Skip to main content

synodic period

synodic period (sĬnŏd´Ĭk), 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 BC 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.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"synodic period." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. 23 Oct. 2017 <http://www.encyclopedia.com>.

"synodic period." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (October 23, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/synodic-period

"synodic period." The Columbia Encyclopedia, 6th ed.. . Retrieved October 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/synodic-period

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.