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Time required for a celestial body to turn once on its axis; especially, the period of the Earth's rotation.
The sidereal day (see sidereal period ) is the time required for the Earth to rotate once relative to the background of the stars (i.e., the time between two observed passages of a star over the same meridian of longitude). The apparent solar day is the time between two successive transits of the Sun over the same meridian. Because the orbital motion of the Earth makes the Sun seem to move slightly eastward each day relative to the stars, the solar day is about four minutes longer than the sidereal day. The mean solar day is the average value of the solar day, which changes slightly in length during the year as the Earth's speed in its orbit varies.
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[c mediumvioletred] (as used in expressions)
All Saints' Day
All Souls' Day
April Fools' Day
All Fools' Day
Canada Day
Dominion Day
D Day
Day Doris
Day Dorothy
Day Lewis Cecil
Groundhog Day
Hershey Alfred Day
Independence Day
John Day Fossil Beds National Monument
Judgment Day of
Labor Day
May Day
Memorial Day
Decoration Day
Mother's Day and Father's Day
New Year's Day
O'Connor Sandra Day
Sandra Day
Saint Bartholomew's Day Massacre of
Six Day War
Thanksgiving Day
Valentine's Day
Veterans Day
Day of Atonement
Reorganized Church of Jesus Christ of Latter Day Saints
{{link=Holocaust Remembrance days">Holocaust Remembrance days
Hundred Days
July Days
June Days
Seven Days' Battles