A planet stops in its tracks

For the week ending January 8, 2011

Here’s your chance to see a planet stop in its tracks and turn around. Well, not really. It’s an illusion of perspective, like telephone poles next to the highway going by real fast while a hill out in the distance seems to go by real slow.

Saturn will be in Virgo all year. But it’s not holding still. Now until around January 22nd, you’ll get to see Saturn heading eastward as it slows down, virtually stops, and begins going westward. The ringed planet will continue that apparent westward motion until the first week in June when it has a rendezvous with a star named Porrina. And then, you’ll get to see Saturn slow down, stop, and reverse direction again.

When you see a planet going eastward from night to night, it’s in direct or prograde motion. That’s an astronomer’s way of saying the object is going in the direction it’s supposed to. When it goes westward, it’s in retrograde motion. And for a few nights, when it’s slowing down to change direction, the object is said to be stationary.

If you could stand somewhere outside the solar system and watch the planets going around the sun, you’d see that each follows a path like a car goes around a race track. They speed up and slow down just a little as they make their circuits. But the only changes in direction are the continuous turns they make on the way around. They don’t start off counterclockwise, stop, then go clockwise.

The illusion of planets going stationary and going through retrograde motion happens because we don’t have the up close perspective of a racing fan by the track. Imagine how things might look though if you could look at the race track edge-on from far away. The cars would be zipping back and forth, but you wouldn’t see that they’re going around anything. The distance would force your observation into a two-dimensional view. That’s sort of what happens to us as we look at the night sky and see planets going back and forth.

This month Saturn rises around midnight. If you look then, you’ll see it in the east. If you wait a few hours (looking closer to sunrise), you’ll see Saturn and Virgo higher in the sky. If you watch from night to night, you’ll see Saturn slowing down almost to a halt in the middle of Virgo. Binoculars may help you appreciate this spectacle better, because you’ll be able to see the dimmer stars that Saturn is moving in front of.

The same illusory change of motion happens with all the planets. The race track analogy isn’t complete without telling you this: we’re looking upon the other planets not like a far off observer would look at the race track, but like a race car driver would look upon the other race cars in the race.

Except, of course, we can take time to enjoy that view because we don’t have to watch where we’re going.