The period of the earth's revolution on its axis. --
   ordinarily divided into twenty-four hours. It is measured by the
   interval between two successive transits of a celestial body over the
   same meridian, and takes a specific name from that of the body. Thus,
   if this is the sun, the day (the interval between two successive
   transits of the sun's center over the same meridian) is called a solar
   day; if it is a star, a sidereal day; if it is the moon, a lunar day.
   See Civil day, Sidereal day, below.