Lecture 8: The Phases of the Moon

Key Ideas:

The Moon always keeps the same face towards the Earth. Phases of the Moon: Lunar Sidereal & Synodic Periods:

Our Nearest Celestial Neighbor

The Moon is a natural satellite of the Earth.

Its orbit around the Earth is elliptical:

Synchronous Rotation

The Moon's rotation period is equal to its orbital period:

As a consequence, the Moon always keeps the same face towards the Earth.

The synchronization of the Moon's rotation and orbit is caused by strong tidal forces from the Earth that effectively "locks" the Moon's orientation relative to the Earth.

[Note: The degree of synchronization is not perfect for two reasons. First, the Moon's orbit is elliptical rather than circular, so that the Moon's orbital speed is faster at perigee and slower at apogee. This mis-match in the exact orbital and rotation rates results in an apparent east-west "rocking" motion of the Moon by about 7.9 degress over the course of a month. The second is that the axis of the moon's rotation is tilted by about 7 degrees relative to its orbital plane (like the Earth's 23.5 degrees). This leads to an additional north-south nodding motion over the course of a month. The combined rocking and nodding motion motion is called "libration". You can see libration in the lunation movie below.]

Phases of the Moon

The Moon produces no visible light of its own

During the month, we see a complete cycle of Phases:

The phase of the Moon depends on the fraction of the sunlit hemisphere visible to us.

Moon Phases
Graphical depiction of the phases (click to see full-size)
[Lunation Movie. This movie shows one month of lunar phases. Note how the moon appears larger at perigee and smaller at apogee. Also note the apparent nodding and rocking motions due to "librations" as mentioned above.]

New Moon & Full Moon

New Moon:

Full Moon:

Quarter Moon

Quarter Moons occur when the Earth, Moon, & Sun are at right angles:

First Quarter:

Last Quarter:

With New Moon and Full Moon, they help to divide the Lunar Month into quarters.

Waxing & Waning

Waxing: increasing illumination

Waning: decreasing illumination

Moonrise, Moonset...

You don't see all moon phases at all times

Times of rising and setting depend on the details of the Earth-Sun-Moon configuration as viewed from the surface of the rotating Earth.

Moonrise and Moonset during Full Moon:

(Click on the image to view at full scale [Size: 28Kb])

Other examples were given in class (Better, work out the approximate moonrise and moonset times for the current phase of the Moon, and then go outside and see if your predictions are correct!)

The View from the Moon

Question: What would an astronaut on the Lunar near side see during one month?


Lunar Sidereal Period

The Lunar Sidereal Period is the time it takes the Moon to complete one orbit around the Earth with respect to the stars.

The Sidereal Period of the moon is measured watching its motions against the background stars:

The Moon on 1999 Oct 2: Last Quarter Moon in Gemini

27.3 days later on 1999 Oct 29: Waning Gibbous Moon in Gemini

Note: The background constellation is the same 27 days later (Gemini), but the Moon is in a different, earlier phase (Waning Gibbous, which comes before Last Quarter phase).

Lunar Synodic Period

The Lunar Synodic Period is the time between successive New Moons.

This is the month used by Lunar Calendars:

Why are they different?

While the Moon orbits the Earth, the Earth orbits the Sun.
(Click on the image to view at full scale [Size: 9Kb])

A Note on Terminology:

Sidereal is derived from the Latin word for star (sidus). A Sidereal Period denotes the time it takes for an object (for example, the Moon) to return to the same place as seen with respect to the stars.

Synodic is derived from the Greek word synodos, meaning a "coming together" (e.g., a Church "Synod"). The Synodic Period of the Moon is therefore the time between the apparent coming together of the Sun and Moon on the same side of the sky at successive New Moons.

Synodic periods always require you to combine two Sidereal periods. In this case, we are combining the orbit of the Moon around the Earth (the Moon's Sidereal Period), and the Earth's orbit around the Sun (the Earth's Sidereal Period) together to compute the Synodic Period.

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Updated: 2008 January 21
Copyright © Richard W. Pogge, All Rights Reserved.