lew (11/08/82)
Actually, there are two definitions of the sidereal day. The "true sidereal day", which is what it sounds like, and the "equinoctial sidereal day", which is the time between transits of the first point of Aries. These are different because of the precession of the equinoxes. Perhaps surprisingly, the "equinoctial sidereal day" is the default definition for the unspecified "sidereal day". Reference - "Norton's Star Atlas" Lew Mammel, Jr. ihuxr!lew