Crescent Venus

¡SkyCaramba! Weekly astronomy blog for the week ending January 11, 2014

It took about two millennia for people all around the world to accept the idea that the earth and other planets go around the sun instead of all the other objects going around the earth. The heliocentric (sun-centered) model was so controversial in some places, offering it as true got some people in trouble. But some evidence for it has been visible to earthbound humans all along.

In ancient Greece, lived an astronomer and mathematician named Aristarchus. He’s famous for his attempts to measure the angles and apparent sizes of the moon and sun. Aristarchus tried to calculate their sizes and distances. His results were way off because the best observing equipment of his time didn’t provide measurements accurate enough for the task. But he was right about the principles he used. Modern equipment and the same mathematics produce fairly accurate results. Aristarchus lived from about 310 to 230 B.C.

Aristarchus proposed a solar system model with the sun at the center and all the planets going around it. Other departures from the geocentric (Earth-centered) notion had been around. One had Mercury and Venus going around the sun, but the sun and everything else going around the earth. Aristarchus is credited for seriously developing the idea that all the solar system objects go around the sun.

The idea that at least Mercury and Venus must be orbiting the sun is easy to get from observation. Neither planet is ever visible all night. Each reaches only so far into the evening sky before turning around, heading toward the sun, and then reemerging into the morning sky where it also goes only so far before turning around.

The other planets go the entire way around the sky. However, from time to time, they also change directions. Some ancient observers thought those planets must be going around something else that is going around the earth. They didn’t realize they were observing an illusion. Those planets appeared to be going backward against the very distant stars while the earth sped through its orbit in a different direction.

Heliocentrism came up from time to time in intellectual circles in Catholic and Muslim areas over the next several centuries. So did the other idea with only Mercury and Venus orbiting the sun. Finally, Nicolaus Copernicus wrote about his solar system model in the 1500s. Other scientists had to be careful when examining his idea though. To the Catholic Church, the idea that the sun was the center of it all was on par with the idea that God is at the center of all Creation. Proposing anything else was heresy.

Then in 1610, Galileo Galilee looked at Venus through a telescope. The crescent he saw meant he was looking at a round object mostly illuminated from the side not facing him. The way the crescent became a smaller semicircle and then an even smaller gibbous figure meant it was indeed going around the thing lighting it up and getting farther away. Eventually, science and the Church came to accept the heliocentric model.

This month, you get a chance to see what Galileo saw, if you have a telescope. Look at Venus in the evening sky early this week. You’ll see a thin crescent. By this weekend, it goes through inferior conjunction, passing between Earth and the sun. When it emerges in the morning sky late next week, you’ll see a crescent again. Think of Galileo when you do.