The triangle _BAC_ is right-angled at _A_; the side _AC_ is the radius of the earth, and the hypothenuse is the distance of the body from the centre of the earth. When the parallax _ABC_ is known, the length of _CB_ can easily by found by trigonometrical computation. We have seen (32) that the parallax of a heavenly body grows less and less as the body passes from the horizon towards the zenith. The parallax of a body and its altitude are, however, so related, that, when we know the parallax at any altitude, we can readily compute the horizontal parallax. The usual method of finding the parallax of one of the nearer heavenly bodies is first to find its parallax when on the meridian, as seen from two places on the earth which differ considerably in latitude: then to calculate what would be the parallax of the body as seen from one of these places and the centre of the earth: and then finally to calculate what would be the parallax were the body on the horizon. [Illustration: Fig. 102.] Thus, we should ascertain the parallax of the body _B_ (Fig. 102) as seen from _A_ and _D_, or the angle _ABD_. We should then calculate its parallax as seen from _A_ and _C_, or the angle _ABC_. Finally we should calculate what its parallax would be were the body on the horizon, or the angle _AB'C_. The simplest method of finding the parallax of a body _B_ (Fig. 102) as seen from the two points _A_ and _D_ is to compare its direction at each point with that of the same fixed star near the body. The star is so distant, that it will be seen in the same direction from both points: hence, if the direction of the body differs from that of the star 2° as seen from one point, and 2° 6' as seen from the other point, the two lines _AB_ and _DB_ must differ in direction by 6'; in other words, the angle _ABD_ would be 6'. The method just described is the usual method of finding the parallax of the moon. 88. _The Apparent Size of the Moon._--The _apparent size_ of a body is the visual angle subtended by it; that is, the angle formed by two lines drawn from the eye to two opposite points on the outline of the body. The apparent size of a body depends upon both its _magnitude_ and its _distance_. The apparent size, or _angular diameter_, of the moon is about thirty-one minutes. This is ascertained by means of the wire micrometer already described (19). The instrument is adjusted so that its longitudinal wire shall pass through the centre of the moon, and its transverse wires shall be tangent to the limbs of the moon on each side, at the point where they are cut by the longitudinal wire. The micrometer screw is then turned till the wires are brought together. The number of turns of the screw needed to accomplish this will indicate the arc between the wires, or the angular diameter of the moon. [Illustration: Fig. 103.]