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The full moon occurs when the Sun and Moon are located on opposite sides of the Earth. In this situation, the face of the Moon visible from the Earth is completely illuminated by the Sun. More specifically, the full moon occurs when the geocentric apparent longitudes of the Sun and the Moon are 180 degrees apart. This is a fancy way of saying that the Sun and the Moon are on opposite sides of the sky.
The Moon takes 27.3 days to orbit the Earth. But because the Moon is orbiting around the Earth in the same direction that the Earth is orbiting the Sun, the Moon takes an additional 2.2 days to return to the same position in the sky, where it’s perfectly lined up with the Sun. That’s why the amount of time it takes to go from a full moon to a full moon is 29.5 days long. Astronomers call this length of time a lunar month.
One interesting side note, the month of February only has 28 days. Since that’s less than the 29 day lunar month, there are some years where February doesn’t have a single full moon. The last time this happened was in 1999, and it’s expected to happen again in 2018.
When the Moon is full, it’s at its brightest. Astronomers measure the brightness of an object using a term called apparent magnitude. The apparent magnitude of the full Moon is -12.7. When the Moon is only at its first quarter, its brightness is -10.0, which is a reduction of 12x. Ancient peoples carefully recorded the times from full moon to full moon since those were some of the few times that they could actually see and get work done in the night – before we had artificial illumination.
A blue moon occurs when a single month has two full moons. The second full moon in a calendar month is known as a blue moon. Blue moons tend to occur every 2.7 years.
Want to know when the next full moon is going to happen? Here’s a calculator from the US Navy’s Observatory.