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A Solar Day is the period of time it takes the Sun to move from any certain meridian of longitude till it returns to the same meridian the next day. Owing to the variable motion of the earth in its orbit, and 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. It commences at midnight when the Sun is at the nadir. Clocks are regulated to show its beginning and end, also its equal divisions into twenty-four hours. There is thus a difference between sun-time and clock-time.

From the time when the Sun is nearest to the earth (perihelion) December 24th, to the time when it is farthest from the earth (aphelion) June 21st, clock-time is in advance of sun-time. From June 21st to December 24th, the Sun is in advance of the clock, the greatest difference being 16 minutes in the beginning of November.

When the unequal motion of the earth in its orbit and the obliquity of the ecliptic act together, the difference between sun-time and clock-time is greatest but four times a year, April 15th, June 15th, September 1st and December 24th, they agree.

A Sidereal Day is the time which elapses between a fixed star’s leaving a eertain degree of longitude