synodic period (sĭnŏd`ĭk), in astronomy, length of time during which a body in the solar system makes one orbit orbit, in astronomy, path in space described by a body revolving about a second body where the motion of the orbiting bodies is dominated by their mutual gravitational attraction.
..... Click the link for more information. of the sun relative to the earth, i.e., returns to the same elongation elongation, in astronomy, the angular distance between two points in the sky as measured from a third point. The elongation of a planet is usually measured as the angular distance from the sun to the planet as measured from the earth.
..... Click the link for more information. . 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 29 1-2 days long; it is longer than the sidereal month. The moon is full when it is at opposition opposition, in astronomy, alignment of two celestial bodies on opposite sides of the sky as viewed from earth. Opposition of the moon or planets is often determined in reference to the sun.
..... Click the link for more information. . 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 calendar [Lat., from Kalends], system of reckoning time for the practical purpose of recording past events and calculating dates for future plans. The calendar is based on noting ordinary and easily observable natural events, the cycle of the sun through the seasons
..... Click the link for more information. and was used in placing the intercalary month in the ancient Greek calendar. For the inferior planets inferior planet, planet whose orbit lies inside that of the earth. There are two inferior planets, Mercury and Venus. They always seem to be close to the sun in the sky; the greatest elongation of Mercury is 28°, and that of Venus, 47°.
..... Click the link for more information. the synodic period is longer than the sidereal period, but for the superior planets superior planet, planet whose orbit lies outside that of the earth. The superior planets are Mars, Jupiter, Saturn, Uranus, and Neptune.
..... Click the link for more information. it is shorter.

synodic period

Time required for a body in the solar system to return to the same or about the same position relative to the Sun as seen from Earth. The Moon's synodic period is the time between successive recurrences of the same phase (e.g., the period between one full moon and the next). A planet's synodic period is the time required for Earth to overtake it as both go around the Sun or (in the case of fast-moving Mercury or Venus) for the other planet to overtake Earth. See also sidereal period.

synodic period [sə′näd·ik ′pir·ē·əd]

(astronomy)

The time period between two successive astronomical conjunctions of the same celestial objects.