ELI ON EARTH: Jupiter Reversing Course

Image: Jupiter's spot, photographed by Voyager 1, courtesy of NASA

Some might question the wisdom of Eli on Earth continuing to report on astronomical events when East Lansing’s sky is consistently the color and clarity of cement. But as the author knows personally, East Lansing does have the occasional clear night when you can find and see an Iridium flare or check out an eclipse or a clear day where you can observe sunspots.

So we persist, and have come today to tell you this: In the December through April skies of East Lansing, Jupiter will be moving backwards across the sky.

As is the case with the moon, the planets rise and set at different times of the day and are seen in different parts of the sky at different times in their orbit. As the planets move in their orbit, their position compared to the stars in the East Lansing sky changes. For most of the year, the planets appear to move farther and farther east of the stars as seen from earth.  The result is that for an observer in East Lansing the planets should generally be slightly farther east of a reference star than they were the night before.

While the planets continue in their elliptical orbit around the sun throughout the year, their path is not always a smooth eastward march across the East Lansing sky. There are times each year when planets slow down or even track backwards (westward) from night to night. In the case of Jupiter, this retrograde motion happens when the earth passes Jupiter while both orbit the sun.

Earth travels about twice fast as Jupiter around the sun and is five times closer to the sun than Jupiter, so each year the earth passes Jupiter.  As the earth comes inside and passes Jupiter, from the surface of the earth Jupiter looks like it is going backwards.  It is a little like being a passenger on train in a dark tunnel and passing another, slower train.  It looks like the other train is going backwards.  When East Lansing passes Jupiter, it looks like Jupiter is moving backwards, and that is how retrograde motion happens for Mars, Jupiter, and Saturn.  Once earth has moved far enough ahead of Jupiter, the earth and Jupiter are so far apart that Jupiter once again appears to move across the sky from east to west.

Jupiter is now entering a retrograde (backward) phase. Starting December 8 and continuing through April 8, 2015, Jupiter will be moving backward across the sky. On a clear night you can look east just before bed and the brightest objects in the sky will be Jupiter and the star Regulus, with Jupiter slightly west of Regulus. You can measure the distance between the two by holding your fingers or fist at arms length between them. From now until April, Jupiter will be slightly more west of Regulus each night. After April 8, 2015, Jupiter will again move east relative to Regulus each night.

It takes a good deal of attention to detail to notice and chart the position and brightness changes of planets in the sky. The movement of planets relative to the stars and each other are complex and inspired the name planet from the Greek word πλανήτης for wanderer (thank you, Wikipedia).

For thousands of years people have tracked, charted, and tried to explain the movements of the planets across the sky. The movement of the sun and the moon through the sky give every indication that they both orbit the earth. If you assume that the planets also orbit the earth, it is very difficult to explain the retrograde (backward) motion and changes in brightness of planets. Before the Copernican system was widely accepted, retrograde motion of the planets was a serious challenge to proponents of an earth-centered planetary system. To explain retrograde motion, pre-Copernican astronomers decided the planets moved in circles (called epicycles) as they circled the earth. This circular motion with a circular orbit was the accepted system in European thought until the acceptance of the sun-centered Copernican system, which explains the retrograde movement of planets without epicycles.

There have actually been sun-centered solar system explanations for planetary movement in Ancient Greece, in India, and in the Islamic world for hundreds and even thousands of years, but in the west political and religious considerations prevented the acceptance of a sun-centered solar system.

Here’s an ELi on Earth tip:  You can show off your knowledge of astronomical history by using the phrase “adding epicycles” as a pejorative comment when people add complexity to save their preferred world-view from the facts.

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