See solar time for more information on the difference to the apparent solar day.

It is based on the apparent solar day, the interval between two successive returns of the Sun to the local meridian.

As a consequence hours varied a little, as the length of an apparent solar day varies throughout the year.

An apparent solar day can be 20 seconds shorter or 30 seconds longer than a mean solar day.

So apparent solar days are shorter in March and September than in June or December.

One twenty-fourth of the apparent solar day (between one noon and the next, or between one sunset and the next).

Earth's rotation period relative to the Sun (true noon to true noon) is its true solar day or apparent solar day.

The duration of daylight varies during the year but the length of a mean solar day is nearly constant, unlike that of an apparent solar day.

Before the middle of the 1st millennium BC, the water clocks were only adjusted to agree with the apparent solar day, thus were no better than the shadow cast by a gnomon (a vertical pole), except that they could be used at night.

Because the Earth's orbit around the sun is elliptical, and because of the obliquity of the Earth's axis relative to the plane of the orbit (the ecliptic), the apparent solar day varies a few dozen seconds above or below the mean value of 24 hours.