Page:Astronomy for Everybody.djvu/83

the latter. This is clone by graduated circles, one of which is attached to each axis. One of these circles has degrees and fractions of a degree marked upon it, so as to show the declination of that point in the heavens at which the telescope is pointed. The other, attached to the polar axis, and called the hour circle, is divided into twenty-four hours, and these again into sixty minutes each. When the astronomer wishes to find a star, he simply looks at the sidereal clock, subtracts the right ascension of the star from the sidereal time, and thus gets its "hour angle" at the moment, or its distance east or west of the meridian. He sets the declination circle at the declination of the star, that is, he turns the telescope until the degree on the circle seen through a magnifying aparatus is equal to the declination of the star; and then he turns the instrument on the polar axis until the hour circle reads its hour angle. Then, starting his clockwork, he has only to look into the telescope and there is the object.

If all this seems a complicated operation to the reader, he has only to visit an observatory and see how simply it is all done. He may thus in a few minutes gain a practical idea of sidereal time, hour angle, declination, etc., which will make the whole subject much clearer than any mere description.

Let us return to some interesting matters, mostly historical, connected with the making of telescopes. The great difficulty, which requires special native skill of the