Universe | Definition & Facts

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class="md-content"> <div class="md-article-container template-desktop infinite-pagination"> <div class="infinite-scroll-container article last"> <article class="article-content container-lg qa-content px-0 pt-0 pb-40 py-lg-20 content md-expanded" data-topic-id="139365"> <div class="grid gx-0"> <div class="col-auto"> <div class="topic-left-rail md-article-drawer position-relative d-flex border-right-sm border-left-sm open"> <div class="drawer d-flex flex-column open"> <div class="left-rail-section-content"> <div class="topic-left-rail-header text-truncate bg-gray-50 position-relative text-right d-flex align-items-center"> <div class="tlr-title px-20 py-15 text-left"> <em class="material-icons text-gray-400 d-lg-none" data-icon="toc"></em> <a class="font-serif font-weight-bold text-black link-blue" href="https://www.britannica.com/science/universe">universe</a> </div> <button aria-label="Close" class="js-sections-close-button btn-link btn-sm btn d-lg-none position-absolute top-0 p-10 right-0" > <em class="material-icons font-26" data-icon="close"></em> </button> </div> <div class="section-content pl-10 pr-20 pl-sm-50 pr-sm-60 pl-lg-5 pr-lg-10 pt-10 pt-lg-0 bg-gray-50 clear-catfish-ad"> <div class="toc mb-20"> <div class="font-serif font-14 font-weight-bold mx-15 mb-15 mt-20"> Table of Contents </div> <ul class="list-unstyled my-0" data-level="h1"><li data-target="#ref1"><div class="pl-25"><a class="link-gray-900 w-100" href="/science/universe">Introduction</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref27539"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/universe#ref27539">Earliest conceptions of the universe</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref27540"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/universe#ref27540">Astronomical theories of the ancient Greeks</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref27541"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/universe/The-system-of-Aristotle-and-its-impact-on-medieval-thought">The system of Aristotle and its impact on medieval thought</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref27542"><div class="d-flex align-items-center"><div class="ml-25"></div><a class="w-100 link-gray-900" href="/science/universe/The-Copernican-revolution">The Copernican revolution</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref27543"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/science/universe/Perceptions-of-the-20th-century">Perceptions of the 20th century</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref27544"><a class="w-100 link-gray-900" href="/science/universe/Perceptions-of-the-20th-century#ref27544">Kapteyn’s statistical studies</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref27545"><a class="w-100 link-gray-900" href="/science/universe/Perceptions-of-the-20th-century#ref27545">Shapley’s contributions</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref27546"><a class="w-100 link-gray-900" href="/science/universe/Hubbles-research-on-extragalactic-systems">Hubble’s research on extragalactic systems</a></li></ul></div></li></ul> <a class="toc-extra-link link-gray-900" href="https://www.britannica.com/science/universe/additional-info">References & Edit History</a> <a class="toc-extra-link link-gray-900" href="/facts/universe">Related Topics</a> </div> <div class="tlr-media-slider pb-10 mb-30"> <a class="section-header link-gray-900 font-serif font-14 font-weight-bold mb-10 mx-10" href="https://www.britannica.com/science/universe/images-videos">Images & Videos</a> <div class="slider js-slider position-relative d-inline-flex align-items-center mw-100 "> <div class="slider-container js-slider-container overflow-hidden d-flex overflow-hidden text-nowrap ml-15"> <a href="/video/universe/203957" data-href="/media/1/139365/203957" class="media-overlay-link d-inline-block mr-5"> <div class="position-relative --aspect-ratio: 16/9"> <img loading="lazy" src="https://cdn.britannica.com/00/185400-138-B50A215F/universe.jpg?w=400&h=225&c=crop" alt="Examine the observable universe's place within the whole universe" class="col-100" /> <div class="btn btn-sm btn-white btn-circle position-absolute shadow" style="top: 50%; left:50%; transform: translate(-50%, -50%)"> <em class="material-icons font-14" 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Shu</div> <div class="editor-description font-12 font-serif mt-5 clamp-description text-black">Professor of Physics, University of Calfornia, San Diego. Author of <i>The Physical Universe.</i></div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link gtm-byline font-12 byline-contributor text-decoration-underline"> Frank H. Shu</span></div> <div class="font-serif font-12 text-gray-700"> <span class="qa-fact-checked-by">Fact-checked by</span> <div class="editor-popover popover p-0"> <a class="d-block p-20 font-12" href="/editor/The-Editors-of-Encyclopaedia-Britannica/4419" > <div class="editor-title font-16 font-weight-bold">The Editors of Encyclopaedia Britannica</div> <div class="editor-description font-12 font-serif mt-5 text-black">Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. 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"> <span class="text-gray-700"> Last Updated: <time datetime="2024-11-27T00:00:00CST" >Nov 27, 2024</time> •</span> <a class="byline-edit-history" href="https://www.britannica.com/science/universe/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 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href="/biography/Saul-Perlmutter" topicid="1518949">Saul Perlmutter</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> <div class=""> <div class="js-fact mb-10 line-clamp clamp-3"> <dl> <dt>Related Topics: </dt> <dd><a href="/science/star-astronomy" topicid="563395">star</a></dd> <dd><a href="/science/eclipse" topicid="178098">eclipse</a></dd> <dd><a href="/science/galaxy" topicid="223818">galaxy</a></dd> <dd><a href="/place/Sun" topicid="573494">Sun</a></dd> <dd><a href="/science/gravity-physics" topicid="242523">gravity</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/universe"> See all related content </a> </div> </div> </div> </div><div class="bg-gray-50 p-15 rounded module-spacing recent-news d-flex flex-column float-false"> <div> <h2 class="font-weight-bold font-14 m-0 d-inline"> News <span class="text-gray-600">•</span> </h2> <div class="recent-news-item first-recent-news-item d-inline"> <a class="font-14 gtm-ap-news-link" href="https://www.space.com/the-universe/scientists-find-highest-energy-cosmic-electrons-ever-seen" rel="nofollow">Scientists find highest energy cosmic ray electrons ever seen</a> <span class="font-14 text-gray-600"> <span>•</span> Nov. 25, 2024, 11:53 PM ET (Space.com) <button class="btn btn-link d-inline p-0 font-12 js-toggle-recent-news"> <span class="text-gray-500">...</span><span>(Show more)</span> </button> </span> </div> </div> <div class="rest-of-recent-news-items"> <div class="recent-news-item mt-5"> <a class="font-14 gtm-ap-news-link" href="https://www.sciencealert.com/scientists-discover-monster-galaxies-lurking-in-the-early-universe" rel="nofollow">Scientists Discover Monster Galaxies Lurking in The Early Universe</a> <span class="font-14 text-gray-600"> <span>•</span> Nov. 18, 2024, 9:53 PM ET (ScienceAlert) </span> </div> <div class="recent-news-item mt-5"> <a class="font-14 gtm-ap-news-link" href="https://www.sciencealert.com/this-black-hole-is-eating-stuff-at-over-40-times-the-theoretical-limit" rel="nofollow">This Black Hole Is Eating Stuff at Over 40 Times The Theoretical Limit</a> <span class="font-14 text-gray-600"> <span>•</span> Nov. 4, 2024, 5:06 AM ET (ScienceAlert) </span> </div> <button class="js-toggle-recent-news d-flex btn btn-unstyled font-14 pr-10 rounded-sm mt-10" aria-label="Toggle additional news items"> Show less <em class="material-icons" data-icon="expand_less"></em> </button> </div> </div><span class="marker before-article"></span><section data-level="1" id="ref1"><span class="marker PREMOD1 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="203957" data-asm-type="video"><div class="md-assembly-wrapper card-media " data-type="video" video-id="185400"><a data-id="185400" class="gtm-assembly-link d-flex justify-content-center" style="--aspect-ratio: 16/9" href="/video/universe/-203957"><img src="https://cdn.britannica.com/00/185400-138-B50A215F/universe.jpg?w=800&h=450&c=crop" alt="Examine the observable universe's place within the whole universe" loading="lazy"><script type="application/json"> { "sources": [ { "file" : "//content.jwplatform.com/manifests/mMJeLTfP.m3u8" } ], "image": "https://cdn.britannica.com/00/185400-138-B50A215F/universe.jpg" ,"tracks": [ { "file" : "//assets-jpcust.jwpsrv.com/tracks/UEZIUWEw", "label": "English" } ] ,"adfile": "//content.jwplatform.com/manifests/t1e6VW3d.m3u8" } </script><div class="btn btn-xl btn-white btn-circle position-absolute shadow" style="top: 50%; transform: translateY(-50%)"><em class="material-icons" data-icon="play_arrow"></em></div></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><span class="md-assembly-title font-weight-bold mr-5 d-inline font-sans-serif md-video-caption" video-control="185400">Examine the observable universe's place within the whole universe</span><span>Learn about defining and measuring the observable universe within the “whole” universe.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><a class="font-14 mt-10 d-inline-block" href="/science/universe/images-videos">See all videos for this article</a></figcaption></figure></div></div><p class="topic-paragraph"><strong><span id="ref1023774"></span>universe</strong>, the whole <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="cosmic" href="https://www.britannica.com/dictionary/cosmic" data-type="EB">cosmic</a> system of <a href="https://www.britannica.com/science/matter" class="md-crosslink " data-show-preview="true">matter</a> and <a href="https://www.britannica.com/science/energy" class="md-crosslink " data-show-preview="true">energy</a> of which <a href="https://www.britannica.com/place/Earth" class="md-crosslink " data-show-preview="true">Earth</a>, and therefore the <a href="https://www.britannica.com/topic/human-being" class="md-crosslink " data-show-preview="true">human</a> race, is a part. Humanity has traveled a long road since societies imagined Earth, the <a href="https://www.britannica.com/place/Sun" class="md-crosslink " data-show-preview="true">Sun</a>, and the <a href="https://www.britannica.com/place/Moon" class="md-crosslink " data-show-preview="true">Moon</a> as the main objects of creation, with the rest of the universe being formed almost as an afterthought. Today it is known that Earth is only a small ball of rock in a space of unimaginable vastness and that the birth of the <a href="https://www.britannica.com/science/solar-system" class="md-crosslink " data-show-preview="true">solar system</a> was probably only one event among many that occurred against the backdrop of an already mature universe. This humbling lesson has unveiled a remarkable fact, one that endows the minutest particle in the universe with a rich and noble heritage: events that occurred in the first few minutes of the creation of the universe 13.7 billion years ago turn out to have had a profound influence on the birth, life, and death of <a href="https://www.britannica.com/science/galaxy" class="md-crosslink " data-show-preview="true">galaxies</a>, <a href="https://www.britannica.com/science/star-astronomy" class="md-crosslink " data-show-preview="true">stars</a>, and <a href="https://www.britannica.com/science/planet" class="md-crosslink " data-show-preview="true">planets</a>. Indeed, a line can be drawn from the forging of the matter of the universe in a primal “<a href="https://www.britannica.com/science/big-bang-model" class="md-crosslink " data-show-preview="true">big bang</a>” to the gathering on Earth of <a href="https://www.britannica.com/science/atom" class="md-crosslink " data-show-preview="true">atoms</a> versatile enough to serve as the basis of <a href="https://www.britannica.com/science/life" class="md-crosslink " data-show-preview="true">life</a>. The <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="intrinsic" href="https://www.merriam-webster.com/dictionary/intrinsic" data-type="MW">intrinsic</a> harmony of such a worldview has great philosophical and <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="aesthetic" href="https://www.merriam-webster.com/dictionary/aesthetic" data-type="MW">aesthetic</a> appeal, and it may explain why public interest in the universe has always endured.</p><span class="marker MOD1 mod-inline"></span><span class="marker PREMOD2 mod-inline"></span><p class="topic-paragraph">The “<a href="https://www.britannica.com/topic/observable-universe" class="md-crosslink " data-show-preview="true">observable universe</a>” is the region of <a href="https://www.britannica.com/science/space-physics-and-metaphysics" class="md-crosslink " data-show-preview="true">space</a> that humans can actually or theoretically observe with the aid of technology. It can be thought of as a bubble with Earth at its centre. It is <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="differentiated" href="https://www.merriam-webster.com/dictionary/differentiated" data-type="MW">differentiated</a> from the entirety of the <a href="https://www.britannica.com/science/universe" class="md-crosslink " data-show-preview="true">universe</a>, which is the whole cosmic system of matter and energy, including the human race. Unlike the observable universe, the universe is possibly <a href="https://www.britannica.com/science/infinity-mathematics" class="md-crosslink " data-show-preview="true">infinite</a> and without spatial edges.</p><span class="marker MOD2 mod-inline"></span><span class="marker PREMOD3 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="159542" data-asm-type="video"><div class="md-assembly-wrapper card-media " data-type="video" video-id="151215"><a data-id="151215" class="gtm-assembly-link d-flex justify-content-center" style="--aspect-ratio: 16/9" href="/video/Scale-universe/-159542"><img src="https://cdn.britannica.com/15/151215-138-AE18D74A/Scale-universe.jpg?w=800&h=450&c=crop" alt="The scale of the universe: Earth's solar system and beyond" loading="lazy"><script type="application/json"> { "sources": [ { "file" : "//content.jwplatform.com/manifests/mlkPhMyT.m3u8" } ], "image": "https://cdn.britannica.com/15/151215-138-AE18D74A/Scale-universe.jpg" ,"tracks": [ { "file" : "//assets-jpcust.jwpsrv.com/tracks/sRWkeg4B", "label": "English" } ] ,"adfile": "//content.jwplatform.com/manifests/CKQZFJVL.m3u8" } </script><div class="btn btn-xl btn-white btn-circle position-absolute shadow" style="top: 50%; transform: translateY(-50%)"><em class="material-icons" data-icon="play_arrow"></em></div></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><span class="md-assembly-title font-weight-bold mr-5 d-inline font-sans-serif md-video-caption" video-control="151215">The scale of the universe: Earth's solar system and beyond</span><span>Scale of the universe.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><a class="font-14 mt-10 d-inline-block" href="/science/universe/images-videos">See all videos for this article</a></figcaption></figure></div></div><p class="topic-paragraph">This article traces the development over time of humanity’s perception of the universe, from prehistoric observations of the night sky to modern calculations on the recessional velocity of galaxies. For articles on component parts of the universe, <em>see</em> <a href="https://www.britannica.com/science/solar-system" class="md-crosslink " data-show-preview="true">solar system</a>, <a href="https://www.britannica.com/science/star-astronomy" class="md-crosslink " data-show-preview="true">star</a>, <a href="https://www.britannica.com/science/galaxy" class="md-crosslink " data-show-preview="true">galaxy</a>, and <a href="https://www.britannica.com/science/nebula" class="md-crosslink " data-show-preview="true">nebula</a>. For an explanation of the scientific study of the universe as a unified whole, <em>see</em> <a href="https://www.britannica.com/science/cosmology-astronomy" class="md-crosslink " data-show-preview="true">cosmology</a>. For an article about the possible existence of other universes, <em>see</em> <a href="https://www.britannica.com/science/multiverse" class="md-crosslink " data-show-preview="true">multiverse</a>.</p><span class="marker MOD3 mod-inline"></span></section> <span class="marker h2"></span><section data-level="1" id="ref27539"> <h2 class="h1">Earliest conceptions of the universe</h2> <span class="marker PREMOD4 mod-inline"></span><p class="topic-paragraph">All scientific thinking on the nature of the universe can be traced to the distinctive geometric patterns formed by the <a href="https://www.britannica.com/science/star-astronomy" class="md-crosslink " data-show-preview="true">stars</a> in the night sky. Even prehistoric people must have noticed that, apart from a daily rotation (which is now understood to arise from the spin of <a href="https://www.britannica.com/place/Earth" class="md-crosslink " data-show-preview="true">Earth</a>), the stars did not seem to move with respect to one another: the stars appear “fixed.” Early <a href="https://www.britannica.com/topic/nomadism" class="md-crosslink " data-show-preview="true">nomads</a> found that knowledge of the <span id="ref391747"></span><a href="https://www.britannica.com/science/constellation" class="md-crosslink " data-show-preview="true">constellations</a> could guide their travels, and they developed stories to help them remember the relative positions of the stars in the night sky. These stories became the mythical tales that are part of most <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="cultures" href="https://www.merriam-webster.com/dictionary/cultures" data-type="MW">cultures</a>.</p><span class="marker MOD4 mod-inline"></span> <span class="marker PREMOD5 mod-inline"></span><p class="topic-paragraph">When nomads turned to farming, an <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="intimate" href="https://www.merriam-webster.com/dictionary/intimate" data-type="MW">intimate</a> knowledge of the constellations served a new function—an aid in timekeeping, in particular for keeping track of the <a href="https://www.britannica.com/science/season" class="md-crosslink " data-show-preview="true">seasons</a>. People had noticed very early that certain celestial objects did not remain stationary relative to the “fixed” stars; instead, during the course of a year, they moved forward and backward in a narrow strip of the sky that contained 12 constellations <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="constituting" href="https://www.merriam-webster.com/dictionary/constituting" data-type="MW">constituting</a> the signs of the <span id="ref391748"></span><a href="https://www.britannica.com/topic/zodiac" class="md-crosslink " data-show-preview="true">zodiac</a>. Seven such wanderers were known to the ancients: the <a href="https://www.britannica.com/place/Sun" class="md-crosslink " data-show-preview="true">Sun</a>, the <a href="https://www.britannica.com/science/moon-natural-satellite" class="md-crosslink " data-show-preview="true">Moon</a>, <a href="https://www.britannica.com/place/Mercury-planet" class="md-crosslink " data-show-preview="true">Mercury</a>, <a href="https://www.britannica.com/place/Venus-planet" class="md-crosslink " data-show-preview="true">Venus</a>, <a href="https://www.britannica.com/place/Mars-planet" class="md-crosslink " data-show-preview="true">Mars</a>, <a href="https://www.britannica.com/place/Jupiter-planet" class="md-crosslink " data-show-preview="true">Jupiter</a>, and <a href="https://www.britannica.com/place/Saturn-planet" class="md-crosslink " data-show-preview="true">Saturn</a>. Foremost among the wanderers was the Sun: day and night came with its rising and setting, and its motion through the zodiac signaled the season to <a href="https://www.britannica.com/plant/plant" class="md-crosslink autoxref " data-show-preview="true">plant</a> and the season to reap. Next in importance was the Moon: its position correlated with the <a href="https://www.britannica.com/science/tide" class="md-crosslink " data-show-preview="true">tides</a>, and its shape changed intriguingly over the course of a month. The Sun and Moon had the power of gods; why not then the other wanderers? Thus probably arose the <a href="https://www.britannica.com/topic/astrology" class="md-crosslink " data-show-preview="true">astrological</a> belief that the positions of the <a href="https://www.britannica.com/science/planet" class="md-crosslink " data-show-preview="true">planets</a> (from the Greek word <em>planetes</em>, “wanderers”) in the zodiac could influence worldly events and even cause the rise and fall of kings. In <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="homage" href="https://www.britannica.com/dictionary/homage" data-type="EB">homage</a> to this belief, <a href="https://www.britannica.com/place/Babylonia" class="md-crosslink " data-show-preview="true">Babylonian</a> priests devised the week of seven days, whose names even in various modern languages (for example, English, French, or Norwegian) can still easily be traced to their origins in the seven planet-gods.</p><a class="link-module shadow-sm d-block qa-quiz-module" href="/quiz/all-about-astronomy" data-link-module-iframe-link=""> <img loading="lazy" src="https://cdn.britannica.com/54/102254-131-ED1A2CE1/Nicolaus-Copernicus.jpg" alt="Nicolaus Copernicus. Nicolas Copernicus (1473-1543) Polish astronomer. In 1543 he published, forward proof of a Heliocentric (sun centered) universe. Coloured stipple engraving published London 1802. De revolutionibus orbium coelestium libri vi." class="rounded-sm mr-15" width="70" /> <div class="line-clamp clamp-5"> <div class="module-title bg-green">Britannica Quiz</div> <div class="font-weight-semi-bold mt-5">All About Astronomy</div> </div> </a><span class="marker MOD5 mod-inline"></span> </section> <span class="marker h3"></span><section data-level="1" id="ref27540"> <h2 class="h1"><span id="ref391746"></span><a href="https://www.britannica.com/science/astronomy" class="md-crosslink " data-show-preview="true">Astronomical</a> theories of the ancient Greeks</h2> <span class="marker PREMOD6 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="13594" data-asm-type="video"><div class="md-assembly-wrapper card-media " data-type="video" video-id="23882"><a data-id="23882" class="gtm-assembly-link d-flex justify-content-center" style="--aspect-ratio: 16/9" href="/video/Ptolemy-theory-solar-system/-13594"><img src="https://cdn.britannica.com/82/23882-138-90AFD59D/Ptolemy-theory-solar-system.jpg?w=800&h=450&c=crop" alt="How did Ptolemy explain retrograde motion?" loading="lazy"><script type="application/json"> { "sources": [ { "file" : "//content.jwplatform.com/manifests/VCekN8bl.m3u8" } ], "image": "https://cdn.britannica.com/82/23882-138-90AFD59D/Ptolemy-theory-solar-system.jpg" ,"tracks": [ { "file" : "//assets-jpcust.jwpsrv.com/tracks/bjct1e5O", "label": "English" } ] ,"adfile": "//content.jwplatform.com/manifests/AAXyRbfv.m3u8" } </script><div class="btn btn-xl btn-white btn-circle position-absolute shadow" style="top: 50%; transform: translateY(-50%)"><em class="material-icons" data-icon="play_arrow"></em></div></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><span class="md-assembly-title font-weight-bold mr-5 d-inline font-sans-serif md-video-caption" video-control="23882">How did Ptolemy explain retrograde motion?</span><span>Ptolemy's theory of the solar system.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><a class="font-14 mt-10 d-inline-block" href="/science/universe/images-videos">See all videos for this article</a></figcaption></figure></div></div><p class="topic-paragraph">The apex in the description of planetary motions during classical antiquity was reached with the <a href="https://www.britannica.com/place/ancient-Greece" class="md-crosslink " data-show-preview="true">Greeks</a>, who were of course superb <a href="https://www.britannica.com/science/geometry" class="md-crosslink " data-show-preview="true">geometers</a>. Like their predecessors, Greek astronomers adopted the natural picture, from the point of view of an observer on <a href="https://www.britannica.com/place/Earth" class="md-crosslink " data-show-preview="true">Earth</a>, that Earth lay motionless at the centre of a rigidly rotating <a href="https://www.britannica.com/science/celestial-sphere" class="md-crosslink " data-show-preview="true">celestial sphere</a> (to which the <a href="https://www.britannica.com/science/star-astronomy" class="md-crosslink " data-show-preview="true">stars</a> were “fixed”), and that the complex to-and-fro wanderings of the <a href="https://www.britannica.com/science/planet" class="md-crosslink " data-show-preview="true">planets</a> in the zodiac were to be described against this unchanging backdrop. They developed an <a href="https://www.britannica.com/science/Ptolemaic-system" class="md-crosslink " data-show-preview="true">epicyclic model</a> that would reproduce the observed planetary motions with quite astonishing accuracy. The model <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="invoked" href="https://www.merriam-webster.com/dictionary/invoked" data-type="MW">invoked</a> small circles on top of large circles, all rotating at individual uniform speeds, and it culminated about 140 <span class="text-smallcaps">ce</span> with the work of <span id="ref391750"></span><a href="https://www.britannica.com/biography/Ptolemy" class="md-crosslink " data-show-preview="true">Ptolemy</a>, who introduced the ingenious <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="artifact" href="https://www.merriam-webster.com/dictionary/artifact" data-type="MW">artifact</a> of displaced centres for the circles to improve the <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="empirical" href="https://www.merriam-webster.com/dictionary/empirical" data-type="MW">empirical</a> fit. Although the <span id="ref391749"></span><a href="https://www.britannica.com/science/Ptolemaic-system" class="md-crosslink " data-show-preview="true">model</a> was purely kinematic and did not attempt to address the <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="dynamical" href="https://www.merriam-webster.com/dictionary/dynamical" data-type="MW">dynamical</a> reasons for why the motions were as they were, it laid the groundwork for the <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="paradigm" href="https://www.merriam-webster.com/dictionary/paradigm" data-type="MW">paradigm</a> that nature is not <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="capricious" href="https://www.merriam-webster.com/dictionary/capricious" data-type="MW">capricious</a> but possesses a regularity and precision that can be discovered from experience and used to predict future events.</p><span class="marker MOD6 mod-inline"></span> <span class="marker PREMOD7 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="120440" data-asm-type="image"><div class="md-assembly-wrapper card-media " data-type="image"><a href="https://cdn.britannica.com/65/465-050-0E0945B1/circumference-method-length-Eratosthenes-angle-Earth-arc.jpg" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center media-overlay-link card-media" data-href="/media/1/139365/120440"><picture><source media="(min-width: 680px)" srcset="https://cdn.britannica.com/65/465-050-0E0945B1/circumference-method-length-Eratosthenes-angle-Earth-arc.jpg"><img src="https://cdn.britannica.com/65/465-050-0E0945B1/circumference-method-length-Eratosthenes-angle-Earth-arc.jpg?w=300" alt="Eratosthenes' method of measuring Earth's circumference" data-width="1600" data-height="1231" loading="eager"></picture><button class="magnifying-glass btn btn-circle position-absolute shadow btn-white top-10 right-10" aria-label="Zoom in"><em class="material-icons link-blue" data-icon="zoom_in"></em></button></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><a class="gtm-assembly-link md-assembly-title font-weight-bold d-inline font-sans-serif mr-5 media-overlay-link" href="https://cdn.britannica.com/65/465-050-0E0945B1/circumference-method-length-Eratosthenes-angle-Earth-arc.jpg" data-href="/media/1/139365/120440">Eratosthenes' method of measuring Earth's circumference</a><span>By knowing the length of an arc (<em>l</em>) and the size of the corresponding central angle (α) that it subtends, one can obtain the radius of the sphere from the relation that the proportion of the length of arc <em>l</em> to Earth's circumference, 2π<em>R</em> (where <em>R</em> is Earth's radius) equals the proportion of the central angle α to the angle subtended by the whole circumference (360°)—i.e., <em>l</em> : 2π<em>R</em> = α : 360.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div></figcaption></figure></div></div><p class="topic-paragraph">The application of the methods of <a href="https://www.britannica.com/science/Euclidean-geometry" class="md-crosslink " data-show-preview="true">Euclidean geometry</a> to planetary <a href="https://www.britannica.com/science/astronomy" class="md-crosslink autoxref " data-show-preview="true">astronomy</a> by the Greeks led to other schools of thought as well. <span id="ref391751"></span><a href="https://www.britannica.com/biography/Pythagoras" class="md-crosslink " data-show-preview="true">Pythagoras</a> (<em>c.</em> 570–<em>c.</em> 490 <span class="text-smallcaps">bce</span>), for example, argued that the world could be understood on rational principles (“all things are numbers”); that it was made of four elements—earth, <a href="https://www.britannica.com/science/water" class="md-crosslink autoxref " data-show-preview="true">water</a>, <a href="https://www.britannica.com/science/air" class="md-crosslink autoxref " data-show-preview="true">air</a>, and fire; that Earth was a sphere; and that the <a href="https://www.britannica.com/place/Moon" class="md-crosslink " data-show-preview="true">Moon</a> shone by reflected <a href="https://www.britannica.com/science/light" class="md-crosslink autoxref " data-show-preview="true">light</a>. In the 4th century <span class="text-smallcaps">bce</span> <span id="ref391752"></span><a href="https://www.britannica.com/biography/Heracleides-Ponticus" class="md-crosslink " data-show-preview="true">Heracleides Ponticus</a>, a follower of Pythagoras, taught that the <a href="https://www.britannica.com/science/spherical-Earth" class="md-crosslink autoxref " data-show-preview="true">spherical Earth</a> rotated freely in space and that <a href="https://www.britannica.com/place/Mercury-planet" class="md-crosslink " data-show-preview="true">Mercury</a> and <a href="https://www.britannica.com/place/Venus-planet" class="md-crosslink " data-show-preview="true">Venus</a> revolved about the <a href="https://www.britannica.com/place/Sun" class="md-crosslink " data-show-preview="true">Sun</a>. From the different lengths of shadows cast in Syene and Alexandria at noon on the first day of summer, <a href="https://www.britannica.com/biography/Eratosthenes" class="md-crosslink " data-show-preview="true">Eratosthenes</a> (<em>c.</em> 276–194 <span class="text-smallcaps">bce</span>) computed the radius of Earth to an accuracy within 20 percent of the modern <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="value" href="https://www.britannica.com/dictionary/value" data-type="EB">value</a>. Starting with the size of Earth’s shadow cast on the Moon during a lunar <a href="https://www.britannica.com/science/eclipse" class="md-crosslink " data-show-preview="true">eclipse</a>, <span id="ref391753"></span><a href="https://www.britannica.com/biography/Aristarchus-of-Samos" class="md-crosslink " data-show-preview="true">Aristarchus of Samos</a> (<em>c.</em> 310–230 <span class="text-smallcaps">bce</span>) calculated the linear size of the Moon relative to Earth. From its measured angular size, he then obtained the distance to the Moon. He also proposed a clever scheme to measure the size and distance of the Sun. Although flawed, the method did enable him to deduce that the Sun is much larger than Earth. This deduction led Aristarchus to speculate that Earth revolves about the Sun rather than the other way around.</p><div class="module-spacing"> <DIV class="marketing-INLINE_SUBSCRIPTION marketing-content" data-marketing-id="INLINE_SUBSCRIPTION"><style> .student-promo-banner-wrapper { container-type: inline-size; margin-bottom: 15px; } @container (min-width: 475px) { .student-promo-banner { flex-direction: row; } .student-promo-banner-img-wrapper { margin-bottom: 0; margin-right: 10px; justify-content: flex-start; } .student-promo-banner-text-wrapper { text-align: left; margin-bottom: 0px; margin-left: 10px; } .student-promo-banner-button-wrapper { margin-right: 0; } }</style> <div class="student-promo-banner-wrapper"> <div class="student-promo-banner d-flex flex-column align-items-center bg-blue rounded p-20"> <div class="student-promo-banner-img-wrapper mb-20 mr-0 d-flex justify-content-center"> <img class="rounded" style="max-width: 100px; min-width: 80px" src="https://cdn.britannica.com/marketing/BlueThistle.webp" /> </div> <div class="student-promo-banner-text-wrapper ml-0 mb-10 text-center text-white"> <div class="h2 mb-10">Get Unlimited Access</div> <div class="h4 font-weight-semi-bold">Try Britannica Premium for free and discover more.</div> </div> <div class="student-promo-banner-button-wrapper d-flex justify-content-center align-items-center ml-auto mr-auto"> <a class="btn btn-m btn-orange" href="https://premium.britannica.com/premium-membership/?utm_source=premium&utm_medium=inline-cta&utm_campaign=black-friday-2024">Subscribe</a> </div> </div> </div> </DIV></div><div class="one-good-fact-module"> </div><span class="marker MOD7 mod-inline"></span> <span class="marker PREMOD8 mod-inline"></span><p class="topic-paragraph">Unfortunately, except for the <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="conception" href="https://www.merriam-webster.com/dictionary/conception" data-type="MW">conception</a> that Earth is a sphere (inferred from Earth’s shadow on the Moon always being circular during a lunar eclipse), these ideas failed to gain general acceptance. The precise reasons remain unclear, but the growing separation between the empirical and aesthetic branches of learning must have played a major role. The unparalleled numerical accuracy achieved by the theory of epicyclic motions for planetary motions lent great empirical validity to the <a href="https://www.britannica.com/science/Ptolemaic-system" class="md-crosslink autoxref " data-show-preview="true">Ptolemaic system</a>. Henceforth, such computational matters could be left to practical astronomers without the necessity of having to <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="ascertain" href="https://www.merriam-webster.com/dictionary/ascertain" data-type="MW">ascertain</a> the physical reality of the model. Instead, absolute truth was to be sought through the <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="Platonic" href="https://www.merriam-webster.com/dictionary/Platonic" data-type="MW">Platonic</a> ideal of pure thought. Even the Pythagoreans fell into this trap; the depths to which they eventually sank may be judged from the story that they discovered and then tried to conceal the fact that the <a href="https://www.britannica.com/science/square-root" class="md-crosslink " data-show-preview="true">square root</a> of 2 is an <a href="https://www.britannica.com/science/irrational-number" class="md-crosslink " data-show-preview="true">irrational number</a> (i.e., cannot be expressed as a ratio of two <a href="https://www.britannica.com/science/integer" class="md-crosslink " data-show-preview="true">integers</a>).</p><span class="marker MOD8 mod-inline"></span> </section> <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":5,"pageId":139365,"pageLength":1325,"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":"PAGINATED","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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