Page:Popular Astronomy - Airy - 1881.djvu/134

120 the equator, (which, as I have said, is 90 degrees from the Pole,) and therefore is necessarily either at the first point of Aries or the first point of Libra. We must, at the same time, by means of the transit instrument, determine the intervals between the passages of the sun and several fixed stars on those days; and then we shall have the interval between the passages of the first point of Aries and those stars. We can then use the first point of Aries so determined, as a starting point for sidereal time; and then instead of measuring our sidereal time from the passage of any star, we shall measure it from the passage of that imaginary point. Now, suppose that our observations of transits on any evening are to be compared with the observation of the transit of the bright star of Aquila; even though we do take that bright star of Aquila as the practical starting point of our observation, yet we do not make our clock to point hours, minutes, and seconds, when the star comes to the meridian, but we put our clock to point 19 hours, 43 minutes, and some seconds; because if we put our clock to point hours, minutes, and seconds, when the first point of Aries is passing the meridian, it shows 19 hours, 43 minutes, and some seconds, when the bright star of Aquila passes the meridian. And that is the use of the first point of Aries, which cannot be seen in the heavens. It is better that an imaginary Zero be chosen for the starting point than any one star; and the first point of Aries is peculiarly convenient, on account of its relation to the sun's path.

The next point of which I omitted to speak is, the difference between a sidereal day and a solar day. I pointed out to you how, by very rough means, the passage of the sun through the stars might be