“Thirty days hath September, April, June and November.” All other months except February have thirty-one days. These are called calendar months. That is because they are no longer linked to the actual cycle of the Moon which was the origin of the month.
Astronomically speaking, how long is a month? It is the length of time that it takes for the Moon to orbit around Earth once. While that definition may seem simple, that length differs depending on how an orbit is defined. While astronomers define five different “months,” most of the confusion that I run into arises from the two are most commonly used.
The Moon completes one (360 degree) orbit of Earth in about 27.3 days. This is called a sidereal month (meaning in relation to the stars or constellations). After one sidereal month, the Moon has completed one orbit of Earth relative to the stars. So, about every 27 days, we see it near the same stars. Even though it has returned to the same place among the stars is not at the same phase. This is NOT how long it takes the Moon to complete one cycle of phases. That takes about 29.5 days. Because Earth is also moving around the Sun, the Moon must travel more than 360 degrees to bring Earth, Moon and Sun back into the same relative positions or alignment (as they were in the previous month). This is called a synodic month.
Here is an animation that demonstrates this.
As sidereal and synodic are unfamiliar words to most people, when I speak of the Moon’s orbital period, I usually say, “the Moon takes about 29.5 days to go around Earth relative to the Sun” or “27.3 days to go around Earth relative to the stars.” This clarifies which orbital period I am using.
Here is an example. On November 21, 2010, the full moon was seen near the Pleiades, a star cluster in the constellation of Taurus. On December 18, (27 days later), the Moon will again appear near the Pleiades. However, the Moon will not be full but will be at a gibbous phase because it is not opposite the Sun in the sky. It will be full on December 21, having moved eastward relative to the stars over the additional 2.2 days, into the constellation Gemini.
I have occasionally seen 28 days given as the Moon’s orbital period (especially in elementary schools). I guess that this is sort of an average between the sidereal and synodic months, but this is not astronomically accurate and should not be used.