Page:Popular Astronomy - Airy - 1881.djvu/138

124 rising nearly at sunset, and setting nearly at sunrise, and if we watch his course among the stars from day to day, or if we determine his place on different days by observations with the transit instrument and mural circle, we find that at this time Jupiter is moving among the stars towards the right hand.

It is convenient to have astronomical terms to describe this direction, without speaking of the right hand or left hand. Now, I have explained to you that the appearance of the stars, at different months of the year shows that the sun must be supposed to move through the stars towards the left; and the moon moves visibly towards the left. Astronomers therefore have agreed to describe this kind of motion by the term "direct," and the opposite motion by the term "retrograde." Now, the planets sometimes move in a retrograde direction: thus, when Mercury or Venus is in that part of its orbit which is nearest to the earth, its motion, as referred to the stars, is retrograde. And the apparent motion of Jupiter at the present time, from the description of it which I have just given, is retrograde; and so in all cases is that of Mars, Jupiter, Saturn, Uranus, Neptune, and the smaller planets, when they are seen on the side opposite to the sun. At other times their apparent motions are direct with respect to the stars.

All these motions are tolerably explained by the construction adopted by Ptolemy and the Greek astronomers: taking the assumption that the earth was fixed; that there was something like a bar, (Figure 26,) of which one end was fixed in the earth and which turned round in a year; that that bar carried the sun, and carried also the centres of the orbits of Mercury, Venus, Mars, Jupiter, and all the planets; and that all the planets revolved in their