ecliptic and the latitude of the sun is nothing. We will now describe the details of the instrument. There is first a large circle, E, B, C, Fig. 9 (which is taken from a drawing of this kind of instrument as constructed subsequently by Tycho Brahe), fixed in the plane of the meridian, having its poles, D, C, pointing to the poles of the heavens; inside this there is another circle, F, I, H, turning on the pivots D, C, and carrying fixed to it the circle, O, P, arranged in a plane at right angles to the points I, K, which are placed at a distance from C and D equal to the obliquity of the ecliptic; so that I and K represent the poles of the ecliptic, and the circle, O, P, the ecliptic itself. There is then another circle, R, M, turning on the pivots I and K, representing a meridian of latitude, and along which it is measured. Then, as the sun is on that part of the ecliptic nearest the north pole, in summer, its position is represented by the point F on the ecliptic, and by N at the winter solstice; so, knowing the time of the year, the sight Q can be placed the same number of degrees from F as the sun is from the solstice, or in a similar position on the circle O P as the sun occupies on the ecliptic. The circle can then be turned round the axis C, D, till the sight Q, and the sight opposite to it, Q´, are in line with the sun. The circle, O, R, will then be in the plane of the ecliptic, or of the path of the earth round the sun. The circle, R, M, is then turned on its axis, I, K, and the sights, R, R, moved until they point to the moon. The distance Q, L, measured along O, P, will then be the difference in longitude of the moon and sun, and its latitude, L, R, measured along the circle R, M. But why should he use the moon? His object was to determine the longitude of the stars, but his only method was to refer to the motion of the sun, which could be laid down in tables, so that its longitude or distance from the vernal equinox was always known. But we do not see the stars and the sun at the same time; therefore in the day time, while the moon was above the horizon, he determined the difference of longitude between the sun and the moon, the longitude of the sun or its distance from the vernal equinox being known by the time of the year; and after the sun had set he determined the difference of longitude between the moon and any particular star; and so he got a fair representation of the longitude of the stars, and succeeded in tabulating the position of 1,022 of them. It is to the use of this instrument that we owe the discovery of the precession of the equinoxes. [Illustration: FIG. 10.—Diagram Illustrating the Precession of the Equinoxes. ]