The solar time is based on the course of the day of the sun and determines the use of the bright day by humans. Before the introduction of mechanical clocks , the time of day was determined solely from the time of the sun.
In contrast to the time of day, the time of the sun does not pass quite evenly because the sun seems to run unevenly through the sky. Causes are
- the annual movement of the earth on an elliptical , d. H. non-circular orbit around the sun and
- the earth's axis that is not perpendicular to this orbit .
The solar time "goes" twice a year up to a maximum of about a quarter of an hour compared to the evenly passing time "before" or "after", which is quantitatively represented with the help of the equation of time .
The evenly passing time can also be imagined as the result of an imaginary mean sun moving evenly through the sky . This gave rise to the term mean solar time as a counterpart to the true solar time "made" by the true sun , which can also be displayed on modern sundials (see adjacent illustration).
Relation to the local time
The solar time depends on the place of observation. Noon or 12 noon is solar time when the sun passes through the meridian . Since the meridian is tied to the geographical longitude of a place, places on different degrees of longitude have different solar times that are the same on one degree of longitude. The solar time is thus a local time , analogous to the distinction given above between true local time (WOZ) and mean local time (MONT).
Relation to the zone time
The mean local time indicated by the mechanical clocks was initially identical to the mean solar time. With the formation of time zones the disturbing juxtaposition of average local times of neighboring towns were in favor of valid in broader zones time zones removed. In memory of the time difference between places at different degrees of longitude, the term local time is still in use as a synonym for zone time, but a place has become a relatively wide zone. The connection to the time of day determined by the sun has largely been lost. There are time zones through which the longitude on which the mean sun “makes” mean local time does not run. It is logical that when local time is used as a synonym for zone time, the role of the sun as a "time maker" is usually ignored.