synodic period [key], in astronomy, length of time during which a body in the solar system makes one orbit of the sun relative to the earth, i.e., returns to the same elongation. Because the earth moves in its own orbit, the synodic period differs from the sidereal period, which is measured relative to the stars. The synodic period of the moon, which is called the lunar month, or lunation, is 291⁄2 days long; it is longer than the sidereal month. The moon is full when it is at opposition. One sidereal month later it will not yet be full, since it must travel further in its orbit around the earth to reach the point of opposition, which has moved relative to the stars because of the earth's motion. Since the calendar month is not equal to the lunar month, the full moon does not occur on the same day every month. The length of time between recurrences of the full moon on the same date is 235 lunar months, or 19 years. This period, called the Metonic cycle, was discovered by the Greek astronomer Meton in 433 b.c. It is used in determining the date of Easter in the Gregorian calendar and was used in placing the intercalary month in the ancient Greek calendar. For the inferior planets the synodic period is longer than the sidereal period, but for the superior planets it is shorter.
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