Blue Moon, 4 planets to shine during the final weekend of May

The rare lunar event won't happen again until 2028, and it comes with a bonus for those who step outside to look up.

May is ending with a rare celestial event: a Blue Moon.

The second full moon of the month will rise on the night of Saturday, May 30, into the early morning of Sunday, May 31. The month's first full moon, the Flower Moon, rose on May 1.

Despite the name, the moon will not turn blue. The term "Blue Moon" is tied to the calendar when two full moons fall within the same month, the second of which earns the nickname. It happens because the moon's cycle lasts about 29.5 days, slightly shorter than most months, so every few years it allows two full moons to bookend the same month.

How rare is a Blue Moon?

The phrase "once in a blue moon" is used to describe something that happens rarely, and the astronomy behind it holds up. On average, a calendar Blue Moon occurs roughly once every two to three years, making May 31 an occasion worth stepping outside for.

This month features the first time two full moons have risen in the same month since August 2023, and it won't happen again until December 2028.

People who step outside at the end of the month to catch a glimpse of the rare lunar event should also be able to spot Venus and Jupiter in the western sky about an hour after sunset. Meanwhile, early risers should be able to see Mars and Saturn in the eastern sky about an hour before daybreak.

The other type of Blue Moon

There is also a second, lesser-known definition of a Blue Moon that predates the calendar version.

In a typical year, each astronomical season contains three full moons. Occasionally, a season will have four. When that happens, the third full moon of the year is called a seasonal Blue Moon.

The calendar-based definition of two full moons in one month is the more widely recognized of the two today, though both remain in use.

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