A day calculated from midnight to midnight: : The civil day applies to two 12-hour periods, A.M. an P.M., but these are never used astronomically so the 24-hour clock is used.

The number of consecutive days the stand-alone system will meet a defined load without solar energy input.

This term is related to system availability.

1. Time that is measured by the stars rather than by the sun: Sidereal time is based on the sidereal day, which is the unit of measurement.

The sidereal time is a unit of time used in astronomy, equal to the period of time in which the Earth makes one rotation relative to the stars.
2. The time taken between successive meridian passages of the same star (23 hours, 56 minutes, 4.091 seconds): The sidereal day is about four minutes shorter than the twenty-four hours of the "solar day".

The interval between two successive noons: Since the Sun is moving across the sky background at a rate of about one degree per day in an easterly direction, the solar day is slightly longer than the sidereal day. Also, the Sun's rate of motion varies, and it is greatest at perihelion.

The mean solar day is equal to 24 hours, 3 minutes, 36.555 seconds.