THE SOLAR SYSTEM (WHTM) — Right now, six planets in our solar system are moving “in retrograde”, and will be for a while. Here they are in their order from the sun:

  • Mercury from Sept. 10 to Oct. 2
  • Jupiter from July 28 to Nov. 23
  • Saturn June 4 to Oct. 23
  • Uranus Aug. 24 to Jan. 23
  • Neptune June 28 to DeC. 4
  • Pluto April 29 to Oct. 8 (Yeah, I know, Pluto is a “dwarf planet” blah blah blah. It’s still in an orbit, and it still displays retrograde motion.)

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Having so many planets moving in retrograde at once is getting a lot of people excited. Astrology sites are full of explanations of how retrograde motions are affecting horoscopes, with talk of “stagnation in the cosmos”, “retrograde energy swaying”, “challenges to relationships” and advice to “avoid new ventures”, “stick to routine jobs”, “evaluate how we are transforming our lives”. There’s also a lot of bad science of the “a planet slows down” variety.

Certainly a lot of fuss and bother for an optical illusion. So what’s really going on? What happens when a planet travels retrograde? Let’s start at the very beginning:

Planets (including Earth) orbit around the sun. This idea is called heliocentrism. (Helios was the god of the sun in Greek mythology.)

Because the planets orbit the sun at different distances, planets have years of different lengths:

Mercury88 days
Venus225 days
Mars687 days 1.88 years
Jupiter4,333 days11.86 years
Saturn10,759 days 29.46 years
Uranus30,687 days84.01 years
Neptune60,190 days164.79 years
Pluto90,520 days248.59 years
Orbits of Planets in days and years

Planets with shorter years will “overtake” slower-moving planets.

Planets with different orbit speeds (NASA)

As a faster orbiting planet overtakes a slower orbiting planet, it will appear (from the vantage point of the faster-moving planet) as if the slower moving planet stops moving forward, then drifts backward, even though it is still moving at its original speed.

Graphic showing the retrograde motion of Mars. Notice how Mars seems from Earth to travel in a loop. (NASA)

We can actually see this effect on Earth. Imagine two cars travel the same direction in two different lanes. One travels slightly faster than the other. As the faster car overtakes the slower, it will seem (from the vantage point of the faster car) as if the slower car stops, then moves backward, even though its actual speed has not changed. (This thought experiment works with different modes of transportation, from walking to bicycles to airplanes.)

It’s worth noting that one of the reasons heliocentrism replaced geocentrism (the notion that the Earth is in the center of the Solar System) is that geocentrism couldn’t explain the retrograde motion of planets. With heliocentrism, explaining retrograde motion was simple.