. Astronomy for high schools and colleges . de at different points on the earths surface arecompared by reducing them all to the centre of the earth. We may also suppose the point G to be the sun and thecircle 8 S to be the earths orbit around it. The paral-lax will then be the difference between the directions ofthe body as seen from the earth and from the sun. Thisis termed the annual j[)arallax, because, owing to the an-nual revolution of the earth, it goes through its periodin a year, always supposing the body observed to be atrest. (3.) A yet more restricted parallax is the horizontal PAR
. Astronomy for high schools and colleges . de at different points on the earths surface arecompared by reducing them all to the centre of the earth. We may also suppose the point G to be the sun and thecircle 8 S to be the earths orbit around it. The paral-lax will then be the difference between the directions ofthe body as seen from the earth and from the sun. Thisis termed the annual j[)arallax, because, owing to the an-nual revolution of the earth, it goes through its periodin a year, always supposing the body observed to be atrest. (3.) A yet more restricted parallax is the horizontal PARALLAX. 51 jpa/rallax of a heavenly body. The parallax first describedin the last paragraph varies with the position of the ob-server on the surface of the earth, and has its greatestvalue when the body is seen in the horizon of the ob-server, as may be seen by an inspection of Fig. 19, inwhicli the angle GPS attains its maximum when the UneP S i^ tangent to the earths surface, in which case Pwill appear in the horizon of the observer at HORIZONTAL PARALLAX. The horizontal parallax depends upon the distance of abody in the following manner: In tlie triangle OPS,right-angled at S, we have 08= CP sin CPS. If, then, we put p, the radius of the earth C S; r, the distance of the body P from the centre of theearth ; TT, the angle S P C, or the horizontal parallax,we shall have, P p = r sm TT; r = — sm 7t Since the earth is not perfectly spherical, the quantity pis not absolutely constant for all parts of the earth, and itsgreatest value is usually taken as that to which the hori-zontal value shall be referred. This greatest value is, aswe shall hereafter see, the radius of the equator, and the 52 ASTRONOMY. corresponding value of the parallax is therefore called theequatorial horizontal parallax. When the distance r of the body is known, the equa-torial horizontal parallax can be found by the first of theabove equations ; when the parallax can be observed, thedistance r is found
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