Rare seasonal blue moon to light up the night sky this weekend

A nearly full Blue Moon rises over the mountain top in Vancouver, B.C., Sunday, Nov. 1, 2020. THE CANADIAN PRESS/Jonathan Hayward

Toronto -

A "​once in a blue moon" event is upon us, as a "seasonal blue moon" is set to light up the night sky this weekend.

Each season (winter, spring, summer and fall) typically has three full moons that occur around 30 days apart. But on rare occasions, four full moons can occur in the same season. When that happens, the third full moon out of the four is called a seasonal blue moon, NASA says.

The first full moon of the summer came on June 24, three days after the summer solstice and the start of the summer season. There was also a second full moon in July. The fourth full moon is set to appear on Sept. 20, just two days before the autumn equinox, which marks the end of summer and beginning of fall.

That leaves the Aug. 20 full moon as the third full moon of the summer. The moon expected to reach its peak fullness on Sunday morning, but will still appear very full on Saturday and Sunday night.

Sunday's blue moon also coincides with the "Sturgeon Moon," the traditional nickname for the August full moon. The Old Farmer's Almanac says that the August full moon got its nickname from the giant sturgeon fish in the Great Lakes that were most easily caught at this time of year.

The seasonal blue moon is one of the two kinds of blue moons. The other type of blue moon is known as a "monthly" or "calendrical" blue moon, which takes place whenever there are two full moons in a single calendar month. The last blue moon was a monthly blue moon that coincided with the Halloween weekend in October 2020.

Despite the name, the full moon won't appear blue in colour. The origins of the name are unclear, but in 1937, a blue moon was defined by the Maine Farmers' Almanac as taking place when there are 13 full moons in a year.

This definition was incorrectly reported in an article published in the American magazine Sky & Telescope in 1946. In the 1980's, several U.S. radio shows repeated the incorrect 1946 definition, leading to the modern understanding of blue moons that we know today.

The next blue moon will be a monthly blue moon in August 2023. The one after that will be a seasonal blue moon taking place in December 2025.