Thinkin' About Space: Retrograde motion of planets

As Earth moves around the Sun, the Sun makes a slow progression through the sky. Generally, you can’t see it because the sunlight scatters off of the atmosphere and turns the sky an opaque blue. Each day it moves a little bit farther east. 

However, the movement is entirely due to Earth’s orbit. As we loop past the Sun, it appears to be moving back in the sky. The Moon also has eastern movement, though it is much more apparent. In about an hour, the Moon moves one lunar diameter east, which taken over the course of a month completes one orbit of Earth. At no point does the Moon or the Sun move west, although I am of course neglecting the east-west motion that is due to Earth’s rotation. 

Looking down at Earth and the Moon from a point far above the solar system, you’ll notice the Moon orbits in a counter-clockwise motion. Earth moves about the Sun in a counter-clockwise direction, as do all other major bodies in the solar system. This motion is said to be prograde. 

From Earth, the motions of the planets mostly follow a prograde path, moving east in wide circuits. In addition, many of the satellites throughout the solar system orbit their primaries in a prograde direction. Triton, a moon of Neptune, orbits in the opposite direction. From the “surface” of Neptune, Triton would appear to move east in comparison to the background stars. It and any other object that moves east (again, neglecting the rotation of the parent body) is said to have retrograde motion.

Ancient astronomers thought that the Sun, Moon, stars and all the planets orbit Earth. This seemed to follow from Earth being the most important object in the universe and therefore the natural focal point of all motion. A large amount of time was spent adjusting observed planetary motions to fit with a geocentric universe. What the astronomers noticed was that at certain points in a planet’s orbit, it slows to a halt and reverses its path. After a time, it halts again and resumes its eastern motion. When a planet is moving west, it is said to be “in retrograde.” 

As the ancient astronomers considered the planets to be gods, whatever was important enough to make a god stop in its tracks and retreat for a time was cause for alarm. At the time, astrology was a popular practice, mainly to tell royalty when to invade or not, what their offspring’s future might be and so on. In fact, astrologers were the ones to make the first accurate star charts as mistakes in interpretations were often fatal.

An astronomer named Nicolaus Copernicus was the first to notice that three of the five planets moved in a retrograde fashion around the time of their opposition. Opposition is when Earth is directly in line between the Sun and the outer planet. Copernicus’ idea that the Sun, not the Earth, was the center of the solar system was supported by this idea. Retrograde motion, at least for outer planets, is caused by Earth passing the planet in its shorter orbit. 

A way to picture the motion is this: As Earth moves around the Sun, an observer on the surface will be looking at Mars over their left shoulder. Night after night, Earth moves farther around the Sun until it is in position between the Sun and Mars. The observer would see Mars directly in front of them. Carried further by the Earth’s orbit, Mars would appear to lag behind, until the observer would see it on their right side. There is then a point where Mars would stop moving to the right and would begin to move left, or prograde, again. That is caused by Earth moving in its orbit towards the far side of the Sun. Mars will continue to move prograde as Earth moves around the Sun, even as Mars falls in a line on the other side of the Sun.

A top-down view of retrograde motion (via Universe Today)

Another type of retrograde motion is reserved for the interior planets, though this one isn’t an optical illusion. I wrote earlier on the motions of Venus, so I’ll save time by referencing that article. Suffice to say, as Venus moves from western elongation, around behind the Sun and to eastern elongation, it is moving prograde. When it reaches elongation and moves in front of the Sun, it is moving towards the west and is thus in retrograde.

Currently, there are four of the visible planets in retrograde. Mars — though being the planet I described and the planet most people associate with retrograde motion — is the lone holdout. The outer planets, Jupiter and Saturn, are in retrograde once a year when Earth passes them. Mars moves quicker and isn’t overtaken by Earth as often, so it is in retrograde every two years or so (this year it won’t be in retrograde at all, but 2018 is the year of its “Great Opposition” and will be in retrograde around that time). Venus spends half of its 225-day orbit in retrograde, and quick Mercury spends half of its 88-day orbit with the same motion.

The orbits of Mercury and Venus (via NASA)

Retrograde motion is nothing more than an optical illusion caused by Earth’s motion around the Sun. The easiest retrograde motion to see, currently, is that of Jupiter. On April 7, Jupiter was at opposition and was at its brightest. This past week has done little to diminish its glow, so it shines brightly after sunset above a blue star called Spica. They’re about an 'outstretched thumb' apart and growing as Jupiter continues moving west. By the end of the summer, however, Jupiter will end its retrograde motion and move back toward Spica until the two are close once again. 

Jupiter's motions for 2017. It is in retrograde from February to June (via Jeffrey Hunt)

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