As the Moon orbits the Earth, we see part of the Moon that
is illuminated. When the Earth is between the Sun and the Moon, the Moon reflects back light to the Earth from its full disk, rather than just part of it. This is the Full Moon. Compare the following two diagrams
| Sun | O o
Quarter Moon (Last Quarter) - aka Half Moon
/ \ o
| Sun | O
In both cases, the side facing the Sun is illuminated, however in the Half Moon phase, we can only see the half of the disc of the Moon. The "Half Moon" and "Quarter Moon" confuses many - the "Half Moon" refers to half the disk, while "Quarter Moon" refers to a quarter of the orbit.
The Earth goes around the Moon in 27.3 days - this is known as the Sidereal Period. This is how long it takes for the Moon to go around the Earth with respect to the stars. The Synodic Period is 29.5 days and measures the time between two New Moons. This is the period used for Lunar calendars. The reason for this difference is that the Earth has moved around the sun over the course of the month. While the Earth - Moon - Distant Star line is the same after 27.3 days (the movement of the Earth doesn't have any noticeable effect upon distant stars), the the Earth - Moon - Sun line requires one to take into account the orbit of the Earth with respect to the Sun.
Realizing that the Synodic Period is 29.5 days, it is possible for there to be a month (February) without a full Moon. This has happened in 1809, 1847, 1866, 1885, 1915, 1934, 1961, 1999 and will occur again in 2018, 2037, 2067, and 2094...
Occasionally, the Moon will pass through the shadow of the Earth. This will only happen when the Moon is full, and we will see a lunar eclipse. During this time, the full Moon will appear to dim or change color (depends on how much dust is in the atmosphere). The next Lunar Eclipse is May 16, 2003 - however, it will only be visible in South and Central America along with Antarctica. Following that is November 9, 2003, which will be visible from the North, South, and Central America.
While it would seem that the Moon should often be in the shadow of the Earth, this is not the case. The Moon's orbit is inclined to the plane of the Earth's orbit about the Sun by 5 degrees. In effect, this can be seen as a bobbing up and down compared to the Earth. It is rare for there to be a Full Moon when the Moon is on the same plane as the Earth's orbit to the Sun.
Because of the importance of the full Moon in various cultures, they have been given names. The most common one known is the "Blue Moon" which occurs when a second full Moon occurs in a month or the third Full Moon when there are four Full Moons in a season. The specific definition is debated. Some various Moon names (many of these are from North American Indians):
- January Moon After Yule, Wolf Moon, or Old Moon
- February Snow Moon or Hunger Moon
- March Sap Moon, Crow Moon, or Lenten Moon
- April Grass Moon or Egg Moon
- May Milk Moon or Planting Moon
- June Rose Moon, Flower Moon, or Strawberry Moon
- July Thunder Moon or Hay Moon
- August Grain Moon or Green Corn Moon
- September Fruit Moon or Harvest Moon
- October Harvest Moon or Hunter's Moon
- November Hunter's Moon, Frosty Moon, or Beaver Moon
- December Moon Before Yule or Long Night Moon.
The Harvest Moon is the Full Moon that appears closest to the Autumnal Equinox
and thus may appear in late September or early October.