![]() ![]() Laura Driessen (author provided) There’s a retrograde most of the time However, the pseudoscientific practice of astrology continues to ascribe a deeper meaning to this illusion.ĭiagram of the dates in 2023 that each planet (and the two dwarf planets Pluto and Ceres) will be in retrograde. So, humans found out retrograde motion was an optical illusion 500 years ago. Astronomers like Apollonius around 300 BCE saw the planets going backwards, and explained this by adding more circles called epicycles. Retrograde motion bamboozled ancient astronomers since humans started looking up in space, and we only officially figured it out when Copernicus proposed in 1543 that the planets are orbiting the Sun (though he wasn’t the first astronomer to propose this heliocentric model).īefore Copernicus, many astronomers thought Earth was the centre of the universe and the planets were spinning around us. Laura Driessen (author provided) A well-known illusion From Astro’s view, it appears that I’m going backwards as he overtakes me. At the bottom we can see the side-on view. At the top we can see the top-down view of the oval. So we see them in retrograde approximately once a year as we whip around the Sun so much faster than they do.ĭiagram of me (Laura) and Astro running around the oval, this time from Astro’s perspective. ![]() The other planets are so far from the Sun and travelling so slowly compared to Earth that it’s almost like they’re standing still. Mars is in retrograde once every two years. This is what happens when we look up at the sky and see one of the outer planets in retrograde. But as he starts to pass me, it seems like I’m going backwards or left (retrograde) while he continues to run forwards to the right. At this moment it seems like we’re both going the same direction, to the right. He’s running around the oval and he starts catching me up from behind. Astro is definitely not a deep thinker, but let’s imagine for a moment that he is and think about what he sees as he runs around the oval. To work this out, we need to swap our perspective. The planets outside our orbit (Mars, Jupiter, Saturn, Uranus, and Neptune) also go into retrograde. The dwarf planet Ceres orbits between Mars and Jupiter. This means Venus is in retrograde twice every three years.ĭiagram of the orbits of the planets (and dwarf planet Pluto) in the Solar System. Venus also orbits inside our orbit of the Sun, zipping around once every 224.7 days. Laura Driessen (author provided)īecause Mercury’s orbit is inside Earth’s orbit, seeing it from our planet is like me watching Astro run.īut Mercury isn’t the only planet to do this. From my point of view it looks like Astro is running right-to-left when he’s on the opposite side of the oval to me, but it looks like he’s running left-to-right when he’s on the same side of the oval to me. This happens because Astro is going much faster than me, and is inside my “orbit” of the oval.ĭiagram of me (Laura) and Astro running around the oval, from my point of view. But when he gets to the same side of the oval as me, it suddenly looks like he’s running right instead of left (retrograde). If we’re both going anti-clockwise around the cricket pitch, when Astro is on the opposite side of the oval to me it looks like he’s going left while I’m jogging right. If I take Astro for a run on my local cricket oval, he does super-speed laps on the inside while I run much more slowly around the outside. Astro is a whippet, or a mini-greyhound, and he has a need for speed. ![]() Let’s use my dog Astro to help explain what’s happening when we see a planet in retrograde. What we are talking about is apparent retrograde motion, when to us on Earth it looks like a planet is moving across the sky in the opposite direction to its usual movement.īecause Mercury is closest to the Sun and has the fastest orbit, it appears to move backwards in the sky more often than any other planet. However, the planets never actually change direction. Retrograde motion means a planet is moving in the opposite direction to normal around the Sun. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie A matter of orbits Image of Mercury from MESSENGER’s Wide Angle Camera. But what does it mean when we say Mercury is “in retrograde”? So it seems a bit rough that we blame Mercury for all our problems three to four times a year when it’s in retrograde. Mercury will also be the first planet destroyed when the Sun expands on its way to becoming a red giant in about 5 billion years. Mercury is the closest planet to the Sun, whipping around our star every 88 days compared to Earth’s 365.25 days.
0 Comments
Leave a Reply. |