Once on a Blue Moon

Is a common way of saying not very often, but what exactly is a Blue Moon? According to the popular definition, it is the second Full Moon to occur in a single calendar month. The average interval between Full Moons is about 29.5 days, the length of an average month is roughly 30.5 days. more.....

This makes it very unlikely that any given month will contain two Full Moons, though it does sometimes happen. On average, there will be 41 months that have two Full Moons in every century, so you could say that once in a Blue Moon actually means once every 2.5 years. more.....

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When is the next one?

Are they realy blue?

It is caused by dust or smoke high in the Earth's atmosphere. The dust is thrown up by major volcanic eruptions such as Mount St. Helens smoke can come from large forest fires.

Whether it's dust or smoke, the tiny particles have a strange effect on the moonlight passing through them. They scatter the light in every direction, but red light is scattered more strongly than blue light, so that less red light passes directly through the dust or smoke. Thus the Moon has a blue tinge.