Mean Solar Day. A standard measurement of "clock time" derived from the average of the annual series of true solar day sequences.
Mean Solar Time. A compensatory measurement based on the mean solar day.
Because of the Earth's motion in orbit the mean solar day is 4 minutes longer than the sidereal day and because of the Moon's daily motion in orbit the lunar day is 50 minutes longer than the solar day.
Solar Day: The complete rotation of the earth in relation to the sun. Although it varies, an average has determined a mean solar day of 24 hours. It is universally used for civil purposes.
Word of the Day: ...
The interval between two successive passages of the vernal equinoxes (q.v.) over the meridian, being about 4 minutes shorter than a mean solar day. Sidereal hour is 1/24th part of the sidereal day.
SIDEREAL LIGHT ...
the obliquity of the ecliptic, the Sun's path, the solar days are not all of equal length, but as the purposes of social and civil life require a uniform division an average is struck of all solar days in a year, and this is called a Mean Solar Day.
 See also: Sun, Moon, Solar, House, Earth
  
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