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The period between two successive transits of a particular meridian on the earth by the first point of the Aries or the successive transit of vernal equinox. Since the first point of Aries is not fixed in space, and it precesses along the equinoctial 11.26 s of the arc each year, the sidereal day measured relative to Aries is minutely shorter than that measured relative to a fixed point in space. These two days are known as practical and theoretical sidereal days, respectively. An equinoctial day is the time over the upper branch of any meridian and is equal to 24 h of mean sidereal time or 23h 56m4.09054s of mean solar time. Also called a sidereal day.