Day

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Day

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1. The duration of one rotation of the earth, or another celestial body, on its axis. </dd>
A day is measured by successive transits of a reference point on the celestial sphere over the meridian, and each type takes its name from the reference used. Thus, for a solar day the reference is the sun; a mean solar day if the mean sun; and an apparent solar day if the apparent sun. For a lunar day the reference is the moon; for sidereal day the vernal equinox; for a constituent day an astre fictif or fictitious star. The expression lunar day refers also to the duration of one rotation of the moon with respect to the sun. A Julian day is the consecutive number of each day, beginning with January 1, 4713 BC. </dd>
2. A period of 24 hours beginning at a specified time, as the civil day beginning at midnight, or the astronomical day beginning at noon. </dd>

References

This article is based on NASA's Dictionary of Technical Terms for Aerospace Use