Does the full moon appear in the same place every month?

On Fri, 20 Aug 1999 you wrote:

> What a cool web site!! Thanks for having it available. I have
> a question about the moon: does the full moon appear in the
> same place in the sky every 28 days? I'm specifically interested
> in where it will be next month, 9/25/99.

Hi Tom,

The location of the full moon in the sky changes from month to month due
to the Earth's revolution about the sun. Remember that a full moon occurs
when the moon is on the opposite side of the Earth from the sun. As the
moon orbits the Earth, the Earth orbits the sun, and the sun moves slowly
through the sky (at the rate of about one degree per day - it's no
accident that a full circle has 360 degrees, and the year almost the same
number of days). So, in the time between full moons, the sun has moved
through about 30 degrees of the sky. So, in September, the full moon will
be found in the constellation of Aquarius, but in October the full moon
will be in Pisces.
There are therefore two ways of referring to the moon's orbital
period about the Earth. The length of time it takes the moon to return to
the same position among the stars is called the sidereal month, and lasts
about 27 days. The length of time it takes the moon to go from full moon
to full moon is the synodic month, and lasts a little over 29 days.

Thanks for writing!
Sincerely,
Kurtis Williams

Back to the Ask An Astronomer page.