Throughout history, people familiar with the night sky have been aware of the seasonal progression of the constellations. For example, northern hemisphere observers know that the Big Dipper is high in the sky on spring evenings and close to the northern horizon in the fall. They also know that Orion will be visible in the winter, but not in the summer. It's the same every year. Not so with the planets!
During the winter of 2009, we saw that Venus shone brilliantly in the evening sky. However in the winter of 2008, it was not visible in the evening. If you wanted to see Venus then, you had get up in the morning before sunrise. Mars was the bright planet visible on winter evenings in 2008, but not in 2009. In fact, Mars will not appear in the evening sky in 2009 until the fall.
Investigating the movements of the planets and trying to predict their positions occupied astronomers until the 17th century A.D.
In those days, the term planet had a slightly different meaning from the one we know today. It was a Greek term that meant "wandering star". The Sun and Moon, as well as Mercury, Venus, Mars, Jupiter and Saturn were all considered to be planets, but the Earth was not. (Our days of the week are named after those original seven planets. You can readily recognize that Saturday, Sunday and Monday relate to Saturn, the Sun and Moon, but if you are wondering about Tuesday, Wednesday, Thursday and Friday, just think of their French names: mardi, mercredi, jeudi and vendredi.)
Before the 16th century, it was generally assumed that all of the planets and stars circled around the Earth. The model that was widely used for predicting their positions was developed by Ptolemy, a Greek astronomer who lived in Alexandria circa AD 100-170. It was a complicated model because the paths traced out by the planets, particularly Mercury, Venus and Mars, were complex. Mercury and Venus never moved far from the Sun in the sky; consequently, they were never visible at midnight. Ptolemy accounted for this by adding more circles. According to his arrangement, the planets moved in circles that moved in circles around the Earth! He was not concerned with explaining why the planets moved the way they did; he just wanted to derive a method for predicting their positions. And for many centuries, his model was the accepted method.
In 1543, Copernicus published his famous book, "On the Revolutions of the Heavenly Spheres". In it, he developed a heliocentric model, one in which the planets circled around the Sun, not the Earth. This new model was still complex, but it provided a more appealing explanation for the unusual motions of Mercury and Venus. They never move far from the Sun in the sky because their orbits are closer to the Sun than Earth's is.
Like Ptolemy, Copernicus was not concerned with why the planets moved the way they did. He just wanted to develop a geometric model that reproduced their motions. However, his work started a scientific revolution. In the century after Copernicus died, astronomy was transformed. Astronomers were no longer preoccupied with predicting planetary positions; they sought to understand why the planets moved the way they did.
Next time, we will discuss how this happened.