PARALLAX

(Gr. irrrpa'Walis, change.) A change of place or of aspect. The term is used in astronomy to denote the difference between the apparent place of a celestial object and its true place, or that in which it would be seen if the observer were placed at the centre to which the motion is referred. When the point of reference is the centre of the earth, the change or aspect is called the diurnal parallax; when it is the centre of the earth's orbit, the change is called the annual parallax. Diurnal parallax.— Let C be the centre of the earth, A the place of the observer, Z his zenith, and S a celestial body. On observing the body from A, it will be seen in the direction A S, making the.ingle Z A S with the zenith. But if the observer could be placed at C, he would see the body in the direction C S, making the angle Z C S with the. zenith. The difference between these two angles Z A S and Z C S is the parallax of S, which, therefore, is equal to the angle A S C. Hence it appears that the parallax of a celestial body is the angle comprised | between two lines drawn from the body, the one to the I centre of the earth, and the other to a point on its surface. On account of the immense distance of the fixed stars, the ' diurnal parallax is altogether insensible with regard to I them. It may amount to a degree in respect of the moon; 1 but the greatest parallax of the nearest planet does not exIt is evident, from the inspection of the figure, that although the distanceceed 30". of the object S remain the same, the angle A S C is not a constant quantity, but is greatest when S is seen in the direction of the horizon A H, and diminishes as the altitude of S increases, until it vanishes altogether at the zenith, where the two lines A S and C S merge into the line C Z. In order to discover the law of this varia tion, let a = C A, the semidiameter of the earth; <f=CS, the distance of the observed object from the centre; Z = Z A S, the apparent zenith distance; and P=ASC, th« [s. 904]