Cycle
Related Category: Astronomy: General
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 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.
