Is There a Seasonal Blue Moon This Month?

This month, we have a "Blue Moon" on May 31, now popularly defined as a second Full Moon in a calendar month.  But there is also the traditional "Seasonal Blue Moon." 

To understand the complexities of the seasonal variety, you have to step away from our standard 12-month calendar and look at the year the way early astronomers, farmers, and religious officials did. They calculated by a year that was divided strictly by the solstices and equinoxes.

A true astronomical season is the three months between these celestial markers. For example, from the Summer Solstice in June to the Autumnal Equinox in September is the season of Summer.

Because a calendar season is three months long, it naturally contains three full moons. Each of these Full Moons historically had specific names based on folklore and agriculture—like the Harvest Moon, Hunter’s Moon, or Snow Moon—which helped people track planting, harvesting, and seasonal changes.

The lunar cycle (the time from one Full Moon to the next) lasts roughly 29.5 days. If you multiply that by three, a typical season takes about 88.5 days to complete. However, a calendar season is slightly longer, lasting roughly 91 to 92 days. Because of that small 3-day gap, the Full Moons slowly drift backward against the solar calendar. Every 2 to 3 years, the gap catches up, and four full moons manage to squeeze into a single season instead of the usual three.

Well, when a season contained four Full Moons, it created a major naming problem. If you just named them in order, the traditional names would get pushed out of sync with the actual seasons. For example, the "Harvest Moon" might suddenly occur too early, before the crops were actually ready to be brought in. 

To fix this, the Maine Farmers' Almanac introduced a clever rule: the third Full Moon in a four-moon season is designated the Blue Moon. By making the third one a wild card "Blue Moon," the fourth and final full moon of that season could keep its proper traditional name, keeping the rest of the calendar perfectly aligned for the upcoming year.

Complicated, right? 

Because this phenomenon relies on the alignment of the solar year and the lunar cycle, it follows a strict cosmic rhythm known as the Metonic Cycle. The Moon's phases repeat on the exact same calendar days every 19 years. During those 19 years, there are exactly 235 full moons but only 76 seasons. Do the math and you're left with 7 extra Full Moons. Therefore, a Seasonal Blue Moon occurs precisely 7 times every 19 years, making it a true astronomical rarity that perfectly embodies the phrase "once in a Blue Moon."