Calendar of moon phases for the year 2012. Over the course of the year, the Moon will orbit the Earth once every 27.3 days. This orbit means that, when viewing the moon from the Earth, it will pass through various phases (known as lunar phases), during which different portions of the celestial body will be visible. These phases are commonly known as: Full Moon, New Moon, First Quarter or Last Quarter.
"Full Moon" refers to those times when the entire moon is visible to an observer on Earth. This occurs when the Moon is on the opposite side of the Earth as the Sun
. "New Moon," in addition to being one of the more popular entries in the "Twilight" series, refers to those times when nearly none of the moon is visible for an Earth observer. It occurs when the Moon and the Sun are on the same side of the Earth.
The "first quarter moon" means the Northern Hemisphere will see the right half of the moon, and the Southern Hemisphere will see the left half. And "last quarter" is, of course, the opposite of that. Other in between phases are sometimes referred to as "Waxing Crescent," "Waning Crescent," "Waxing Gibbous" and "Waning Gibbous," though these distinctions really just refer to the periods between the main four lunar phases. There are other names for lunar phases that occur less frequently. For example, a "Harvest Moon" is a full moon that happens close to the autumnal equinox (September 21st, the beginning of fall.) A "Blue Moon" is a second full moon that occurs within the space of a single month, an event that only happens about once every 3 years. (Hence the cliche "once in a Blue Moon.")
Traditionally, each full moon out of a given year was given an individual name based on the month in which it appears. (July featured the "Thunder Moon," November the "Frosty Moon," and so on.) These terms have typically fallen out of favor.