Page:Popular Astronomy - Airy - 1881.djvu/105

Rh than that the axis of. the earth should change its position. So strong is the tendency of rotation to preserve the position of the axis unaltered, both in parallelism to itself, and with respect to its position in the rotating body, that we may assert boldly that the earth's axis has always been in the same general position within the earth ever since the earth first received motion. And its position, as regards the place among the stars to which it points, is affected only by the very slow motion called Precession of the Equinoxes (of which more will be said hereafter); and even this does not affect its inclination to the plane of the ecliptic.

Having got through this part of my subject, I will now proceed to speak of the apparent motion of the planets. The movements of the planets are extremely complex. At the present time, Venus is what is called the morning star; she is to be seen before sunrise. If you watch her movements through the stars, you will see that her motion is in the same direction as the motion of the sun. You will remember what I said in regard to the motion of the sun: that it moves in regard to the stars, in a direction opposite to the movement of the hands of a watch. You cannot see the stars surrounding the sun, though astronomers can see them with their telescopes; but by watching the stars from month to month, you find that the stars appear to move away from the sun towards the right, or that the sun moves among the stars towards the left. That is called direct motion. If you look at Venus at the present time, you will see that she is moving in a direct motion faster than the sun. She will go behind the