Phases of the Moon


The Moon changes in position on the celestial sphere each day. It also changes in appearance from a full Moon to a thin crescent. These changes are called phases.

The Moon's light comes from the Sun, and the sunlight is reflected off the Moon's surface.

The phase of the Moon that we see depends on the orientation of the Earth and Moon relative to the Sun.

A. Sidereal period of the Moon

The Moon revolves around the Earth in the same direction that the Earth rotates.

To circle the Earth once (relative to the stars) takes 27.32 days. This is called the SIDEREAL PERIOD of the Moon (or the sidereal month).

The Moon rises in the east and sets in the west daily, but its position in the sky moves EASTWARD by about 13 degrees per day.

(360 degrees divided by 27.32 days = 13.177 degrees per day)

This means that if the moon is on the meridian next to a star, at the same time the next evening, the moon will be 13 degrees east of the star (or east of the meridian).

(Note that the stars are moving too, rising almost 4 minutes earlier each day. The stars vary night to night by less than one degree, only 0.986 degrees.).

The Earth turns at 15 degrees per hour, so it takes less than 1 hour to turn 13 degrees and return the Moon to the same spot.

The Moon rises and sets almost 1 hour later each night.

The same side of the Moon always faces the Earth. WHY?

The Moon's orbital period is equal to its rotational period.

In other words, the Moon turns on its axis at the same rate as it revoles around the Earth.

What is it like on the back side of the Moon?

It is NOT always dark there. The backs ide of the Moon is only fully dark when we are seeing a full moon from Earth. The back side has phases just like the side facing Earth. When the Moon is at the phase called "new", it is fully lit on the back side.

Note: As seen from the Moon, the Earth also goes through phases.

B. Phases of the Moon

The length of time from New Moon to New Moon is called the LUNAR MONTH or SYNODIC PERIOD of the Moon. It is 29.53 days.

The Moon takes 27.32 days to orbit the Earth (with respect to the stars), but it takes LONGER (29.53 days) to go through a cycle of phases. WHY?

This is because the Earth-Moon system has moved around the sun by about 27 degrees over the course of the month. The Moon will have gone around the Earth once with respect to the stars, but it needs to travel further to line back up the same way with the sun.


Be able to draw a sketch of each of these phases.

Know the orientation of the Earth-Moon-Sun system for each phase.

Know approximate rising, meridian, and setting times for each phase (assume sunrise is at 6 am and sunset is at 6 pm).

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This page created by Pamela J. W. Gore
January 22, 1996