Apparent solar time or
true solar time is the hour angle of the Sun. It is based on the
apparent solar day, which is the interval between two successive returns of the Sun to the local
meridian. Note that the solar day starts at
noon, so apparent solar time 00:00 means noon and 12:00 means
midnight. Solar time can be measured by a
sundial.
The length of a solar day varies throughout the year for two reasons. First, Earth's
orbit is an
ellipse, not a
circle, so the Earth moves faster when it is nearest the Sun (
perihelion) and slower when it is farthest from the Sun (
aphelion) (see
Kepler's laws of planetary motion). Second, due to Earth's
axial tilt, the Sun moves along a
great circle (the
ecliptic) that is tilted to Earth's
celestial equator. When the Sun crosses the equator at both
equinoxes, the Sun is moving at an angle to the equator, so the projection of this tilted motion onto the equator is slower than its
mean motion; when the Sun is farthest from the equator at both
solstices,
the Sun moves parallel to the equator, so the projection of this
parallel motion onto the equator is faster than its mean motion (see
tropical year).
Consequently, apparent solar days are shorter in March (26–27) and
September (12–13) than they are in June (18–19) or December (20–21).
These dates are shifted from those of the equinoxes and solstices by
the fast/slow Sun at Earth's perihelion/aphelion.