Sidereal Month Versus Lunar Month (Synodic)

The sidereal month and synodic month are both based on the lunar cycle.
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The words "month" and "moon" are cognates of each other. The Julian and Gregorian calendars have twelve months with 28 to 31 days, yet they are roughly based on the cycle of the Moon or lunar month. The lunar month is still used in many cultures and by astronomers and other scientists. However, there are multiple ways of defining what, exactly, constitutes a month using the Moon.

Synodic Lunar Month

Usually, when someone refers to a lunar month, they mean the synodic month. This is the lunar month defined by the visible phases of the Moon. The month is the time between two syzygies, which means it's the length of time between successive full moons or new moons. Whether this type of lunar month is based on the full moon or new moon varies according to the culture. The lunar phase depends on the Moon's appearance, which in turn is related to its position with respect to the Sun as viewed from Earth. The Moon's orbit is elliptical rather than perfectly round, so the length of a lunar moon varies, ranging from 29.18 days to 29.93 days and averaging 29 days, 12 hours, 44 minutes, and 2.8 seconds. The synodic lunar month is used to calculate lunar and solar eclipses.

Sidereal Month

The sidereal lunar month is defined according to the Moon's orbit with respect to the celestial sphere. It is the length of time for the Moon to return to the same position with respect to the fixed stars. The length of the sidereal month is 27.321 days or 27 days, 7 hours, 43 minutes, 11.5 seconds. Using this type of month, the sky can be divided into 27 or 28 lunar mansions, which feature specific stars or constellations. The sidereal month is used in China, India, and the Middle East.

Although the synodic and sidereal months are most common, there are other ways of defining lunar months:

Tropical Month

The tropical month is based on the vernal equinox. Because of the Earth's precession, the Moon takes slightly less time to return to an ecliptic longitude of zero than to return to the same point with respect to the celestial sphere, yielding a tropical month of 27.321 days (27 days, 7 hours, 43 minutes, 4.7 seconds).

Draconic Month

The draconic month is also called the draconitic month or the nodical month. The name refers to a mythical dragon, which lives at the nodes where the plane of the lunar orbit intersects the plane of the ecliptic. The dragon eats the sun or moon during eclipses, which occur when the Moon is near a node. The draconic month is the average length of time between successive transits of the Moon through the same node. The plane of the lunar orbit gradually rotates westward, so the nodes slowly rotate around the Earth. A draconic month is shorter than a sidereal month, with an average length of 27.212 days (27 days, 5 hours, 5 minutes, 35.8 seconds).

Anomalistic Month

Both the orientation of the Moon in its orbit and the shape of the orbit change. Because of this, the diameter of the Moon changes, depending mainly on how close to perigee and apogee it is (the apsides). The Moon takes longer to return to the same apsis because it moves ahead one revolution, defining the anomalistic month. This month averages 27.554 days. The anomalistic month is used together with the synodic month to predict whether a solar eclipse will be total or annular. The anomalistic month may also be used to predict how large the full moon will be.

Length of Lunar Month in Days

Here's a quick comparison of the average length of different types of lunar months. For this table, a "day" is defined as 86,400 seconds. Days, like lunar months, may be defined in different ways.

Lunar Month Length in Days
anomalistic 27.554 days
draconic 27.212 days
sidereal 27.321 days
synodic 29.530 days
tropical 27.321 days