Time with respect to the Sun; technically, the hour angle of the Sun plus 12 hours, added to make the day begin at midnight rather than noon. The true Sun is the basis of apparent solar time, as shown on a sundial, but this runs irregularly because the Earth's orbit is not circular and the Sun's path (the ecliptic) is inclined to the celestial equator. For accurate timekeeping purposes mean solar time is used, based on the hour angle of the fictitious mean sun. Solar time, which is the basis of all civil timekeeping, loses about 4 min a day against sidereal time, so that a star will rise about 4 min earlier each night.
Subjects: Astronomy and Astrophysics.