Laboratory Astronomy (Paperback)

Book may have numerous typos, missing text, images, or index. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. 1901. Not illustrated. Excerpt: ... of some known star after nightfall, free from any error involved in reading the hour-circle. As before, a star should be chosen at nearly the same declination, so that the interval of time may be very nearly equal to the difference in right ascension between the sun and the star, even if the instrument is not very exactly in the meridian. Observation of several different stars will enable us to determine whether the instrument actually does describe the plane of the meridian as it is rotated about the horizontal axis (see Chapter VIII); and by the observation of stars near the pole, as described on page 81, we may determine whether the declination circle reads exactly 0 when the telescope points to the equator, as should be the case. THE MERIDIAN CIRCLE An instrument which is to be used in this manner, however, is not usually so constructed that it can be pointed at any point in the heavens. Thus, it is unnecessary that it should consist of so many moving parts as the equatorial instrument, and steadiness, strength, and ease of manipulation are very much increased by constructing it as shown in Fig. 40, which represents a very small instrument built on the plan of the meridian circle of the fixed observatory. The strong horizontal axis revolves in two Y's, which are set in strong supports in an east and west line. The axis is enlarged towards the center, and through the center passes at right angles the telescope tube. The axis carries at one end a graduated circle perpendicular to the axis of rotation. If the axis of the telescope is perpendicular to the axis of rotation, and if the latter is adjusted horizontally east and west, the telescope may be brought into any position of the meridian plane, but must always be directed to some point of the latter. A poi...