cycle, in astronomy, period of time required for the recurrence of some celestial event. The length of a cycle may be measured relative to the sun or to the fixed stars (see sidereal time ). A frequently observed cycle is the day , during which the sun seems to circle around the earth due to the earth's rotation on its axis; although the length of the day varies, the average day is defined as exactly 24 hr of mean solar time . Another important cycle is the year , during which the earth completes an orbit of the sun. The solar year is measured from one vernal equinox to the next and is equal to 365 days, 5 hr, 48 min, 46 sec of mean solar time (see calendar ). The sidereal year, measured relative to the stars, differs in length from the solar year due to the precession of the equinoxes . The moon goes through a cycle of phases as it orbits the earth, completing a cycle from one full moon to the next in about 29 1⁄2 days, or one lunar month (see synodic period ). The moon completes an orbit of the earth relative to the stars in one sidereal month, which is about 2 days shorter than the lunar month. Every 18 years, 11 1⁄3 days the earth, moon, and sun are in very nearly the same relative positions; for this reason, solar and lunar eclipses recur in a cycle with this period. This cycle was known to the Chaldaeans (fl. 1000–540 BC) and was called the saros by them. Halley's comet reappears in a cycle whose period is about 75 years. Astronomers also make use of various other cycles, e.g., those of sunspots and variable stars.
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