Because of the eccentricity of the moon's orbit, its distance from the
surface of the earth varies throughout the cycle of its orbit. The
closest approach ("perigee") is 363,104 km and the furthest retreat
("apogee") is 405,696 km.
Around perigee (when the gravitational force between earth and moon is
strongest), the moon moves faster in its orbit, having a maximum
orbital speed of 1082 m/s. Around apogee (when the gravitational force
is weakest), the moon moves slower, having a minimum orbital speed of
Therefore, at perigee the moon will move more rapidly from the angle
at which a given phase occurs to the angle at which the next phase
occurs (a sun-moon angle of 180 degrees for full moon, 90 degrees for
quarter moon and 0 degrees for new moon). At apogee the moon will move
more slowly from phase to phase.
You can view the dates of perigee and apogee for 2006 at the following page:
"The Sun, The Moon, Earth and Eclipses"
(scroll to the section titled "APSIDES OF THE MOON")
There you will see that an apogee occurred on January 17 2006 (during
your "slow" phase change) and a perigee occurred on June 16 2006
(during your "fast" phase change).
The variation in the orbital motion also manifests itself as
longitudinal libration. Because the moon's rotation is constant but
its orbital motion is not, one of them can "get ahead of" the other or
behind it, enabling us to see a few degrees around the edges of the
closest lunar hemisphere.
Libration does affect the perceived phase, because although the
terminator (line between dark and light) is not changed by the angle
at which we view it, the terminator will occur in a different part of
the perceived disk if we are looking from a different angle. A great
animation of this can be found by clicking the "animation" link on the
"The Moon's Orbit"
I trust you find this answer satisfying, otherwise please request clarification.
"Moon - Wikipedia"
"Libration of the Moon"
Google Search Strategy:
"moon phases" "varies because"
"lunar orbit" libration
Request for Answer Clarification by
15 May 2006 10:06 PDT
You know that an answer is good when it seems obvious in retrospect. Great job!
I'd like one clarification though:
I asked "isn't the moon phase cycle supposed to be 29.53 days?"
Obviously, the differences due to speed at perigee/apogee would cancel
themselves out at the end of each full cycle, and it makes sense that
its duration would be a regular 29.53 days.
But, when calculating times from full to full moon during my "fast"
and "slow" phase changes, there is still a difference:
From and including: Saturday, January 14, 2006 at 9:48:00 AM
To, but not including : Monday, February 13, 2006 at 4:44:00 AM
The duration is 29 days, 18 hours, 56 minutes and 0 seconds
From and including: Sunday, June 11, 2006 at 6:03:00 PM
To, but not including : Tuesday, July 11, 2006 at 3:02:00 AM
The duration is 29 days, 8 hours, 59 minutes and 0 seconds
There's still a ~10 hour difference. Why?
And the strict answer to "is the moon phase cycle 29.53 days?" would be NO, right?
I'd like to know what's its variation, if that's still within the
scope of my question. (an answer like "from 29.49 to 29.54 days" would
I'm asking this because I'm working on a lunar calendar by the way.
Great answer otherwise, thanks!