Earth’s period of orbital revolution around the Sun with reference to the fixed stars is called the sidereal year: 356.25636 days. However, the positions of the stars are gradually changing because of precession. The solar or tropical year is measured between successive passages through the mean equinox, which takes account of precession and is equal to 365.24219 days. This is the most commonly adopted definition of the year as it is the one that relates directly to seasonal changes.
The Moon keeps the same face towards Earth because its sidereal period of axial rotation of 27.322 days is the same as its orbital period. The lunar or synodic month, i.e. the mean period between complete successive occurrences of identical lunar phases, is 29.53059 days.
Calendars of the world have usually been set by the Sun and Moon, measuring the day, month and year. As the year (≈365¼ days) and the month (≈29½ days) are not commensurate, early astronomers found it necessary to assign different numbers of days to successive months or years to make their calendar year correspond with the solar year.
In ancient times people thought that celestial phenomena, especially the planetary motions, were related to their own destinies. This belief, astrology, encouraged the development of mathematical schemes for predicting planetary motions and this helped to advance the science of astronomy.
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