<P> On September 23, 1950, several muskeg fires that had been smoldering for several years in Alberta, Canada, suddenly blew up into major--and very smoky--fires . Winds carried the smoke eastward and southward with unusual speed, and the conditions of the fire produced large quantities of oily droplets of just the right size (about 1 micrometre in diameter) to scatter red and yellow light . Wherever the smoke cleared enough so that the sun was visible, it was lavender or blue . Ontario, Canada, and much of the east coast of the United States were affected by the following day, and two days later, observers in Britain reported an indigo sun in smoke - dimmed skies, followed by an equally blue moon that evening . </P> <P> The key to a blue moon is having lots of particles slightly wider than the wavelength of red light (0.7 micrometer)--and no other sizes present . It is rare, but volcanoes sometimes produce such clouds, as do forest fires . Ash and dust clouds thrown into the atmosphere by fires and storms usually contain a mixture of particles with a wide range of sizes, with most smaller than 1 micrometer, and they tend to scatter blue light . This kind of cloud makes the moon turn red; thus red moons are far more common than blue moons . </P> <P> The following blue moons occur between 2009 and 2021 . These dates use UTC as the timezone; exact dates vary with different timezones . </P> <P> Using the Maine Farmers' Almanac definition of blue moon (meaning the third full moon in a season of four full moons, but referenced to astronomical rather than equal seasons), blue moons have occurred or will occur on: </P>

When was the last time there was a full moon on easter