The sidereal month.
- Based on the moon's position in relation to the stars.
- 27 days, 7 hours, 43 mins, 11.5 seconds. long.
A synodic month.
- Which is the average period of moon's journey with respect to the sun as seen from earth.
- 29 days, 12 hours, 44 mins, 2.9 seconds.
- The synodic month is used to calculate eclipse cycles.
The Draconitic month
- The time it takes the moon to cross the ecliptic, ascending- drop down and cross (descending, and then cross the ecliptic once more, ascending.
- A Draconitic month is slightly shorter than the sidereal month, 27 days 5 hours, 5 mins, 35.8 seconds.
- The plane of the moon's orbit precesses over a full circle in about 18.6 years.
An anomalistic month.
- Because- the orbit of the moon around the earth is an ellipse not a circle.
- An anomalistic month is slightly longer than the other months.
- The position of the extreme point, when it is furthest away (the line of apsides: perigee and apogee) makes one complete circle (lunar precession) in about 9 years.
- The longer month has an average of 27 d 13 h 18 min 33.2 s.
- This affects the size of the moon as seen from earth, and when eclipses take place.
Finally, the tropical month
- It is customary to specify positions of celestial bodies with respect to the vernal equinox.
- Because of precession, this point moves back slowly along the ecliptic.
- Therefore it takes the Moon less time to return to an ecliptic longitude of zero than to the same point amidst the fixed stars: 27.321582 days (27 d 7 h 43 min 4.7 s).
- This slightly shorter period is known as tropical month.
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