As astronomers, we’re used to watching objects move across the sky. But much of this is due to the apparent movement of the celestial sphere<\/a> when seen from Earth.<\/p> Take the stars<\/a>, for example. You can see them moving across the night sky in a matter of hours, but this is of course down to the fact that Earth is spinning while it orbits the Sun.<\/p> The stars aren’t really speeding across the sky. They just appear to be, from our perspective, and photographers are found of capturing this in long-exposure images known as star trails<\/a>.<\/p> This same apparent motion can be said for the planets, too.<\/p> And once in a while, if you observe the movements of the planets along the ecliptic<\/a> night after night, you might notice one suddenly appearing to stop, then beginning to move backwards.<\/p> We call this apparent backwards movement ‘retrograde motion’.<\/p> The planets normally move from west to east relative to the stars.<\/p> If you look skywards at the same time of night on two different nights, you\u2019ll notice that the planets have moved to the east.<\/p> This is because all the planets, as viewed from above, orbit the Sun in an anticlockwise direction. This is known as normal or direct motion.<\/p> It\u2019s generally only comets<\/a>, with eccentric orbits, that travel around the Sun in a clockwise, or retrograde, direction.<\/p> Every once in a while a planet will also appear to move in a retrograde direction.<\/p> Mars retrograde<\/a> is a well-known and well-observed example.<\/p> Its motion will first appear to slow down, then stop for a short while, and then start up again, but in the opposite, westward direction.<\/p> Eventually it will stop again and resume its eastward movement.<\/p> Nicolaus Copernicus<\/a> was an early astronomer known for arguing that the Sun and not the Earth was at the centre of the Solar System (heliocentrism vs geocentrism<\/a>).<\/p> He reasoned that planets further away from the Sun would move more slowly than closer ones.<\/p> As a faster planet overtakes Earth, its motion against the stars, as seen from Earth, reverses.<\/p> The same explanation works for the outer planets too, only now it is the Earth that overtakes the other planet.<\/p> Before Copernicus, astronomers had difficulty in marrying their belief that the Earth was the centre of the Solar System with the retrograde motion of the planets.<\/p> Their solution involved the planets each moving in mini-orbits called epicycles, while they travelled around the Earth on their main orbits.<\/p> The combined effect of these two motions would occasionally produce a westward motion of the planet when viewed from Earth.<\/p> This article originally appeared in the March 2006 issue of <\/strong><\/em>BBC Sky at Night Magazine<\/strong>.<\/strong><\/em><\/p> <\/body><\/html>\n Why do some planets appear to move backwards in the night sky? <\/p>\n","protected":false},"author":24,"featured_media":65071,"template":"","categories":[1],"acf":{"readingTimeMinutes":"4"},"uagb_featured_image_src":{"full":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2024\/10\/why-planets-seem-to-slow-down-stop-and-reverse-direction-in-the-sky-retrograde-motion-explained.jpg",1500,1000,false],"thumbnail":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2024\/10\/why-planets-seem-to-slow-down-stop-and-reverse-direction-in-the-sky-retrograde-motion-explained-150x150.jpg",150,150,true],"medium":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2024\/10\/why-planets-seem-to-slow-down-stop-and-reverse-direction-in-the-sky-retrograde-motion-explained-300x200.jpg",300,200,true],"medium_large":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2024\/10\/why-planets-seem-to-slow-down-stop-and-reverse-direction-in-the-sky-retrograde-motion-explained-768x512.jpg",768,512,true],"large":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2024\/10\/why-planets-seem-to-slow-down-stop-and-reverse-direction-in-the-sky-retrograde-motion-explained-1024x683.jpg",800,534,true],"1536x1536":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2024\/10\/why-planets-seem-to-slow-down-stop-and-reverse-direction-in-the-sky-retrograde-motion-explained.jpg",1500,1000,false],"2048x2048":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2024\/10\/why-planets-seem-to-slow-down-stop-and-reverse-direction-in-the-sky-retrograde-motion-explained.jpg",1500,1000,false]},"uagb_author_info":{"display_name":"importmanagerhub@sprylab.com","author_link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/author\/importmanagerhubsprylab-com\/"},"uagb_comment_info":0,"uagb_excerpt":"Why do some planets appear to move backwards in the night sky?","_links":{"self":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/rss_feed\/65070"}],"collection":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/rss_feed"}],"about":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/types\/rss_feed"}],"author":[{"embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/users\/24"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/media\/65071"}],"wp:attachment":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/media?parent=65070"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/categories?post=65070"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}Movement of the planets<\/strong><\/h2>
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Why retrograde happens<\/strong><\/h2>
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