"as a kind of placeholder"
By Blaine Friedlander
May 17, 2019 at 12:58 PM EDT
When the moon is full on Saturday, Earth’s lunar companion becomes a “blue” moon — in a floral, seasonal way.
Popular folklore holds that the second full moon in a given month is a blue moon, but this Saturday’s blue moon has another definition. It is the third of four full moons in a given season. In most seasons, there are just three full moons.
But this spring, there are four full moons between the vernal equinox on March 20 and the summer solstice on June 21. The third of the four full moons gets the blue designation — as a kind of placeholder — so named moons like the harvest moon, the snow moon and the wolf moon don’t become out of sync with their usual companion month, said astronomer Geoff Chester of the U.S. Naval Observatory.
The seasonal blue moons occur every two to three years on average. The next one is set for Aug. 22, 2021.
Chester said this seasonal blue moon — which is the original definition of a blue moon — was first dreamed up by the Maine Farmer’s Almanac in the early 20th century.
Thus, May’s usual flower moon becomes a blue flower moon, if we conform to the farmers’ almanacs.
“The blue moon is an interesting bit of scientific trivia for those that keep track of these kinds of things,” Chester said. “There is nothing particularly special about it. Somewhere along the line you’ve got to have an extra moon.”
For Washington, the moon officially becomes full at 5:11 p.m., but starts to rise Saturday at 8:10 p.m., according to the Naval Observatory.