Skywatchers will get to see a blue moon Saturday, but not the one you typically hear talked about. This one is a seasonal blue moon and it is rarer.
The most popular definition of blue moon is the second full moon in a single calendar month. A seasonal blue moon is the third of four full moons in a single season. It happens once every two to three years because most seasons only have three full moons.
When the first full moon of a season falls just after an equinox or solstice, there is a good chance you will have four full moons in that season.
The spring equinox, marking the start of spring, was on March 20. The first full moon of this spring came the day after that on March 21. It was followed by another on April 19. After Saturday's seasonal blue moon, there will be one last full moon on June 17. That's four days before the summer solstice on June 21, marking the start of summer.
The next seasonal blue moon after this will occur on August 22, 2021.