Blue Moon

moon · rare events · sky

Second full moon. A blue moon happens when two full moons occur in the same calendar month. This is pretty rare because most months don't have enough days to fit in two complete cycles of the moon's phases.

Smoke and dust matter. A blue moon can actually look blue when smoke, dust, or ash in Earth's atmosphere filters out red light and lets blue light shine through. This happened after big volcanic eruptions or massive wildfires.

Four full moons season. Another kind of blue moon is when a season (like spring or summer) has four full moons instead of the usual three. The third one in that group gets called a blue moon.

Not really blue. Despite the name, blue moons are usually NOT actually blue at all. The word 'blue' probably comes from an old phrase meaning 'rare' or 'impossible,' not the actual color.

Happens every few years. Because the moon's cycle is about 29.5 days long, a second full moon in one month only occurs every two or three years.