Page:Popular Astronomy - Airy - 1881.djvu/133

Rh the sun is seen at the Spring Equinox. The first point of Libra is that point at which the sun is seen at the Autumnal Equinox. Neither of these points is exactly marked by any star or other mark in the heavens: the first point of Aries is not very far from the third star of Pegasus (Algenib), and the first point of Libra is not far from the bright star in the tail of Leo, towards the bright star of Virgo.

The importance of acquiring a knowledge of these points is this. I have spoken of the method of using the transit instrument in combination with the use of the clock, which I said was to determine the interval of time between the meridian passage of some known bright star, and the meridian passage of any other object which we see in the heavens. It was plain, therefore, that by using it in this way we can determine the interval between the passage of the object and that of any star or every star; and also that we can determine, with the same precision, the intervals of the passages of all the bright stars; and we must do this, if we wish to make our representation of the heavens and of the position of an object in the heavens at all complete. This, however, would be a most tedious way of doing it. There is no good way of doing what is equivalent to this, except by referring every interval of passage to some one imaginary point or Zero: and the imaginary point or Zero which all Astronomers have found it convenient to adopt, is the first point of Aries. It is a point as I have said, which we cannot see in the heavens, but which we can determine by tracing the motion of the sun among the stars. We must observe, by means of the mural circle, on what day or between what days the sun is 90 degrees from the Pole; then the sun is necessarily at the place where his path crosses