Difference Of Days. There is a vast difference in the length of days. A sidereal day is the real and invariable period of the diurnal rotation of the earth on its axis, and contains 23 hours, 56 minutes, 3 5 seconds, of mean solar time ; the lunar day is 24 hours, 48 minutes; and the solar day, which is the mean apparent time of one revolution of the earth on its axis, is 24 hours.