The day as used in tables of ephemerides, generally speaking, identical to the astronomical day. Prior to 1 January 1925, the astronomical day began at noon instead of midnight. Publications like The American Ephemeris gave ephemerides for 0 hour, meaning noon. In publications issued after 1924, the ephermide is still given for 0 hour, but midnight is meant.
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Last revised: 4 May 2008.