The basis of the civil time account is the average sunny day.

Greenwich time is the mean solar time of Greenwich.

The so-called ”solar constant” is, in fact, not constant.

Sidereal time is 3 mins 56 seconds faster per day than mean solar time.

The sidereal day is 3 minutes 56 seconds shorter than the solar day.

A sidereal day is shorter than a solar day by 3 min 56 seconds.

The mean solar day is the period used in calendar computation.

To remind you: The duration of the Thalassan Mean Solar Day is 29.4325 hours ET.

The time denoted as UT0 is sundial time at the prime meridian, corrected for the difference between the real sun and the mean sun (up to +/- 15 min).

A sidereal day is shorter than a solar day by 3 min 56 seconds.

The Earth, for example, has a solar day average of 24 hours and a sidereal day of 23h 56min 4.09s.

Keep in mind that the Hebrew 24 hour day begins at sunset, whereas our new day begins at midnight.

It is counted from Greenwich midnight and its unit is mean solar day.

UTC is thus the modern successor of Greenwich Mean Time, GMT, which was used when the unit of time was the mean solar day.

This “mean solar day” was determined by astronomers from observations of the passage of stars through the sky.