Solar time

The fundamental unit of solar time is the day, based on the synodic rotation period.

[1] A tall pole vertically fixed in the ground casts a shadow on any sunny day.

At one moment during the day, the shadow will point exactly north or south (or disappear when and if the Sun moves directly overhead).

[2] This is "mean solar time", which is still not perfectly constant from one century to the next but is close enough for most purposes.

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

[b] The length of a solar day varies through the year, and the accumulated effect produces seasonal deviations of up to 16 minutes from the mean.

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

The sun has always been visible in the sky, and its position forms the basis of apparent solar time, the timekeeping method used in antiquity.

[19] It is not clear if they knew of the variation in the length of the solar day and the corresponding equation of time.

[20] Apparent solar time grew less useful as commerce increased and mechanical clocks improved.

On a prograde planet like the Earth , the sidereal day is shorter than the solar day . At time 1, the Sun and a certain distant star are both overhead. At time 2, the planet has rotated 360° and the distant star is overhead again (1→2 = one sidereal day). But it is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day). More simply, 1→2 is a complete rotation of the Earth , but because the revolution around the Sun affects the angle at which the Sun is seen from the Earth, 1→3 is how long it takes noon to return. [Note that in this diagram, the relative motion, and corresponding angles, are highly exaggerated for illustrative purposes.]
The Earth's orbit around the Sun, showing its eccentricity
The equation of time—above the x-axis a sundial will appear fast relative to a clock showing local mean time, and below the axis a sundial will appear slow .
Sun and Moon , Nuremberg Chronicle , 1493