Once in a blue moon

The last full moon of summer rose over Sundance on Wednesday evening. It not only marked the end of the season, but the last time a blue supermoon will grace the skies until the year 2037.

This two-for-one event includes a pair of lunar phenomena, the blue moon and the supermoon.

A blue moon is the uncommon occurrence of two full moons in a single calendar month. It gets its name from a 16th-century expression "the moon is blue", which meant something that was impossible.

However, after Krakatoa erupted in 1883, strange sunsets appeared around the world and the moon did indeed appear blue, which led to the expression changing to "once in a blue moon", meaning rare instead of impossible.

Blue moons don't exactly count as "rare", but they don't happen often because the moon's cycle takes 29.5 days to complete. According to NASA, two full moons in a month happens every two to three years.

Much rarer is two phenomena coming together to create a blue supermoon, which hasn't happened since December, 2009.

A supermoon is the colloquial term for a perigean full moon – in other words, the moment when the moon is full at the closest point to Earth in its oval-shaped orbit.

Last week's supermoon was the brightest of the year because the moon was around 17,000 miles closer to the Earth than usual at 222,043 miles, according to the Old Farmer's Almanac.

During a supermoon event, the moon appears around 8% bigger than average and 16% brighter.