Blue Moon tonight, an extra full moon of the month

A Blue Moon, the calendar term for a second full moon in a single month, peaks tonight as the month's extra full moon. Because the lunar cycle lasts 29.5 days, months longer than that can hold two full moons, so Blue Moons occur about once every 2.5 years. Skywatchers will see the full moon rise at sunset; thin clouds can produce a lunar halo and the Moon's apparent size shifts slightly with its distance from Earth. The Blue Moon matters for photographers, cultural observances, and anyone who wants a rare second chance this month to observe a full moon.