Page:Practical astronomy (1902, John Wiley & Sons).djvu/31

 EPHEME1US. 13 with its proper sign to the mean orbit longitude gives the true orbit longitude, after reduction to true equinox of date. The Right Ascension and Declination of the Moon can now be computed for any instant of time, thus : subtract the longitude of the node from the orbit longitude of the moon, and we have the moon's angular distance from her node, represented in the figure by X M l. This, with the inclination i, will give us the moon's latitud e and the angular distance JV 0^ the latter added to the longitude of the node will give the moon's longitude FO,. The latitude, longitude, and obliquity of the ecliptic suffice to compute the right ascension and declination. The radius vector, equatorial horizontal parallax, apparent diameter, etc., are computed as in the case of the sun. THE EPHEMEEIS OF A PLANET. From the tables of a planet its true orbit longitude as seen from the sun is found, as in the case of the moon as seen from the earth. The heliocentric longitude and latitude, and the radius vector are found from the heliocentric orbit longitude, heliocentric longitude of the node, and inclination, in the same way as the geocentric elements of the moon are found from similar data in the lunar orbit. To pass from heliocentric to geocentric coordinates, let P, Fig. 4, be the planet's center, E that of the earth, S that of the sun, and FIG. 4. the projection of P on the plane of the ecliptic. S V and E V aro drawn to the vernal equinox; then let