Page:Practical astronomy (1902, John Wiley & Sons).djvu/27

 EPHEMERIS. 9 9. Equation of Time. If, at the instant when the true sun's mean place coincides with the mean equinox, an imaginary point should leave the reduced place of the mean equinox and travel with uniform motion on the celestial equator, returning to its starting- point at the instant the true sun's mean place next again coincides with the mean equinox, such a point is called a Mean Sun. Time measured by the hour angles of this point is called Mean Solar rime. The angle included between the declination circles passing through the centre of the true sun and this point at any instant is called the Equation of Time for that instant ; its value, at any in- stant, added algebraically to mean or apparent solar time will give the other. As the apparent time can be found by direct observa- tion the equation of time is usually employed as a correction to pass from apparent to mean solar time. Thus in Fig. 2, P M is the me- ridian, S the true sun, S' its mean place, S" the mean sun, VS'" the true R. A. of the true sun, V"S" the mean R. A. of the mean sun = VS' = sun's mean longitude, angle MPS f " or arc MS'" apparent solar time, MS" mean solar time, and S"S'" the Equation of Time = VS'"-(VS" + W"). Hence we have for the Equation of Time, e = True sun's true Right Ascension - (sun's mean longitude-f- equation of equinoxes in R. A.). (17) The mean sun (S") moving in the equator and used in connec- tion with time, must not be confused with the mean sun (S') before referred to, moving in the ecliptic. 10. Referring to the American Ephemeris, we see that Page I of each month contains the Sun's Apparent R. A., Declination, Semi-diameter, Sidereal time of semi-diameter passing the me- ridian, at Greenwich apparent noon, together with the values for their respective hourly changes; the latter being computed from the values of their differential co-efficients. From these we can find the corresponding data for any other meridian. Page II con- tains similar data for the epoch of Greenwich mean moon, and in addition the sidereal time or R. A. of the mean sun. Page III con- tains the sun's true longitude and latitude, the logarithm of the earth's radius vector and the mean time of sidereal noon. The