In the course of the twenty-four hours every star above the horizon of the observatory must come at least once within the range of this telescope, and at that moment the observer points the telescope to the {83} star, and notes the time by his clock when the star crossed the spider's threads, which are fitted in the focus of his eye-piece. He also notes the angle at which the telescope was inclined to the horizon by reading the divisions of his circle. For by these two--the time when the star passed before the telescope and the angle at which the telescope was inclined--he is able to fix the position of the star. "But why should catalogues be repeated? When once the position of a star has been observed, why trouble to observe it again? Will not the record serve in perpetuity?" The answers to these questions have been given by star catalogues themselves, or have come out in the process of making them. The Earth rotates on its axis and revolves round the Sun. But that axis also has a rolling motion of its own, and gives rise to an apparent motion of the stars called +Precession+. Hipparchus discovered this effect while at work on his catalogue, and our knowledge of the amount of Precession enables us to fix the date when the constellations were designed. Similarly, Bradley discovered two further apparent motions of the stars--+Aberration+ and +Nutation+. Of these, the first arises from the fact that the light coming from the stars moves with an inconceivable speed, but does not cross from star to Earth instantly; it takes an appreciable, even a long, time to make the journey. But the Earth is travelling round the Sun, and therefore continually changing its direction of motion, and in consequence there is an apparent change in the direction in which the star is seen. The change is very small, for though the Earth moves 18-½ miles in a second, light travels 10,000 times as fast. Stars therefore are deflected from their true positions by Aberration, by {84} an extreme amount of 20.47" of arc, that being the angle shown by an object that is slightly more distant than 10,000 times its diameter. The axis of the Earth not only rolls on itself, but it does so with a slight staggering, nodding motion, due to the attractions of the Sun and Moon, known as +Nutation+. And the axis does not remain fixed in the solid substance of the Earth, but moves about irregularly in an area of about 60 feet in diameter. The positions of the north and south poles are therefore not precisely fixed, but move, producing what is known as the +Variation of Latitude+. Then star-places have to be corrected for the effect of our own atmosphere, _i.e._ refraction, and for errors of the instruments by which their places are determined. And when all these have been allowed for, the result stands out that different stars have real movement of their own--their +Proper Motions+.