Second | Measurement, Timekeeping, Clock

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They write new content and verify and edit content received from contributors.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link font-12 "> The Editors of Encyclopaedia Britannica</span></div> <div class="last-updated font-12 font-serif"> <a class="byline-edit-history" href="https://www.britannica.com/science/second/additional-info#history" rel="nofollow">Article History</a> </div></div> </div> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm shadow-sm mobile-toc-button gtm-mobile-toc-inline-button d-none d-sm-block js-sections-inline-button module-spacing btn d-lg-none"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="toc"></em> Table of Contents </button> <div class="d-flex d-sm-none flex-row"> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm shadow-sm mobile-toc-button gtm-mobile-toc-inline-button js-sections-inline-button module-spacing"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="toc"></em> Table of Contents </button> <button class="ai-ask-button btn border-2 ai-ask-button btn border-2 module-spacing btn-sm js-inline-ai-ask-button btn-outline-red-400 border-red-400 p-10 ml-5"> Ask the Chatbot a Question </button> </div> <div class="js-qf-module qf-module px-40 px-sm-20 py-15 mx-auto module-spacing font-14 bg-gray-50 rounded"> <div class="facts-list mt-10"> <div class=""> <div class="js-fact mb-10 line-clamp clamp-3"> <dl> <dt>Related Topics: </dt> <dd><a href="/science/time" topicid="596034">time</a></dd> <dd><a href="/science/International-System-of-Units" topicid="291305">International System of Units</a></dd> <dd><a href="/science/minute" topicid="384662">minute</a></dd> <dd><a href="/technology/atomic-second" topicid="41779">atomic second</a></dd> <dd><a href="/topic/leap-second" topicid="1784403">leap second</a></dd> </dl> <button class="js-more-btn d-none btn btn-unstyled font-12 bg-gray-50" aria-label="Toggle more/less fact data"> <em class="js-content link-blue">(Show more)</em> </button> </div> <div class="text-center"> <a class="btn btn-sm btn-link p-0" href="/facts/second"> See all related content </a> </div> </div> </div> </div><span class="marker before-article"></span><section data-level="1" id="ref1"><span class="marker PREMOD1 mod-inline"></span><p class="topic-paragraph"><strong><span id="ref795849"></span>second</strong>, fundamental unit of time, now defined in terms of the <span id="ref795851"></span><a href="https://www.britannica.com/technology/radiation-measurement" class="md-crosslink " data-show-preview="true">radiation</a> <a href="https://www.britannica.com/science/frequency-physics" class="md-crosslink autoxref " data-show-preview="true">frequency</a> at which atoms of the element <span id="ref99254"></span><a href="https://www.britannica.com/science/cesium" class="md-crosslink " data-show-preview="true">cesium</a> change from one state to another.</p><div class="one-good-fact-module"> </div><span class="marker MOD1 mod-inline"></span><span class="marker PREMOD2 mod-inline"></span><p class="topic-paragraph">The second was formerly defined as 1/86,400 of the mean solar day鈥攊.e., the average period of rotation of the <a href="https://www.britannica.com/place/Earth" class="md-crosslink autoxref " data-show-preview="true">Earth</a> on its <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="axis" href="https://www.britannica.com/dictionary/axis" data-type="EB">axis</a> relative to the Sun. In the mid-20th century this definition became inadequate because of the need for increased <a href="https://www.britannica.com/science/precision-measurement" class="md-crosslink autoxref " data-show-preview="true">precision</a> in timekeeping. In 1956 the second was redefined by the International Committee on Weights and Measures as 1/31,556,925.9747 of the length of the tropical (seasonal) <a href="https://www.britannica.com/science/year" class="md-crosslink autoxref " data-show-preview="true">year</a> 1900. In 1967 the <span id="ref99255"></span>13th General Conference on Weights and Measures provisionally defined the second as 9,192,631,770 cycles of radiation associated with the transition between the two hyperfine levels of the ground state of the cesium-133 atom (<em>see</em> <a href="https://www.britannica.com/science/atomic-time" class="md-crosslink " data-show-preview="true">atomic time</a>). The number of cycles of radiation was chosen to make the length of the defined second correspond as closely as possible to that of the now <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="obsolete" href="https://www.britannica.com/dictionary/obsolete" data-type="EB">obsolete</a> astronomically determined second of <span id="ref99257"></span><a href="https://www.britannica.com/science/Ephemeris-Time" class="md-crosslink " data-show-preview="true">Ephemeris Time</a> (defined as the fraction of the tropical year given above). As the rate of rotation of the Earth constantly changes, it is necessary to occasionally add (or theoretically to subtract) a second during the year to ensure the atomic timescale <a href="https://www.britannica.com/science/Coordinated-Universal-Time" class="md-crosslink " data-show-preview="true">Coordinated Universal Time</a> (UTC) stays in synchronization with nature. This represents the sole definition of the second in the <span id="ref99256"></span><a href="https://www.britannica.com/science/International-System-of-Units" class="md-crosslink " data-show-preview="true">International System of Units</a> (SI).</p><span class="marker MOD2 mod-inline"></span></section> <span class="md-signature"><a href="/contributor/Jonathan-D-Betts/5077">Jonathan D. Betts</a></span> <span class="marker end-of-content"></span><span class="marker after-article"></span></div> <div id="chatbot-root"></div> </div> </div> </div> <div class="ai-dialog-placeholder"></div> </div> </div> <aside class="col-md-da-320"></aside> </div> </div> </div> </div> </article></div> </div></div> </div> </main> <div id="md-footer"></div> <noscript><iframe src="//www.googletagmanager.com/ns.html?id=GTM-5W6NC8" height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript> <script type="text/javascript" id="_informizely_script_tag"> var IzWidget = IzWidget || {}; (function (d) { var scriptElement = d.createElement('script'); scriptElement.type = 'text/javascript'; scriptElement.async = true; scriptElement.src = "https://insitez.blob.core.windows.net/site/f780f33e-a610-4ac2-af81-3eb184037547.js"; var node = d.getElementById('_informizely_script_tag'); node.parentNode.insertBefore(scriptElement, node); } )(document); </script>  <script> window.ap3c = window.ap3c || {}; var ap3c = window.ap3c; ap3c.cmd = ap3c.cmd || []; ap3c.cmd.push(function() { ap3c.init('ZO4siT4cLwnykPnzZWJtd3Byb2Q', 'https://engage.email.britannica.com/'); ap3c.track({v: 0}); }); ap3c.activity = function(act) { ap3c.act = (ap3c.act || []); ap3c.act.push(act); }; var s, t; s = document.createElement('script'); s.type = 'text/javascript'; s.src = "https://engage.email.britannica.com/app.js"; t = document.getElementsByTagName('script')[0]; t.parentNode.insertBefore(s, t); </script> <script class="marketing-page-info" type="application/json"> {"pageType":"Topic","templateName":"DESKTOP","pageNumber":1,"pagesTotal":1,"pageId":531340,"pageLength":231,"initialLoad":true,"lastPageOfScroll":false} </script> <script class="marketing-content-info" type="application/json"> [] </script> <script src="https://cdn.britannica.com/mendel-resources/3-130/js/libs/jquery-3.5.0.min.js?v=3.130.14"></script> <script type="text/javascript" data-type="Init Mendel Code Splitting"> (function() { $.ajax({ dataType: 'script', cache: true, url: 'https://cdn.britannica.com/mendel-resources/3-130/dist/topic-page.js?v=3.130.14' }); })(); </script> <script class="analytics-metadata" type="application/json"> {"leg":"D","adLeg":"D","userType":"ANONYMOUS","pageType":"Topic","pageSubtype":null,"articleTemplateType":"SMALL","gisted":false,"pageNumber":1,"hasSummarizeButton":false,"hasAskButton":false} </script> <script type="text/javascript"> EBStat={accountId:-1,hostnameOverride:'webstats.eb.com',domain:'www.britannica.com', json:''}; </script> <script type="text/javascript"> ( function() { $.ajax( { dataType: 'script', cache: true, url: '//www.britannica.com/webstats/mendelstats.js?v=1' } ) .done( function() { try {writeStat(null,EBStat);} catch(err){} } ); })(); </script> <div id="bc-fixed-dialogue"></div> </body> </html>

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Second | Measurement, Timekeeping, Clock | Britannica