A rare blue moon is expected to light up the skies tonight. It will be a full moon which will be at its fullest at around 9:45p.m ET.

There are two definitions of blue moons.

Tonight's blue moon is an extra summer moon, which is the third moon of four in the season that started on the June solstice. In most seasons, there are three full moons, but once in a while there are four moons. The extra moon is called a blue moon. Tonight's full moon is also called a Full Sturgeon Moon, the Full Red Moon, the Green Corn Moon and the Grain Moon.

A blue moon is also defined as the second of two full moons within any given calendar month and is a rare occurrence; hence the phrase "once in a blue moon" which indicates the rarity of an event. All calendar months, aside from February, have 30 or 31 days, resulting in the occasional two full moons within a single month. Under this meaning of the 'blue moon', the last blue moon was on Aug, 31, 2012. The next 'blue moon' will not occur until July 2015.

The name of the event comes from the colors the moon takes on from time to time due to different conditions. After volcanic activity or forest fires, the moon can appear to take on a bluish or sometimes lavender coloration. This is because of soot and ash particles deposited high up into the Earth's atmosphere.

A rare 'super' supermoon, a huge perigee moon, occurred on June 22 this year, The moon appeared to be bigger than the average full moon as it will be lined up with the sun perfectly so that the moon will appear to be brighter and bigger.

A "perigee" full moon means that it's orbit is closes to the center of Earth in a month. Full moons do no necessarily happen during a perigee but when it does, it means the sun, earth and moon align. Another super moon like this will not be seen until August 2014.