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The sidereal day is..
  • A
    a means to find apparent time by applying the equation of time.
  • B
    of constant duration.
  • C
    shorter in winter than in summer.
  • D
    introduced because of GPS.


The apparent solar day is measured against the real or apparent Sun (the one that "appears" to you) and it is defined as the time between two successive transits of the Sun over the same meridian. It is not of constant length.

  • This interval is about four minutes longer than the sidereal day, largely because of the sun's apparent annual motion eastward along the ecliptic (actually, the earth's "westward" motion along its orbit), which motion delays the sun's return to meridional transit.

The mean solar day is the averaged length of an apparent solar day over the year. It is of constant length and related to light and darkness. Used as the civil day, being divided into hours, minutes and seconds of "mean" time.

A sidereal day is measured against a distant star and is of nearly constant length.

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