Astronomical Coordinates And Timekeeping I 561

vernal equinox to vernal equinox, remembering that the equinox moves while it is happening. A sidereal year has 365.2564 mean solar days, while a tropical year has 365.2422 mean solar days.

Our definition of the year also brings us back to a definition for a universal time, which really has a con stant rate. We base ephemeris time on the rate of the mean Sun at the beginning of the year 1900. Ephemeris time is a certain fraction of the tropical year 1900 (ephemeris year), which contains 365.242 199 mean solar days, so the ephemeris second is defined as 1/31 55 6 9 2 5 974 of an ephemeris year.

Appendix G

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