Page:Practical astronomy (1902, John Wiley & Sons).djvu/28

10 PRACTICAL ASTRONOMY. obliquity, precession, and sun's mean horizontal parallax for the year, are found on page 278 of the Ephemeris. All these consti- tute an Ephemeris of the Sun. From the hourly changes the elements for any meridian can be readily computed. THE EPHEMEKIS OF THE MOON. The Ephemeris of the Moon consists of tables giving the Moon's Right Ascension and Declination for every hour of Greenwich mean time, with the changes for each minute; the Apparent Semi-diameter, Horizontal Parallax, Time of upper transit on the Greenwich Meridian, and Moon's Age. In order to compute these, it is first necessary to find the True Longitude of the Moon, its True Latitude, the Longitude of the Moon's Node, the Inclination of the Moon's Orbit to the Ecliptic, and the Longitude of Perigee. l c The Elements of the Lunar Orbit. Let DC be the intersection of the celestial sphere by the plane of the lunar orbit; VB the FIG. 3. ecliptic, and VA the equinoctial ; V the mean vernal equinox, N the ascending node, P the Perigee, all relating to some assumed epoch. Also let M l , M z , M. B , J/ 4 , be the geocentric places of the moon's center at the four times, t l9 t z ,t 3 , # 4 . These places are