Time is measured by observing the position of the heavenly bodies. Units of time are periods, that is intervals of time, after the lapse of which the same order of things returns. In astronomy the fundamental, invariable unit of time is the sidereal day i.e. the period, during which the earth revolves once about its axis, measured by help of two successive returns of any fixed star to the meridian, the motion of the fixed stars depending only upon the daily rotation of the earth about its axis and not, like that of the sun and its satellites, upon the progressive advance of the earth in its orbit round the sun.
The position of the sun, moon and planets is determined by comparing them with that of the fixed stars and thus the mean solar day is fixed by help of the sidereal day. The mean solar day in the average of all the solar days throughout the year and exceeds the sidereal day by an average difference of four minutes. Since the solar days are not equal in length, the sun's motion in its apparent orbit and distance from the equator being different in different seasons, the apparent or real solar day does not exactly coincide with the mean solar day.
The duration of the longest apparent (real) solar day is 24h 0m 30s mean time, and that of the shortest 23h 59m 39s mean time; in consequence of all this the mean and apparent solar days commence only 4 times a year (about 14 April, 14 June, 31 August and 23 December) at the same absolute moment, the small differences between the mean and apparent solar day producing, by summation from day to day, that both kinds of days begin at different absolute moments during the whole year, except only on the above mentioned 4 days.
This difference between the commencement of the mean and apparent day or between mean and apparent time is called the Equation of time and its absolute greatest value is 16 minutes.
In the Calendar of all nations without exception, the fundamental unit is the solar day, because the sun is the source of all forces displayed on our planet, whether mechanical, chemical or vital. All other periods of time or so called cycles, applied as units of time, for instance the week, the synodical month, the tropical year, the cycles of 60 years, 60 days etc. are composed of the solar day and its subdivisions.
For this reason we will next occupy ourselves with the cycles, used
