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 291/2 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, 111/3 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 B.C.) 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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