Page:Practical astronomy (1902, John Wiley & Sons).djvu/19



Ephemeris.&mdash;The numerical values of the coördinates of the principal celestial bodies, together with the elements of position of the circles of reference, are recorded for given equidistant instants of time in an Astronomical Ephemeris.

The "American Ephemeris and Nautical Almanac" is published by the United States Government, generally three years in advance of the year of its title, and comprises three parts, viz.:

Part I.&mdash;Ephemeris for the Meridian of Greenwich, which gives the heliocentric and geocentric positions of the major planets, the ephemeris of the sun, and other fundamental astronomical data for equidistant intervals of mean Greenwich time.

Part II.&mdash;Ephemeris for the Meridian of Washington, which gives the ephemerides of certain fixed stars, sun, moon, and major planets, for transit over the meridian of Washington, and also the mean places of the fixed stars, with the data for their reduction.

Part III.&mdash;Phenomena, which contains prediction of phenomena to be observed, with data for their computation.

To construct the ephemeris of the sun it is necessary to compute its tables: these are
 * 1) The table of Epochs.
 * 2) The table of Longitudes of Perigee.
 * 3) The table of Equations of the Center, and its corrections.
 * 4) The table of the Equations of the Equinoxes in Longitude.