Jupiter is one of the 5 planets visible with the unaided eye, and so it has been known for thousands of years. But the movement of Jupiter and the other planets was a mystery until just a few hundred years ago. Jupiter moves across the sky in a very predictable pattern, but every now and then it reverses direction in the sky, making a tiny loop against the background stars – this is Jupiter retrograde.
Of course, Jupiter isn’t actually moving backwards in the sky – it orbits the Sun in the same counter-clockwise direction as the other planets. So what’s going on?
In ancient times, astronomers thought that the Sun, the Moon, the planets and the stars orbited around the Earth. This helped explain the movement of the planets, but there was a problem. The planets would occasionally reverse direction in the sky – move in a retrograde direction from the way they normally go. To explain these movements, astronomers developed a complicated model of orbiting spheres, where the planets followed a spiral path around the Earth.
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This model was turned on its ear by Copernicus in the 1500s when he proposed that the planets orbited around the Sun. This also elegantly explained why Jupiter has a retrograde motion, as well as the other planets. Jupiter is following a roughly circular orbit around the Sun, but it takes 12 years to complete an orbit; while Earth takes just a year for an orbit.
The retrograde motion of Jupiter actually comes from Earth catching up to Jupiter in its orbit. As Earth passes Jupiter in orbit, we’re looking back at it as we go by. Think of a car passing another car on the highway. You see the car up ahead, and then as you pass it, the car appears to be moving backwards from your point of view. It’s not actually going backwards, of course, it’s all in your perspective.
Each Jupiter retrograde period lasts about 4 months, and happen every 9 months. Consider the orbit of the Earth and Jupiter, and you can understand that this is how long it takes Earth to complete an orbit around the Sun and then catch up with Jupiter again.
Astrologers think that Jupiter retrograde indicates some kind of change of luck and fortune, but there is nothing in the science of astronomy that supports that view at all.
Want more information on Jupiter? Here’s a link to Hubblesite’s News Releases about Jupiter, and here’s NASA’s Solar System Exploration Guide.
We have recorded a podcast just about Jupiter for Astronomy Cast. Click here and listen to Episode 56: Jupiter.
Wikipedia: Retrograde Motion