• Extract from Wikipedia: There are several ways to consider how much time it takes the Moon to complete one orbit. The sidereal month is the time it takes to make one complete orbit with respect to the fixed stars, which is about 27.3 days. In contrast, the synodic month is the time it takes the Moon to reach the same phase, which takes about 29.5 days. The synodic period is longer than the sidereal period because the Earth-Moon system moves a finite distance in its orbit around the Sun during each sidereal month, and a longer time is required to achieve the same relative geometry. Other definitions for the duration of a lunar month include the time it takes to go from perigee to perigee (the anomalistic month), from ascending node to ascending node (the Draconitic month), and from two successive passes of the same ecliptic longitude (the Tropical month). As a result of the slow precession of the lunar orbit, these latter three periods are only slightly different than the sidereal month. The average length of a calendric month (1/12 of a year) is about 30.4 days. - I am interested to see the other answers to this question. :)
  • Because the Earth moved around the sun. If the Earth was stationary they'd be the same. I wonder if I can get one figure from the other... 27 1/3 is the time it takes for the moon to revolve round to the same point in the sky and would be the time for phases if the Earth stayed still. The moon's phases take a little longer because after 27 1/3 days from a new moon when the Sun, Moon and Earth were in alignment the Earth has moved around the Sun. The Moon is still in the same position in the sky but (relative to the Earth) the Sun moved! The Earth is moving at one orbit per 365.25 days approx. The Moon at one per 27 1/3 I need a number of days from the New Moon so that the extra fraction of the Moon's orbit that the Moon will have to turn is equal to the fraction of the Earth's orbit i.e. d / 27.333 - 1 = d / 365.25 13.363 d - 365.25 = d 12.363 d = 365.25 d = 29.5 days!

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