Up in the Air

Saturn stumped the ancient stargazers

April 14, 2013 

Daniel B. Caton is a physics and astronomy professor and director of observatories at Appalachian State University.

We will find Saturn dominating our night sky as summer nears. Rising in the east a bit after sunset, Saturn is approaching opposition at the end of this month. At opposition, a planet is opposite the sun: rising at sunset and setting at sunrise.

As we approach opposition, careful plotting of the planet’s position with respect to the stars would show an odd behavior: The planet stops its slow night-to-night eastward march as it inches along its 30-year-long orbit and appears to back up westward for a while before it resumes its forward motion. Throw in the tilt of the planet’s orbital plane and you end up with the do-si-do motion we call a retrograde loop.

These motions vexed astronomers back in the days of the geocentric model of the solar system proposed by Aristotle and Ptolemy. The Earth-centered model was an idea that did not work well and had to be fitted out with all manner of add-ons to make it work. The geocentric model was patched with epicycles, deferents and equants – weird sub-loop motions and reference points – to make them reproduce the observed motion of the planets.

Galileo would eventually make the observations needed to verify Copernicus’ heliocentric model, and the planets’ motions were explained without the need for patches.

Retrograde motion is an optical illusion caused by Earth passing the slower outer planets at opposition. Let’s put it another way: If Kyle Busch were doing 128 mph on a public street, he would see slower cars moving forward far ahead of him, but as he passed them they would appear to back up for a while before their forward motion was again obvious in his rearview mirror.

Galileo also discovered the rings of Saturn, although his low-power telescope’s minimal optics showed a poor image. At first, he interpreted the rings as two moons on opposite sides of the planet. Because of the tilt of the planet, those “moons” disappeared from his view the next season, and he had to wait until the rings were no longer edge-on for further observations. In the end, he thought they were some kind of “arms” or handles on the planet.

If you have a small telescope, this will be a good year to look at the rings since the rings are well positioned. Even the smallest telescope will show the rings and often a few of Saturn’s moons.

Daniel B. Caton is a physics and astronomy professor and director of observatories at Appalachian State University. Email: catondb@appstate.edu. More on this month’s column: www.upintheair.info.

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