The Heavens Above: A Popular Handbook of Astronomy

by J. A. Gillet; W. J. Rolfe

To find the altitude of a star, the reading of the vertical circle is first ascertained when the telescope is pointed horizontally, and again when the telescope is pointed at the star. The difference between these two readings of the vertical circle will be the altitude of the star.

12. The Vernier.—To enable the observer to read the fractions of the divisions on the circles, a device called a vernier is often employed. It consists of a short, graduated arc, attached to the end of an arm c (Fig. 17), which is carried by the axis, and turns with the telescope. This arc is of the length of nine divisions on the circle, and it is divided into ten equal parts. If 0 of the vernier coincides with any division, say 6, of the circle, 1 of the vernier will be 1/10 of a division to the left of 7, 2 will be 2/10 of a division to the left of 8, 3 will be 3/10, of a division to the left of 9, etc. Hence, when 1 coincides with 7, 0 will be at 6-1/10; when 2 coincides with 8, 0 will be at 6-2/10; when 3 coincides with 9, 0 will be at 6-3/10, etc.

Fig. 17.

To ascertain the reading of the circle by means of the vernier, we first notice the zero line. If it exactly coincides with any division of the circle, the number of that division will be the reading of the circle. If there is not an exact coincidence of the zero line with any division of the circle, we run the eye along the vernier, and note which of its divisions does coincide with a division of the circle. The reading of the circle will then be the number of the first division on the circle behind the 0 of the vernier, and a number of tenths equal to the number of the division of the vernier, which coincides with a division of the circle. For instance, suppose 0 of the vernier beyond 6 of the circle, and 7 of the vernier to coincide with 13 of the circle. The reading of the circle will then be 6-7/10.

13. Hour Circles.—Great circles perpendicular to the celestial equator are called hour circles. These circles all pass through the poles of the heavens, as shown in Fig. 18. EQ is the celestial equator, and P and P' are the poles of the heavens.

The point A on the equinoctial (Fig. 19) is called the vernal equinox, or the first point of Aries. The hour circle, APP', which passes through it, is called the equinoctial colure.

Fig. 18.

14. Declination and Right Ascension.—The declination of a heavenly body is its distance north or south of the celestial equator. The polar distance of a heavenly body is its distance from the nearer pole. Declination and polar distance are measured on hour circles, and for the same heavenly body they are complements of each other.