But rest assured, there’s no scientific evidence that any planet’s apparent retrograde motion affects you at all. This frequency is probably the reason that Mercury’s is the most famous of the retrogrades, although its recent popularity as an explanation for things going awry may have roots in pop culture. But as Mercury, for example, speeds past Earth in its super-fast orbit and starts to turn around the Sun to pass to the other side, we see it slow in its east-to-west motion and start to move in the opposite direction as it takes that turn.īecause Mercury only takes 88 days to orbit the Sun compared to Earth’s 365, it overtakes us in this way three to four times a year, appearing to move backwards for about three weeks at a time. Most of the time, both planets move from east to west in Earth’s skies. We mostly see them in the morning or evening, when they aren’t either directly between us and the Sun or on the far side of the Sun. Because these planets orbit between Earth and the Sun, we tend not to see them high in the night sky. Retrograde motion works a little differently for Venus and Mercury. It’s also the best time for observing a planet, since it’s at its closest point to Earth, making it look bigger and brighter. The point in time when Earth first catches up to a farther-out planet is also called opposition, and is when that planet is on the opposite side of the Earth from the Sun. Retrograde motion is technically called “apparent retrograde motion” since it’s not an actual change in the planet’s motion through space, but rather an effect caused by our perspective from Earth. An eccentric orbit had a center different from the Earth and accounted well for changes in a planets brightness. Where this did not fit, Ptolemy proposed an eccentric. This explained retrograde motion while keeping the planets in their circular orbits around the Earth. This happens when Earth overtakes a slower-moving planet in its orbit around the Sun, or is overtaken by a faster-moving planet. He argued that planets move on two sets of circles, a deferent and an epicycle. When we say that a planet is in retrograde it means that from the perspective of Earth, a planet’s motion across the sky goes backwards night after night compared to its usual direction for a period of time. What does it mean for a planet to be in retrograde? If you’ve seen the term “Mercury in retrograde” and aren’t sure what to make of it, here are the facts. It’s because Mercury enters into a period of retrograde orbit, which is popularly believed to have astrological significance. This isn’t because of new scientific discoveries being made, or new missions launching to study the rocky world. Not until the 16th century - when the Polish astronomer Copernicus placed the sun at the center of the solar system - did all that retrograde motion suddenly make sense.Three or four times a year, the planet Mercury becomes a trending topic online. But it was impossible for them to come up with a solution that also fit with the popular idea that Earth was the center of the solar system. ![]() Retrograde motion was even visible to early astronomers, who were thoroughly confused when they saw this and struggled to explain it. Since they don't, every couple of years, Mars temporarily gets left behind. If Earth and Mars orbited at the same pace and remained in fixed positions relative to each other throughout their orbits, Mars would always look like it was moving in the same, east-to-west direction. If you observe and mark the position of Mars night after night during retrograde, you'll see a shape emerge - sometimes it's a closed loop and sometimes it's more of a zigzag - all depending on where the planets are on their tilted axes. A swiftly tilting planetĪnd if that isn't weird enough, because Earth and Mars have different tilts to their orbital paths, the shape of the path tracking Mars' backward motion can change between retrograde events. As our orbital path carries us past the Red Planet, we experience the illusion that Mars is pulling away from us, rather than the reality - that Earth is moving away from Mars.Īfter a couple of months of this, our perception of how our planets are moving hits the reset button, and Mars appears to resume its forward movement. Every 26 months, Earth catches up to Mars and moves past it. We're both in motion, but Mars has farther to go to make it all the way around. Mars needs 687 Earth days to make a complete circuit. ![]() It takes Earth 365 days to orbit the sun.
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