Why is May 31 full moon a blue moon?

What makes this blue moon “the second per month”

Astronomers and popular calendars use different definitions of “blue moon,” but this week’s rare blue moon fits the calendar-based version. Because dates on the calendar are anchored to the sky—especially to the full moon—this month’s lunar timing matters.

In the story, the full moon on 31 May is described as the second full moon of the calendar month. That is the key condition for the “blue moon” label in the common calendar definition: when a month contains two full moons, the second one is called a blue moon.

Why it’s worth knowing

This labeling matters mainly for how people plan and talk about sky events. It also highlights a subtle disconnect: the astronomical full moon itself doesn’t change because we assign names to it. Instead, the naming happens because the calendar month boundaries are human constructs that don’t align perfectly with the lunar cycle.

In practical terms:

• The full moon frequency is regular astronomically.
• The calendar varies by month length.
• When the lunar cycle lines up such that two full moons land in one calendar month, one gets the “blue moon” name.

The story also emphasizes that the term “blue moon” doesn’t necessarily mean the moon looks blue to the naked eye. The color interpretation is a misconception; the phrase is a naming convention tied to the timing of full moons.

If you’re tracking skywatch events, this approach helps you interpret what’s happening even when public media uses different blue-moon definitions. For this week’s event, the determining factor is the second full moon in the same calendar month.