Assignment 1

The length of the month is determined by the regular variation in the appearance of the Moon, i.e., by the changing phase of the Moon. The changing of the phases of the Moon defines the Synodic Month . The different phases of the Moon are caused by changes in the relative orientation of the Earth, the Moon, and the Sun caused by the orbital motion of the Moon about the Earth. The Moon orbits in the west-to-east direction about the Earth, that is, the Moon orbits about the Earth in the CCW sense as viewed from the NCP.

The Moon's orbital plane is inclined at angle of ~5 degress to the ecliptic plane (i.e., it is nearly co-planar with the orbit of the Earth), and the Moon's orbital period about the Earth (the sidereal revolution period which defines the sidereal month) is 27.3 days. The synodic period (the period of the lunar phases) is 29.5 days.

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The month based on the phases of the Moon (the synodic month) is longer than the true month, the sidereal month. Why is this so?

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A star rises around 4 minutes earlier from night to night. (Why?) Does the Moon rise later or earlier from night to night? Explain.