one day of solar time, the time between successive noons; the time in which a planet such as Earth apparently rotates once around its axis, relative to the sun (on Earth about 24 hours as an average - the length of solar day is not constant throughout the year)
The rotational period of Earth; the time in which Earth rotates exactly 360 degrees around its axis, measured relative to the stars (about 23 hours, 56 minutes, 4 seconds).