finally being redefined in 1967/8 in terms of the cesium atomic clock standard (see below). </p><p>Although ET is no longer directly in use, it leaves a continuing legacy. Its successor time scales, such as TDT, as well as the atomic time scale <a href="/wiki/International_Atomic_Time" title="International Atomic Time">IAT (TAI)</a>, were designed with a relationship that "provides continuity with ephemeris time".<sup id="cite_ref-ESAAp42_16-0" class="reference"><a href="#cite_note-ESAAp42-16"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup> ET was used for the calibration of atomic clocks in the 1950s.<sup id="cite_ref-Mark1958_17-0" class="reference"><a href="#cite_note-Mark1958-17"><span class="cite-bracket">[</span>17<span class="cite-bracket">]</span></a></sup> Close equality between the ET second with the later <a href="/wiki/Second#International_second" title="Second">SI second</a> (as defined with reference to the cesium atomic clock) has been verified to within 1 part in 10<sup>10</sup>.<sup id="cite_ref-refMark1988_18-0" class="reference"><a href="#cite_note-refMark1988-18"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup> </p><p>In this way, decisions made by the original designers of ephemeris time influenced the length of today's <a href="/wiki/Second#International_second" title="Second">standard SI second</a>, and in turn, this has a continuing influence on the number of <a href="/wiki/Leap_second" title="Leap second">leap seconds</a> which have been needed for insertion into current broadcast time scales, to keep them approximately in step with <a href="/wiki/Solar_time#Mean_solar_time" title="Solar time">mean solar time</a>. </p> <div class="mw-heading mw-heading2"><h2 id="Definition_(1952)"><span id="Definition_.281952.29"></span>Definition (1952)</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Ephemeris_time&action=edit&section=2" title="Edit section: Definition (1952)"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Ephemeris time was defined in principle by the orbital motion of the Earth around the Sun<sup id="cite_ref-ESAAp79_12-4" class="reference"><a href="#cite_note-ESAAp79-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup> (but its practical implementation was usually achieved in another way, see below). Its detailed definition was based on <a href="/wiki/Simon_Newcomb" title="Simon Newcomb">Simon Newcomb</a>'s <a href="/wiki/Newcomb%27s_Tables_of_the_Sun" title="Newcomb's Tables of the Sun"><i>Tables of the Sun</i></a> (1895),<sup id="cite_ref-NewcSun_5-1" class="reference"><a href="#cite_note-NewcSun-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> implemented in a new way to accommodate certain observed discrepancies: </p><p>In the introduction to <i>Tables of the Sun,</i> the basis of the tables (p. 9) includes a formula for the Sun's <a href="/wiki/Mean_longitude" title="Mean longitude">mean longitude</a> at a time, indicated by interval T (in units of Julian centuries of 36525 mean solar days<sup id="cite_ref-19" class="reference"><a href="#cite_note-19"><span class="cite-bracket">[</span>19<span class="cite-bracket">]</span></a></sup>), reckoned from Greenwich Mean Noon on 0 January 1900: </p> <dl><dd>Ls = 279° 41' 48".04 + 129,602,768".13T +1".089T<sup>2</sup> . . . . . (1)</dd></dl> <p>Spencer Jones' work of 1939<sup id="cite_ref-HSJ1939_10-1" class="reference"><a href="#cite_note-HSJ1939-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> showed that differences between the observed positions of the Sun and the predicted positions given by Newcomb's formula demonstrated the need for the following correction to the formula: </p> <dl><dd>ΔLs = + 1".00 + 2".97T + 1".23T<sup>2</sup> + 0.0748B</dd></dl> <p>where "the times of observation are in Universal time, not corrected to Newtonian time," and 0.0748B represents an irregular fluctuation calculated from lunar observations.<sup id="cite_ref-Clem1948-172_20-0" class="reference"><a href="#cite_note-Clem1948-172-20"><span class="cite-bracket">[</span>20<span class="cite-bracket">]</span></a></sup> </p><p>Thus, a conventionally corrected form of Newcomb's formula, incorporating the corrections on the basis of mean solar time, would be the sum of the two preceding expressions: </p> <dl><dd>Ls = 279° 41' 49".04 + 129,602,771".10T +2".32T<sup>2</sup> +0.0748B . . . . . (2)</dd></dl> <p>Clemence's 1948 proposal, however, did not adopt such a correction of mean solar time. Instead, the same numbers were used as in Newcomb's original uncorrected formula (1), but now applied somewhat prescriptively, to define a new time and time scale implicitly, based on the real position of the Sun: </p> <dl><dd>Ls = 279° 41' 48".04 + 129,602,768".13E +1".089E<sup>2</sup> . . . . . (3)</dd></dl> <p>With this reapplication, the time variable, now given as E, represents time in ephemeris centuries of 36525 <b>ephemeris days</b> of 86400 <b>ephemeris seconds</b> each. The 1961 official reference summarized the concept as such: "The origin and rate of ephemeris time are defined to make the Sun's mean longitude agree with Newcomb's expression"<sup id="cite_ref-21" class="reference"><a href="#cite_note-21"><span class="cite-bracket">[</span>21<span class="cite-bracket">]</span></a></sup> </p><p>From the comparison of formulae (2) and (3), both of which express the same real solar motion in the same real time but defined on separate time scales, Clemence arrived at an explicit expression, estimating the difference in seconds of time between ephemeris time and mean solar time, in the sense (ET-UT): </p><p><span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \delta t=+24^{s}.349+72^{s}.3165T+29^{s}.949T^{2}+1.821B}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>δ</mi> <mi>t</mi> <mo>=</mo> <mo>+</mo> <msup> <mn>24</mn> <mrow class="MJX-TeXAtom-ORD"> <mi>s</mi> </mrow> </msup> <mn>.349</mn> <mo>+</mo> <msup> <mn>72</mn> <mrow class="MJX-TeXAtom-ORD"> <mi>s</mi> </mrow> </msup> <mn>.3165</mn> <mi>T</mi> <mo>+</mo> <msup> <mn>29</mn> <mrow class="MJX-TeXAtom-ORD"> <mi>s</mi> </mrow> </msup> <mn>.949</mn> <msup> <mi>T</mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msup> <mo>+</mo> <mn>1.821</mn> <mi>B</mi> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \delta t=+24^{s}.349+72^{s}.3165T+29^{s}.949T^{2}+1.821B}</annotation> </semantics> </math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/9770d89572d9b1cf58c1901e041e645fe02e076c" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.505ex; width:50.337ex; height:2.843ex;" alt="{\displaystyle \delta t=+24^{s}.349+72^{s}.3165T+29^{s}.949T^{2}+1.821B}"></span> . . . . . (4)<sup id="cite_ref-Clem1948-172_20-1" class="reference"><a href="#cite_note-Clem1948-172-20"><span class="cite-bracket">[</span>20<span class="cite-bracket">]</span></a></sup> </p><p>with the 24.349 seconds of time corresponding to the 1.00" in ΔLs. Clemence's formula (today superseded by more modern estimations) was included in the original conference decision on ephemeris time. In view of the fluctuation term, practical determination of the difference between ephemeris time and UT depended on observation. Inspection of the formulae above shows that the (ideally constant) units of ephemeris time have been, for the whole of the twentieth century, very slightly shorter than the corresponding (but not precisely constant) units of mean solar time (which, besides their irregular fluctuations, tend to lengthen gradually). This finding is consistent with the modern results of Morrison and Stephenson<sup id="cite_ref-morr3_22-0" class="reference"><a href="#cite_note-morr3-22"><span class="cite-bracket">[</span>22<span class="cite-bracket">]</span></a></sup> (see article <a href="/wiki/%CE%94T_(timekeeping)" title="ΔT (timekeeping)">ΔT</a>). </p> <div class="mw-heading mw-heading2"><h2 id="Implementations">Implementations</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Ephemeris_time&action=edit&section=3" title="Edit section: Implementations"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <div class="mw-heading mw-heading3"><h3 id="Secondary_realizations_by_lunar_observations">Secondary realizations by lunar observations</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Ephemeris_time&action=edit&section=4" title="Edit section: Secondary realizations by lunar observations"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Although ephemeris time was defined in principle by the orbital motion of the Earth around the Sun,<sup id="cite_ref-23" class="reference"><a href="#cite_note-23"><span class="cite-bracket">[</span>23<span class="cite-bracket">]</span></a></sup> it was usually measured in practice by the orbital motion of the Moon around the Earth.<sup id="cite_ref-24" class="reference"><a href="#cite_note-24"><span class="cite-bracket">[</span>24<span class="cite-bracket">]</span></a></sup> These measurements can be considered as secondary realizations (in a <a href="/wiki/Metrology" title="Metrology">metrological</a> sense) of the primary definition of ET in terms of the solar motion, after a calibration of the mean motion of the Moon with respect to the mean motion of the Sun.<sup id="cite_ref-guin88_25-0" class="reference"><a href="#cite_note-guin88-25"><span class="cite-bracket">[</span>25<span class="cite-bracket">]</span></a></sup> </p><p>Reasons for the use of lunar measurements were practically based: the Moon moves against the background of stars about 13 times as fast as the Sun's corresponding rate of motion, and the accuracy of time determinations from lunar measurements is correspondingly greater. </p><p>When ephemeris time was first adopted, time scales were still based on astronomical observation, as they always had been. The accuracy was limited by the accuracy of optical observation, and corrections of clocks and time signals were published in arrear. </p> <div class="mw-heading mw-heading3"><h3 id="Secondary_realizations_by_atomic_clocks">Secondary realizations by atomic clocks</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Ephemeris_time&action=edit&section=5" title="Edit section: Secondary realizations by atomic clocks"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A few years later, with the invention of the <a href="/wiki/Atomic_clock" title="Atomic clock">cesium atomic clock</a>, an alternative offered itself. Increasingly, after the calibration in 1958 of the cesium atomic clock by reference to ephemeris time,<sup id="cite_ref-Mark1958_17-1" class="reference"><a href="#cite_note-Mark1958-17"><span class="cite-bracket">[</span>17<span class="cite-bracket">]</span></a></sup> cesium atomic clocks running on the basis of ephemeris seconds began to be used and kept in step with ephemeris time. The atomic clocks offered a further secondary realization of ET, on a quasi-real time basis<sup id="cite_ref-guin88_25-1" class="reference"><a href="#cite_note-guin88-25"><span class="cite-bracket">[</span>25<span class="cite-bracket">]</span></a></sup> that soon proved to be more useful than the primary ET standard: not only more convenient, but also more precisely uniform than the primary standard itself. Such secondary realizations were used and described as 'ET', with an awareness that the time scales based on the atomic clocks were not identical to that defined by the primary ephemeris time standard, but rather, an improvement over it on account of their closer approximation to uniformity.<sup id="cite_ref-26" class="reference"><a href="#cite_note-26"><span class="cite-bracket">[</span>26<span class="cite-bracket">]</span></a></sup> The atomic clocks gave rise to the <a href="/wiki/International_Atomic_Time" title="International Atomic Time">atomic time scale</a>, and to what was first called Terrestrial Dynamical Time and is now <a href="/wiki/Terrestrial_Time" title="Terrestrial Time">Terrestrial Time</a>, defined to provide continuity with ET.<sup id="cite_ref-ESAAp42_16-1" class="reference"><a href="#cite_note-ESAAp42-16"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup> </p><p>The availability of atomic clocks, together with the increasing accuracy of astronomical observations (which meant that relativistic corrections were at least in the foreseeable future no longer going to be small enough to be neglected),<sup id="cite_ref-27" class="reference"><a href="#cite_note-27"><span class="cite-bracket">[</span>27<span class="cite-bracket">]</span></a></sup> led to the eventual replacement of the ephemeris time standard by more refined time scales including <a href="/wiki/Terrestrial_time" class="mw-redirect" title="Terrestrial time">terrestrial time</a> and <a href="/wiki/Barycentric_dynamical_time" class="mw-redirect" title="Barycentric dynamical time">barycentric dynamical time</a>, to which ET can be seen as an approximation. </p> <div class="mw-heading mw-heading2"><h2 id="Revision_of_time_scales">Revision of time scales</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Ephemeris_time&action=edit&section=6" title="Edit section: Revision of time scales"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In 1976, the <a href="/wiki/International_Astronomical_Union" title="International Astronomical Union">IAU</a> resolved that the theoretical basis for its then-current (since 1952) standard of Ephemeris Time was non-relativistic, and that therefore, beginning in 1984, Ephemeris Time would be replaced by two relativistic timescales intended to constitute <a href="/wiki/Dynamical_time_scale" title="Dynamical time scale">dynamical timescales</a>: <a href="/wiki/Terrestrial_Time#History" title="Terrestrial Time">Terrestrial Dynamical Time (TDT)</a> and <a href="/wiki/Barycentric_Dynamical_Time" title="Barycentric Dynamical Time">Barycentric Dynamical Time (TDB)</a>.<sup id="cite_ref-28" class="reference"><a href="#cite_note-28"><span class="cite-bracket">[</span>28<span class="cite-bracket">]</span></a></sup> Difficulties were recognized, which led to these, in turn, being superseded in the 1990s by time scales <a href="/wiki/Terrestrial_Time" title="Terrestrial Time">Terrestrial Time (TT)</a>, <a href="/wiki/Geocentric_Coordinate_Time" title="Geocentric Coordinate Time">Geocentric Coordinate Time GCT (TCG)</a> and <a href="/wiki/Barycentric_Coordinate_Time" title="Barycentric Coordinate Time">Barycentric Coordinate Time BCT (TCB)</a>.<sup id="cite_ref-ESAAp42_16-2" class="reference"><a href="#cite_note-ESAAp42-16"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="JPL_ephemeris_time_argument_Teph">JPL ephemeris time argument T<sub>eph</sub></h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Ephemeris_time&action=edit&section=7" title="Edit section: JPL ephemeris time argument Teph"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>High-precision <a href="/wiki/Ephemeris" title="Ephemeris">ephemerides</a> of sun, moon and planets were developed and calculated at the <a href="/wiki/Jet_Propulsion_Laboratory" title="Jet Propulsion Laboratory">Jet Propulsion Laboratory</a> (JPL) over a long period, and the latest available were adopted for the ephemerides in the <a href="/wiki/Astronomical_Almanac" title="Astronomical Almanac">Astronomical Almanac</a> starting in 1984. Although not an IAU standard, the ephemeris time argument T<sub>eph</sub> has been in use at that institution since the 1960s. The time scale represented by T<sub>eph</sub> has been characterized as a <a href="/wiki/Principle_of_relativity" title="Principle of relativity">relativistic</a> coordinate time that differs from <a href="/wiki/Terrestrial_Time" title="Terrestrial Time">Terrestrial Time</a> only by small periodic terms with an amplitude not exceeding 2 milliseconds of time: it is linearly related to, but distinct (by an offset and constant rate which is of the order of 0.5 s/a) from the <a href="/wiki/Barycentric_Coordinate_Time" title="Barycentric Coordinate Time">TCB</a> time scale adopted in 1991 as a standard by the <a href="/wiki/IAU" class="mw-redirect" title="IAU">IAU</a>. Thus for clocks on or near the <a href="/wiki/Geoid" title="Geoid">geoid</a>, T<sub>eph</sub> (within 2 milliseconds), but not so closely TCB, can be used as approximations to Terrestrial Time, and via the standard ephemerides T<sub>eph</sub> is in widespread use.<sup id="cite_ref-EMS1998_4-1" class="reference"><a href="#cite_note-EMS1998-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> </p><p>Partly in acknowledgement of the widespread use of T<sub>eph</sub> via the JPL ephemerides, IAU resolution 3 of 2006<sup id="cite_ref-29" class="reference"><a href="#cite_note-29"><span class="cite-bracket">[</span>29<span class="cite-bracket">]</span></a></sup> (re-)defined <a href="/wiki/Barycentric_Dynamical_Time" title="Barycentric Dynamical Time">Barycentric Dynamical Time</a> (TDB) as a current standard. As re-defined in 2006, TDB is a linear transformation of <a href="/wiki/Barycentric_Coordinate_Time" title="Barycentric Coordinate Time">TCB</a>. The same IAU resolution also stated (in note 4) that the "independent time argument of the JPL ephemeris <a href="/wiki/DE405" class="mw-redirect" title="DE405">DE405</a>, which is called T<sub>eph</sub>" (here the IAU source cites<sup id="cite_ref-EMS1998_4-2" class="reference"><a href="#cite_note-EMS1998-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup>), "is for practical purposes the same as <a href="/wiki/Barycentric_Dynamical_Time" title="Barycentric Dynamical Time">TDB</a> defined in this Resolution". Thus the new TDB, like T<sub>eph</sub>, is essentially a more refined continuation of the older ephemeris time ET and (apart from the <span class="nowrap">< 2 ms</span> periodic fluctuations) has the same mean rate as that established for ET in the 1950s. </p> <div class="mw-heading mw-heading2"><h2 id="Use_in_official_almanacs_and_ephemerides">Use in official almanacs and ephemerides</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Ephemeris_time&action=edit&section=8" title="Edit section: Use in official almanacs and ephemerides"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Ephemeris time based on the standard adopted in 1952 was introduced into the Astronomical Ephemeris (UK) and the <a href="/wiki/American_Ephemeris_and_Nautical_Almanac" title="American Ephemeris and Nautical Almanac">American Ephemeris and Nautical Almanac</a>, replacing UT in the main ephemerides in the issues for 1960 and after.<sup id="cite_ref-30" class="reference"><a href="#cite_note-30"><span class="cite-bracket">[</span>30<span class="cite-bracket">]</span></a></sup> (But the ephemerides in the Nautical Almanac, by then a separate publication for the use of navigators, continued to be expressed in terms of UT.) The ephemerides continued on this basis through 1983 (with some changes due to adoption of improved values of astronomical constants), after which, for 1984 onwards, they adopted the <a href="/wiki/JPL" class="mw-redirect" title="JPL">JPL</a> ephemerides. </p><p>Previous to the 1960 change, the 'Improved Lunar Ephemeris' had already been made available in terms of ephemeris time for the years 1952—1959<sup id="cite_ref-31" class="reference"><a href="#cite_note-31"><span class="cite-bracket">[</span>31<span class="cite-bracket">]</span></a></sup> (computed by <a href="/wiki/Wallace_John_Eckert" title="Wallace John Eckert">W J Eckert</a> from <a href="/wiki/Ernest_William_Brown" title="Ernest William Brown">Brown</a>'s theory with modifications recommended by Clemence (1948)). </p> <div class="mw-heading mw-heading2"><h2 id="Redefinition_of_the_second">Redefinition of the second</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Ephemeris_time&action=edit&section=9" title="Edit section: Redefinition of the second"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Successive definitions of the unit of ephemeris time are mentioned above (<a href="#History_(1952_standard)">History</a>). The value adopted for the 1956/1960 standard second: </p> <dl><dd>the fraction 1/31 556 925.9747 of the <a href="/wiki/Tropical_year" title="Tropical year">tropical year</a> for 1900 <a href="/wiki/January_0" class="mw-redirect" title="January 0">January 0</a> at 12 hours ephemeris time.</dd></dl> <p>was obtained from the linear time-coefficient in Newcomb's expression for the solar mean longitude (above), taken and applied with the same meaning for the time as in formula (3) above. The relation with Newcomb's coefficient can be seen from: </p> <dl><dd>1/31 556 925.9747 = 129 602 768.13 / (360×60×60×36 525×86 400).</dd></dl> <p><a href="/wiki/Caesium" title="Caesium">Caesium</a> <a href="/wiki/Atomic_clocks" class="mw-redirect" title="Atomic clocks">atomic clocks</a> became operational in 1955, and quickly confirmed the evidence that the rotation of the Earth fluctuated irregularly.<sup id="cite_ref-McCarthySeidelmann2009_32-0" class="reference"><a href="#cite_note-McCarthySeidelmann2009-32"><span class="cite-bracket">[</span>32<span class="cite-bracket">]</span></a></sup> This confirmed the unsuitability of the mean solar second of Universal Time as a measure of time interval for the most precise purposes. After three years of comparisons with lunar observations, <a href="/wiki/William_Markowitz" title="William Markowitz">Markowitz</a> et al. (1958) determined that the ephemeris second corresponded to 9 192 631 770 ± 20 cycles of the chosen cesium resonance.<sup id="cite_ref-Mark1958_17-2" class="reference"><a href="#cite_note-Mark1958-17"><span class="cite-bracket">[</span>17<span class="cite-bracket">]</span></a></sup> </p><p>Following this, in 1967/68, the General Conference on Weights and Measures (CGPM) replaced the definition of the <a href="/wiki/Second#International_second" title="Second">SI second</a> by the following: </p> <blockquote><p>The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. </p></blockquote> <p>Although this is an independent definition that does not refer to the older basis of ephemeris time, it uses the same quantity as the value of the ephemeris second measured by the cesium clock in 1958. This <a href="/wiki/Second#International_second" title="Second">SI second referred to atomic time</a> was later verified by Markowitz (1988) to be in agreement, within 1 part in 10<sup>10</sup>, with the second of ephemeris time as determined from lunar observations.<sup id="cite_ref-refMark1988_18-1" class="reference"><a href="#cite_note-refMark1988-18"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup> </p><p>For practical purposes the length of the ephemeris second can be taken as equal to the length of the second of <a href="/wiki/Barycentric_Dynamical_Time" title="Barycentric Dynamical Time">Barycentric Dynamical Time (TDB)</a> or <a href="/wiki/Terrestrial_Time" title="Terrestrial Time">Terrestrial Time (TT)</a> or its predecessor TDT. </p><p>The difference between ET and UT is called <a href="/wiki/%CE%94T_(timekeeping)" title="ΔT (timekeeping)">ΔT</a>; it changes irregularly, but the long-term trend is <a href="/wiki/Parabola" title="Parabola">parabolic</a>, decreasing from ancient times until the nineteenth century,<sup id="cite_ref-morr3_22-1" class="reference"><a href="#cite_note-morr3-22"><span class="cite-bracket">[</span>22<span class="cite-bracket">]</span></a></sup> and increasing since then at a rate corresponding to an increase in the solar day length of 1.7 ms per century (see <a href="/wiki/Leap_second" title="Leap second">leap seconds</a>). </p><p><a href="/wiki/International_Atomic_Time" title="International Atomic Time">International Atomic Time</a> (TAI) was set equal to <a href="/wiki/Universal_Time" title="Universal Time">UT2</a> at 1 January 1958 0:00:00. At that time, ΔT was already about 32.18 seconds. The difference between Terrestrial Time (TT) (the successor to ephemeris time) and atomic time was later defined as follows: </p> <dl><dd>1977 January 1.000 3725 TT = 1977 January 1.000 0000 TAI, <i>i.e.</i></dd></dl> <dl><dd>TT − TAI = 32.184 seconds</dd></dl> <p>This difference may be assumed constant—the rates of TT and TAI are designed to be identical. </p> <div class="mw-heading mw-heading2"><h2 id="Notes_and_references">Notes and references</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Ephemeris_time&action=edit&section=10" title="Edit section: Notes and references"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <div class="mw-references-wrap mw-references-columns"><ol class="references"> <li id="cite_note-ESAE-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-ESAE_1-0">^</a></b></span> <span class="reference-text"><a href="#refESAE">'ESAE 1961': 'Explanatory Supplement (1961)</a>, esp. p. 9.</span> </li> <li id="cite_note-ESAA-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-ESAA_2-0">^</a></b></span> <span class="reference-text"><a href="#refESAA">'ESAA (1992)': P K Seidelmann (ed).</a>, especially <a rel="nofollow" class="external text" href="https://books.google.com/books?id=uJ4JhGJANb4C&pg=PA41">at pp. 41—42</a> and <a rel="nofollow" class="external text" href="https://books.google.com/books?id=uJ4JhGJANb4C&pg=PA79">at p. 79</a>.</span> </li> <li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><a href="#refGuin1988">B Guinot and P K Seidelmann (1988)</a>, at p. 304—5.</span> </li> <li id="cite_note-EMS1998-4"><span class="mw-cite-backlink">^ <a href="#cite_ref-EMS1998_4-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-EMS1998_4-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-EMS1998_4-2"><sup><i><b>c</b></i></sup></a></span> <span class="reference-text"><a href="#refEMS1998">E M Standish (1998)</a>.</span> </li> <li id="cite_note-NewcSun-5"><span class="mw-cite-backlink">^ <a href="#cite_ref-NewcSun_5-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-NewcSun_5-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><a href="#refNewcSun">S Newcomb (1895)</a>.</span> </li> <li id="cite_note-6"><span class="mw-cite-backlink"><b><a href="#cite_ref-6">^</a></b></span> <span class="reference-text">For the components of the definition including its retrospective aspect, see <a href="#refClem1948">G M Clemence (1948)</a>, esp. p. 172, and <a href="#refESAE">'ESAE 1961': 'Explanatory Supplement (1961)</a>, esp. pages 69 and 87.</span> </li> <li id="cite_note-Clemence1948-7"><span class="mw-cite-backlink">^ <a href="#cite_ref-Clemence1948_7-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Clemence1948_7-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><a href="#refClem1948">G M Clemence (1948)</a>.</span> </li> <li id="cite_note-8"><span class="mw-cite-backlink"><b><a href="#cite_ref-8">^</a></b></span> <span class="reference-text"><a href="#refSitter1927">W de Sitter (1927)</a>.</span> </li> <li id="cite_note-Clemence1971-9"><span class="mw-cite-backlink"><b><a href="#cite_ref-Clemence1971_9-0">^</a></b></span> <span class="reference-text"><a href="#refClem1971">G M Clemence (1971)</a>.</span> </li> <li id="cite_note-HSJ1939-10"><span class="mw-cite-backlink">^ <a href="#cite_ref-HSJ1939_10-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-HSJ1939_10-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><a href="#refHSJ1939">H Spencer Jones (1939)</a>.</span> </li> <li id="cite_note-11"><span class="mw-cite-backlink"><b><a href="#cite_ref-11">^</a></b></span> <span class="reference-text"><a href="#refClem1948">Clemence (1948)</a>, at p. 171.</span> </li> <li id="cite_note-ESAAp79-12"><span class="mw-cite-backlink">^ <a href="#cite_ref-ESAAp79_12-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-ESAAp79_12-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-ESAAp79_12-2"><sup><i><b>c</b></i></sup></a> <a href="#cite_ref-ESAAp79_12-3"><sup><i><b>d</b></i></sup></a> <a href="#cite_ref-ESAAp79_12-4"><sup><i><b>e</b></i></sup></a></span> <span class="reference-text"><a href="#refESAA">ESAA (1992)</a>, see <a rel="nofollow" class="external text" href="https://books.google.com/books?id=uJ4JhGJANb4C&pg=PA79">page 79</a>.</span> </li> <li id="cite_note-13"><span class="mw-cite-backlink"><b><a href="#cite_ref-13">^</a></b></span> <span class="reference-text">At the <a href="/wiki/International_Astronomical_Union" title="International Astronomical Union">IAU</a> meeting in Rome 1952: see <a href="#refESAE">ESAE (1961)</a> at sect.1C, p. 9; also <a href="#refClem1971">Clemence (1971)</a>.</span> </li> <li id="cite_note-14"><span class="mw-cite-backlink"><b><a href="#cite_ref-14">^</a></b></span> <span class="reference-text">ESAA 1992, p. 79: citing decision of <a href="/wiki/International_Committee_for_Weights_and_Measures" class="mw-redirect" title="International Committee for Weights and Measures">International Committee for Weights and Measures</a> (CIPM), Sept 1954.</span> </li> <li id="cite_note-15"><span class="mw-cite-backlink"><b><a href="#cite_ref-15">^</a></b></span> <span class="reference-text"><a href="#refESAA">ESAA (1992)</a>, see <a rel="nofollow" class="external text" href="https://books.google.com/books?id=uJ4JhGJANb4C&pg=PA80">page 80</a>, citing CIPM recommendation Oct 1956, adopted 1960 by the <a href="/wiki/General_Conference_on_Weights_and_Measures" title="General Conference on Weights and Measures">General Conference on Weights and Measures</a> (CGPM).</span> </li> <li id="cite_note-ESAAp42-16"><span class="mw-cite-backlink">^ <a href="#cite_ref-ESAAp42_16-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-ESAAp42_16-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-ESAAp42_16-2"><sup><i><b>c</b></i></sup></a></span> <span class="reference-text"><a href="#refESAA">ESAA (1992)</a>, at <a rel="nofollow" class="external text" href="https://books.google.com/books?id=uJ4JhGJANb4C&pg=PA42">page 42</a>.</span> </li> <li id="cite_note-Mark1958-17"><span class="mw-cite-backlink">^ <a href="#cite_ref-Mark1958_17-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Mark1958_17-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-Mark1958_17-2"><sup><i><b>c</b></i></sup></a></span> <span class="reference-text"><a href="#refMark1958">W Markowitz, R G Hall, L Essen, J V L Parry (1958)</a></span> </li> <li id="cite_note-refMark1988-18"><span class="mw-cite-backlink">^ <a href="#cite_ref-refMark1988_18-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-refMark1988_18-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><a href="#refMark1988">Wm Markowitz (1988)</a>.</span> </li> <li id="cite_note-19"><span class="mw-cite-backlink"><b><a href="#cite_ref-19">^</a></b></span> <span class="reference-text">The unit of <i>mean solar</i> day is left implicit on p. 9 but made explicit on p. 20 of <a href="#refNewcSun">Newcomb (1895)</a>.</span> </li> <li id="cite_note-Clem1948-172-20"><span class="mw-cite-backlink">^ <a href="#cite_ref-Clem1948-172_20-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Clem1948-172_20-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><a href="#refClem1948">Clemence (1948)</a>, p. 172, following <a href="#refHSJ1939">Spencer Jones (1939)</a>.</span> </li> <li id="cite_note-21"><span class="mw-cite-backlink"><b><a href="#cite_ref-21">^</a></b></span> <span class="reference-text"><a href="#refESAE">ESAE (1961)</a> at p. 70.</span> </li> <li id="cite_note-morr3-22"><span class="mw-cite-backlink">^ <a href="#cite_ref-morr3_22-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-morr3_22-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><a href="#morrste2004">L V Morrison & F R Stephenson (2004)</a>; also <a href="#stemorr84">F R Stephenson, L V Morrison (1984)</a>, and <a href="#stemorr95">F R Stephenson, L V Morrison (1995)</a>.</span> </li> <li id="cite_note-23"><span class="mw-cite-backlink"><b><a href="#cite_ref-23">^</a></b></span> <span class="reference-text"><a href="#refClem1948">Clemence (1948)</a>, at pp. 171—3.</span> </li> <li id="cite_note-24"><span class="mw-cite-backlink"><b><a href="#cite_ref-24">^</a></b></span> <span class="reference-text"><a href="#refMark1955">W Markowitz & others (1955)</a>; <a href="#refMark1959">W Markowitz (1959)</a>; also <a href="#refMark1958">W Markowitz, R G Hall, L Essen, J V L Parry (1958)</a>.</span> </li> <li id="cite_note-guin88-25"><span class="mw-cite-backlink">^ <a href="#cite_ref-guin88_25-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-guin88_25-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"> <a href="#refGuin1988">B Guinot & P K Seidelmann (1988)</a>, at p. 305.</span> </li> <li id="cite_note-26"><span class="mw-cite-backlink"><b><a href="#cite_ref-26">^</a></b></span> <span class="reference-text"><a href="#tr32-1306">W G Melbourne & others, 1968</a>, section II.E.4-5, pages 15—16, including footnote 7, noted that the Jet Propulsion Laboratory spacecraft tracking and data reduction programs of that time (including the Single Precision Orbit Determination Program) used, as ET, the current US atomic clock time A.1 offset by 32.25 seconds. The discussion also noted that the usage was "inaccurate" (the quantity indicated was not identical with any of the other realizations of ET such as ET0, ET1), and that while A.1 gave "certainly a closer approximation to uniform time than ET1" there were no grounds for considering either the atomic clocks or any other measures of ET as (perfectly) uniform. Section II.F, pages 18—19, indicates that an improved time measure of (A.1 + 32.15 seconds), applied in the JPL Double Precision Orbit Determination Program, was also designated ET.</span> </li> <li id="cite_note-27"><span class="mw-cite-backlink"><b><a href="#cite_ref-27">^</a></b></span> <span class="reference-text"><a href="#refWink1977">G M R Winkler and T C van Flandern (1977)</a>.</span> </li> <li id="cite_note-28"><span class="mw-cite-backlink"><b><a href="#cite_ref-28">^</a></b></span> <span class="reference-text"><a href="#refIAU1976">IAU resolutions (1976)</a>; see also ESAA (1992) <a rel="nofollow" class="external text" href="https://books.google.com/books?id=uJ4JhGJANb4C&pg=PA41">at p. 41</a>.</span> </li> <li id="cite_note-29"><span class="mw-cite-backlink"><b><a href="#cite_ref-29">^</a></b></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.iau.org/static/resolutions/IAU2006_Resol3.pdf">"IAU 2006 resolution 3"</a> <span class="cs1-format">(PDF)</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=IAU+2006+resolution+3&rft_id=http%3A%2F%2Fwww.iau.org%2Fstatic%2Fresolutions%2FIAU2006_Resol3.pdf&rfr_id=info%3Asid%2Fen.wikipedia.org%3AEphemeris+time" class="Z3988"></span></span> </li> <li id="cite_note-30"><span class="mw-cite-backlink"><b><a href="#cite_ref-30">^</a></b></span> <span class="reference-text"><a href="#refESAA">ESAA 1992</a>, <a rel="nofollow" class="external text" href="https://books.google.com/books?id=uJ4JhGJANb4C&pg=PA612">at p. 612</a>.</span> </li> <li id="cite_note-31"><span class="mw-cite-backlink"><b><a href="#cite_ref-31">^</a></b></span> <span class="reference-text">"Improved Lunar Ephemeris", US Government Printing Office, 1954.</span> </li> <li id="cite_note-McCarthySeidelmann2009-32"><span class="mw-cite-backlink"><b><a href="#cite_ref-McCarthySeidelmann2009_32-0">^</a></b></span> <span class="reference-text"><a href="#McCarthySeidelmann2009">McCarthy & Seidelmann (2009) Ch. 4, "Variable Earth Rotation"</a></span> </li> </ol></div> <div class="mw-heading mw-heading2"><h2 id="Bibliography">Bibliography</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Ephemeris_time&action=edit&section=11" title="Edit section: Bibliography"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><cite id="refClem1948"><a href="/wiki/Gerald_Maurice_Clemence" title="Gerald Maurice Clemence">G M Clemence</a>, <a rel="nofollow" class="external text" href="http://articles.adsabs.harvard.edu/full/1948AJ.....53..169C">"On the System of Astronomical Constants"</a>, <i>Astronomical Journal</i>, vol. 53(6) (1948), issue #1170, pp. 169–179.</cite></li> <li><cite id="refClem1971"><a href="/wiki/Gerald_Maurice_Clemence" title="Gerald Maurice Clemence">G M Clemence</a> (1971), <a rel="nofollow" class="external text" href="http://articles.adsabs.harvard.edu/full/1971JHA.....2...73C">"The Concept of Ephemeris Time"</a>, <i>Journal for the History of Astronomy</i>, vol. 2 (1971), pp. 73–79.</cite></li> <li><cite id="refGuin1988">B Guinot and P K Seidelmann (1988), <a rel="nofollow" class="external text" href="http://adsabs.harvard.edu/abs/1988A%26A...194..304G">"Time scales – Their history, definition and interpretation"</a>, <i>Astronomy and Astrophysics</i>, vol. 194 (nos. 1–2) (April 1988), pp. 304–308.</cite></li> <li><cite id="refESAA">'ESAA (1992)': P K Seidelmann (ed.), <a rel="nofollow" class="external text" href="https://books.google.com/books?id=uJ4JhGJANb4C">"Explanatory Supplement to the Astronomical Almanac"</a>, University Science Books, CA, 1992; <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/0-935702-68-7" title="Special:BookSources/0-935702-68-7">0-935702-68-7</a>.</cite></li> <li><cite id="refESAE">'ESAE 1961': "Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac" ('prepared jointly by the Nautical Almanac Offices of the United Kingdom and the United States of America', HMSO, London, 1961).</cite></li> <li><cite id="refIAU1976"><a href="/wiki/IAU" class="mw-redirect" title="IAU">IAU</a> resolutions (1976): <a rel="nofollow" class="external text" href="http://www.iau.org/static/resolutions/IAU1976_French.pdf">Resolutions adopted by the IAU in 1976 at Grenoble</a>.</cite></li> <li><cite id="refILE">"Improved Lunar Ephemeris", US Government Printing Office, 1954.</cite></li> <li><cite id="refMark1955"><a href="/wiki/William_Markowitz" title="William Markowitz">W Markowitz</a>, R G Hall, S Edelson (1955), <a rel="nofollow" class="external text" href="http://articles.adsabs.harvard.edu/full/1955AJ.....60R.171M">"Ephemeris time from photographic positions of the moon"</a>, <i>Astronomical Journal</i>, vol. 60 (1955), p. 171.</cite></li> <li><cite id="refMark1958"><a href="/wiki/William_Markowitz" title="William Markowitz">W Markowitz</a>, R G Hall, L Essen, J V L Parry (1958), <a rel="nofollow" class="external text" href="http://www.leapsecond.com/history/1958-PhysRev-v1-n3-Markowitz-Hall-Essen-Parry.pdf">"Frequency of cesium in terms of ephemeris time"</a>, Physical Review Letters, vol. 1 (1958), 105–107.</cite></li> <li><cite id="refMark1959"><a href="/wiki/William_Markowitz" title="William Markowitz">W Markowitz</a> (1959), <a rel="nofollow" class="external text" href="http://adsabs.harvard.edu/full/1959AJ.....64..106M">"Variations in the Rotation of the Earth, Results Obtained with the Dual-Rate Moon Camera and Photographic Zenith Tubes"</a>, <i>Astronomical Journal</i>, vol. 64 (1959), pp. 106–113.</cite></li> <li><cite id="refMark1988"><a href="/wiki/William_Markowitz" title="William Markowitz">Wm Markowitz</a> (1988), <a rel="nofollow" class="external text" href="http://articles.adsabs.harvard.edu/full/1988IAUS..128..413M">"Comparisons of ET(Solar), ET(Lunar), UT and TDT"</a>, in A K Babcock & G A Wilkins (eds.), <i>The Earth's Rotation and Reference Frames for Geodesy and Geophysics</i>, IAU Symposia #128 (1988), pp. 413–418.</cite></li> <li><cite id="McCarthySeidelmann2009"><a href="/wiki/Dennis_McCarthy_(scientist)" title="Dennis McCarthy (scientist)">Dennis McCarthy</a> & P. Kenneth Seidelmann (2009), <i>TIME From Earth Rotation to Atomic Physics</i>, Wiley-VCH, Weinheim, <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-3-527-40780-4" title="Special:BookSources/978-3-527-40780-4">978-3-527-40780-4</a>.</cite></li> <li><cite id="tr32-1306">W G Melbourne, J D Mulholland, W L Sjogren, F M Sturms (1968), "<a rel="nofollow" class="external text" href="https://ntrs.nasa.gov/search.jsp?R=19690001472">Constants and Related Information for Astrodynamic Calculations</a>", NASA Technical Report 32-1306, Jet Propulsion Laboratory, July 15, 1968.</cite></li> <li><cite id="morrste2004">L V Morrison, F R Stephenson (2004), <a rel="nofollow" class="external text" href="http://articles.adsabs.harvard.edu/full/2004JHA....35..327M">"Historical values of the Earth's clock error ΔT and the calculation of eclipses"</a>, <i>Journal for the History of Astronomy</i> (<link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><a href="/wiki/ISSN_(identifier)" class="mw-redirect" title="ISSN (identifier)">ISSN</a> <a rel="nofollow" class="external text" href="https://www.worldcat.org/search?fq=x0:jrnl&q=n2:0021-8286">0021-8286</a>), vol. 35(3) (2004), #120, pp. 327–336 (with addendum at <a rel="nofollow" class="external text" href="http://articles.adsabs.harvard.edu/full/2005JHA....36..339M">vol. 36, p. 339</a>).</cite></li> <li><cite id="refNewcSun"><a href="/wiki/Simon_Newcomb" title="Simon Newcomb">Simon Newcomb</a> (1895), <i><a href="/wiki/Newcomb%27s_Tables_of_the_Sun" title="Newcomb's Tables of the Sun">Tables of the Sun</a></i> ("Tables of the Motion of the Earth on its Axis and Around the Sun", in "Tables of the Four Inner Planets", vol. 6, part 1, of <i>Astronomical Papers prepared for the use of the American Ephemeris and Nautical Almanac</i> (1895), at pages 1–169).</cite></li> <li><cite id="refSitter1927"><a href="/wiki/Willem_de_Sitter" title="Willem de Sitter">W de Sitter</a> (1927), <a rel="nofollow" class="external text" href="http://articles.adsabs.harvard.edu/full/1927BAN.....4...21D">"On the secular accelerations and the fluctuations of the longitudes of the moon, the sun, Mercury and Venus"</a>, <i>Bull. Astron. Inst. Netherlands</i>, vol. 4 (1927), pages 21–38.</cite></li> <li><cite id="refHSJ1939"><a href="/wiki/Harold_Spencer_Jones" title="Harold Spencer Jones">H Spencer Jones</a>, <a rel="nofollow" class="external text" href="http://adsabs.harvard.edu/abs/1939MNRAS..99..541S">"The Rotation of the Earth, and the Secular Accelerations of the Sun, Moon and Planets"</a>, in <i>Monthly Notes of the Royal Astronomical Society</i>, vol. 99 (1939), pp. 541–558.</cite></li> <li><cite id="refEMS1998">E M Standish, <a rel="nofollow" class="external text" href="http://articles.adsabs.harvard.edu/full/1998A%26A...336..381S">"Time scales in the JPL and CfA ephemerides"</a>, <i>Astronomy & Astrophysics</i>, vol. 336 (1998), 381–384.</cite></li> <li><cite id="stemorr84">F R Stephenson, L V Morrison (1984), <a rel="nofollow" class="external text" href="http://adsabs.harvard.edu/abs/1984RSPTA.313...47S">"Long-term changes in the rotation of the earth – 700 B.C. to A.D. 1980"</a>, (Royal Society, Discussion on Rotation in the Solar System, London, England, Mar. 8, 9, 1984) <i>Royal Society (London), Philosophical Transactions, Series A</i> (<link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><a href="/wiki/ISSN_(identifier)" class="mw-redirect" title="ISSN (identifier)">ISSN</a> <a rel="nofollow" class="external text" href="https://www.worldcat.org/search?fq=x0:jrnl&q=n2:0080-4614">0080-4614</a>), vol. 313 (1984), #1524, pp. 47–70.</cite></li> <li><cite id="stemorr95">F R Stephenson, L V Morrison (1995), <a rel="nofollow" class="external text" href="http://adsabs.harvard.edu/abs/1995RSPTA.351..165S">"Long-Term Fluctuations in the Earth's Rotation: 700 BC to AD 1990"</a>, <i>Royal Society (London), Philosophical Transactions, Series A</i> (<link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><a href="/wiki/ISSN_(identifier)" class="mw-redirect" title="ISSN (identifier)">ISSN</a> <a rel="nofollow" class="external text" href="https://www.worldcat.org/search?fq=x0:jrnl&q=n2:0080-4614">0080-4614</a>), vol. 351 (1995), #1695, pp. 165–202.</cite></li> <li><cite id="refWink1977">G M R Winkler and T C van Flandern (1977), <a rel="nofollow" class="external text" href="http://articles.adsabs.harvard.edu/full/1977AJ.....82...84W">"Ephemeris Time, relativity, and the problem of uniform time in astronomy"</a>, <i>Astronomical Journal</i>, vol. 82 (Jan. 1977), pp. 84–92.</cite></li></ul> <div class="navbox-styles"><style data-mw-deduplicate="TemplateStyles:r1129693374">.mw-parser-output .hlist dl,.mw-parser-output .hlist ol,.mw-parser-output .hlist ul{margin:0;padding:0}.mw-parser-output .hlist dd,.mw-parser-output .hlist dt,.mw-parser-output .hlist li{margin:0;display:inline}.mw-parser-output .hlist.inline,.mw-parser-output .hlist.inline dl,.mw-parser-output .hlist.inline ol,.mw-parser-output .hlist.inline ul,.mw-parser-output .hlist dl dl,.mw-parser-output .hlist dl ol,.mw-parser-output .hlist dl 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measurement and standards"><abbr title="Discuss this template">t</abbr></a></li><li class="nv-edit"><a href="/wiki/Special:EditPage/Template:Time_measurement_and_standards" title="Special:EditPage/Template:Time measurement and standards"><abbr title="Edit this template">e</abbr></a></li></ul></div><div id="Time_measurement_and_standards" style="font-size:114%;margin:0 4em"><a href="/wiki/Time" title="Time">Time measurement</a> and <a href="/wiki/Time_standard" title="Time standard">standards</a></div></th></tr><tr><td class="navbox-abovebelow" colspan="3"><div> <ul><li><a href="/wiki/Chronometry" title="Chronometry">Chronometry</a></li> <li><a href="/wiki/Orders_of_magnitude_(time)" title="Orders of magnitude (time)">Orders of magnitude</a></li> <li><a href="/wiki/Time_metrology" class="mw-redirect" title="Time metrology">Metrology</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">International standards</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Coordinated_Universal_Time" title="Coordinated Universal Time">Coordinated Universal Time</a> <ul><li><a href="/wiki/UTC_offset" title="UTC offset">offset</a></li></ul></li> <li><a href="/wiki/Universal_Time" title="Universal Time">UT</a></li> <li><a href="/wiki/%CE%94T_(timekeeping)" title="ΔT (timekeeping)">ΔT</a></li> <li><a href="/wiki/DUT1" title="DUT1">DUT1</a></li> <li><a href="/wiki/International_Earth_Rotation_and_Reference_Systems_Service" title="International Earth Rotation and Reference Systems Service">International Earth Rotation and Reference Systems Service</a></li> <li><a href="/wiki/ISO_31-1" title="ISO 31-1">ISO 31-1</a></li> <li><a href="/wiki/ISO_8601" title="ISO 8601">ISO 8601</a></li> <li><a href="/wiki/International_Atomic_Time" title="International Atomic Time">International Atomic Time</a></li> <li><a href="/wiki/12-hour_clock" title="12-hour clock">12-hour clock</a></li> <li><a href="/wiki/24-hour_clock" title="24-hour clock">24-hour clock</a></li> <li><a href="/wiki/Barycentric_Coordinate_Time" title="Barycentric Coordinate Time">Barycentric Coordinate Time</a></li> <li><a href="/wiki/Barycentric_Dynamical_Time" title="Barycentric Dynamical Time">Barycentric Dynamical Time</a></li> <li><a href="/wiki/Civil_time" title="Civil time">Civil time</a></li> <li><a href="/wiki/Daylight_saving_time" title="Daylight saving time">Daylight saving time</a></li> <li><a href="/wiki/Geocentric_Coordinate_Time" title="Geocentric Coordinate Time">Geocentric Coordinate Time</a></li> <li><a href="/wiki/International_Date_Line" title="International Date Line">International Date Line</a></li> <li><a href="/wiki/IERS_Reference_Meridian" title="IERS Reference Meridian">IERS Reference Meridian</a></li> <li><a href="/wiki/Leap_second" title="Leap second">Leap second</a></li> <li><a href="/wiki/Solar_time" title="Solar time">Solar time</a></li> <li><a href="/wiki/Terrestrial_Time" title="Terrestrial Time">Terrestrial Time</a></li> <li><a href="/wiki/Time_zone" title="Time zone">Time zone</a></li> <li><a href="/wiki/180th_meridian" title="180th meridian">180th meridian</a></li></ul> </div></td><td class="noviewer navbox-image" rowspan="9" style="width:1px;padding:0 0 0 2px"><div><span typeof="mw:File"><a href="/wiki/Hourglass" title="Hourglass"><img alt="template illustration" src="//upload.wikimedia.org/wikipedia/commons/thumb/7/70/Marine_sandglass_MMM.jpg/75px-Marine_sandglass_MMM.jpg" decoding="async" width="75" height="175" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/7/70/Marine_sandglass_MMM.jpg/113px-Marine_sandglass_MMM.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/7/70/Marine_sandglass_MMM.jpg/150px-Marine_sandglass_MMM.jpg 2x" data-file-width="1637" data-file-height="3819" /></a></span><br /><span typeof="mw:File"><a href="/wiki/Time_zone" title="Time zone"><img alt="template illustration" src="//upload.wikimedia.org/wikipedia/commons/thumb/6/63/Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png/75px-Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png" decoding="async" width="75" height="152" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/6/63/Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png/113px-Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/6/63/Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png/150px-Aleutian_Islands_with_180th_meridian_and_International_Date_Line_%28cropped%29.png 2x" data-file-width="496" data-file-height="1007" /></a></span></div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Obsolete standards</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a class="mw-selflink selflink">Ephemeris time</a></li> <li><a href="/wiki/Greenwich_Mean_Time" title="Greenwich Mean Time">Greenwich Mean Time</a></li> <li><a href="/wiki/Prime_meridian" title="Prime meridian">Prime meridian</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Time_in_physics" title="Time in physics">Time in physics</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Absolute_space_and_time" title="Absolute space and time">Absolute space and time</a></li> <li><a href="/wiki/Spacetime" title="Spacetime">Spacetime</a></li> <li><a href="/wiki/Chronon" title="Chronon">Chronon</a></li> <li><a href="/wiki/Continuous_signal" class="mw-redirect" title="Continuous signal">Continuous signal</a></li> <li><a href="/wiki/Coordinate_time" title="Coordinate time">Coordinate time</a></li> <li><a href="/wiki/Cosmological_decade" title="Cosmological decade">Cosmological decade</a></li> <li><a href="/wiki/Discrete_time_and_continuous_time" title="Discrete time and continuous time">Discrete time and continuous time</a></li> <li><a href="/wiki/Proper_time" title="Proper time">Proper time</a></li> <li><a href="/wiki/Theory_of_relativity" title="Theory of relativity">Theory of relativity</a></li> <li><a href="/wiki/Time_dilation" title="Time dilation">Time dilation</a></li> <li><a href="/wiki/Gravitational_time_dilation" title="Gravitational time dilation">Gravitational time dilation</a></li> <li><a href="/wiki/Time_domain" title="Time domain">Time domain</a></li> <li><a href="/wiki/Time-translation_symmetry" title="Time-translation symmetry">Time-translation symmetry</a></li> <li><a href="/wiki/T-symmetry" title="T-symmetry">T-symmetry</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Horology" class="mw-redirect" title="Horology">Horology</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Clock" title="Clock">Clock</a></li> <li><a href="/wiki/Astrarium" title="Astrarium">Astrarium</a></li> <li><a href="/wiki/Atomic_clock" title="Atomic clock">Atomic clock</a></li> <li><a href="/wiki/Complication_(horology)" title="Complication (horology)">Complication</a></li> <li><a href="/wiki/History_of_timekeeping_devices" title="History of timekeeping devices">History of timekeeping devices</a></li> <li><a href="/wiki/Hourglass" title="Hourglass">Hourglass</a></li> <li><a href="/wiki/Marine_chronometer" title="Marine chronometer">Marine chronometer</a></li> <li><a href="/wiki/Marine_sandglass" title="Marine sandglass">Marine sandglass</a></li> <li><a href="/wiki/Radio_clock" title="Radio clock">Radio clock</a></li> <li><a href="/wiki/Watch" title="Watch">Watch</a> <ul><li><a href="/wiki/Stopwatch" title="Stopwatch">stopwatch</a></li></ul></li> <li><a href="/wiki/Water_clock" title="Water clock">Water clock</a></li> <li><a href="/wiki/Sundial" title="Sundial">Sundial</a></li> <li><a href="/wiki/Dialing_scales" title="Dialing scales">Dialing scales</a></li> <li><a href="/wiki/Equation_of_time" title="Equation of time">Equation of time</a></li> <li><a href="/wiki/History_of_sundials" title="History of sundials">History of sundials</a></li> <li><a href="/wiki/Schema_for_horizontal_dials" title="Schema for horizontal dials">Sundial markup schema</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Calendar" title="Calendar">Calendar</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Gregorian_calendar" title="Gregorian calendar">Gregorian</a></li> <li><a href="/wiki/Hebrew_calendar" title="Hebrew calendar">Hebrew</a></li> <li><a href="/wiki/Hindu_calendar" title="Hindu calendar">Hindu</a></li> <li><a href="/wiki/Holocene_calendar" title="Holocene calendar">Holocene</a></li> <li><a href="/wiki/Islamic_calendar" title="Islamic calendar">Islamic</a> (lunar Hijri)</li> <li><a href="/wiki/Julian_calendar" title="Julian calendar">Julian</a></li> <li><a href="/wiki/Solar_Hijri_calendar" title="Solar Hijri calendar">Solar Hijri</a></li> <li><a href="/wiki/Astronomical_year_numbering" title="Astronomical year numbering">Astronomical</a></li> <li><a href="/wiki/Dominical_letter" title="Dominical letter">Dominical letter</a></li> <li><a href="/wiki/Epact" title="Epact">Epact</a></li> <li><a href="/wiki/Equinox" title="Equinox">Equinox</a></li> <li><a href="/wiki/Intercalation_(timekeeping)" title="Intercalation (timekeeping)">Intercalation</a></li> <li><a href="/wiki/Julian_day" title="Julian day">Julian day</a></li> <li><a href="/wiki/Leap_year" title="Leap year">Leap year</a></li> <li><a href="/wiki/Lunar_calendar" title="Lunar calendar">Lunar</a></li> <li><a href="/wiki/Lunisolar_calendar" title="Lunisolar calendar">Lunisolar</a></li> <li><a href="/wiki/Solar_calendar" title="Solar calendar">Solar</a></li> <li><a href="/wiki/Solstice" title="Solstice">Solstice</a></li> <li><a href="/wiki/Tropical_year" title="Tropical year">Tropical year</a></li> <li><a href="/wiki/Determination_of_the_day_of_the_week" title="Determination of the day of the week">Weekday determination</a></li> <li><a href="/wiki/Names_of_the_days_of_the_week" title="Names of the days of the week">Weekday names</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Archaeology and geology</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Chronological_dating" title="Chronological dating">Chronological dating</a></li> <li><a href="/wiki/Geologic_time_scale" title="Geologic time scale">Geologic time scale</a></li> <li><a href="/wiki/International_Commission_on_Stratigraphy" title="International Commission on Stratigraphy">International Commission on Stratigraphy</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Astronomical_chronology" title="Astronomical chronology">Astronomical chronology</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Galactic_year" title="Galactic year">Galactic year</a></li> <li><a href="/wiki/Nuclear_timescale" title="Nuclear timescale">Nuclear timescale</a></li> <li><a href="/wiki/Precession" title="Precession">Precession</a></li> <li><a href="/wiki/Sidereal_time" title="Sidereal time">Sidereal time</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Other <a href="/wiki/Unit_of_time" title="Unit of time">units of time</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Instant" title="Instant">Instant</a></li> <li><a href="/wiki/Flick_(time)" title="Flick (time)">Flick</a></li> <li><a href="/wiki/Shake_(unit)" title="Shake (unit)">Shake</a></li> <li><a href="/wiki/Jiffy_(time)" title="Jiffy (time)">Jiffy</a></li> <li><a href="/wiki/Second" title="Second">Second</a></li> <li><a href="/wiki/Minute" title="Minute">Minute</a></li> <li><a href="/wiki/Moment_(unit)" title="Moment (unit)">Moment</a></li> <li><a href="/wiki/Hour" title="Hour">Hour</a></li> <li><a href="/wiki/Day" title="Day">Day</a></li> <li><a href="/wiki/Week" title="Week">Week</a></li> <li><a href="/wiki/Fortnight" title="Fortnight">Fortnight</a></li> <li><a href="/wiki/Month" title="Month">Month</a></li> <li><a href="/wiki/Year" title="Year">Year</a></li> <li><a href="/wiki/Olympiad" title="Olympiad">Olympiad</a></li> <li><a href="/wiki/Lustrum" title="Lustrum">Lustrum</a></li> <li><a href="/wiki/Decade" title="Decade">Decade</a></li> <li><a href="/wiki/Century" title="Century">Century</a></li> <li><a href="/wiki/Saeculum" title="Saeculum">Saeculum</a></li> <li><a href="/wiki/Millennium" title="Millennium">Millennium</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Related topics</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Chronology" title="Chronology">Chronology</a></li> <li><a href="/wiki/Duration_(philosophy)" title="Duration (philosophy)">Duration</a> <ul><li><a href="/wiki/Duration_(music)" title="Duration (music)">music</a></li></ul></li> <li><a href="/wiki/Mental_chronometry" title="Mental chronometry">Mental chronometry</a></li> <li><a href="/wiki/Decimal_time" title="Decimal time">Decimal time</a></li> <li><a href="/wiki/Metric_time" title="Metric time">Metric time</a></li> <li><a href="/wiki/System_time" title="System time">System time</a></li> <li><a href="/wiki/Time_metrology" class="mw-redirect" title="Time metrology">Time metrology</a></li> <li><a href="/wiki/Time_value_of_money" title="Time value of money">Time value of money</a></li> <li><a href="/wiki/Timekeeper" title="Timekeeper">Timekeeper</a></li></ul> </div></td></tr></tbody></table></div>    </div><noscript><img src="https://login.wikimedia.org/wiki/Special:CentralAutoLogin/start?type=1x1" alt="" width="1" height="1" style="border: none; position: absolute;"></noscript> <div class="printfooter" data-nosnippet="">Retrieved from "<a dir="ltr" href="https://en.wikipedia.org/w/index.php?title=Ephemeris_time&oldid=1215926516">https://en.wikipedia.org/w/index.php?title=Ephemeris_time&oldid=1215926516</a>"</div></div> <div id="catlinks" class="catlinks" data-mw="interface"><div id="mw-normal-catlinks" class="mw-normal-catlinks"><a href="/wiki/Help:Category" 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