Blue Moon

Once in 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, whilst the length of an average month is roughly 30.5 days. 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 two-and-a-half years.

When the Moon really does turn blue
We've used the expression Blue Moon simply to mean the second Full Moon in a single calendar month. The Moon won't actually change color, despite the name. However, there have been occasions when the Moon has appeared to be blue in hue. This isn't an astronomical phenomenon. Instead, it is caused by dust or smoke high in the Earth's atmosphere. The dust is thrown up by major volcanic eruptions such as Krakatoa, Mount St. Helens or Mount Pinatubo, whilst smoke can come from large forest fires.

Whether it's dust or smoke, the tiny particles have a strange effect on the moonlight (or sunlight) 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.

Because of the unusual size distribution of the dust particles, this scattering works in the opposite sense to the normal scattering by dust low on the horizon which makes the Sun appear deep red in color as it rises or sets.

When February has no Full Moon
Several people have noticed that February 1999 has no Full Moon. In 1999, January and March both have two Full Moons. The second Full Moon in January occurs late on the 31st, whilst the first Full Moon of March falls close to the beginning of the month, on the 2nd.

You might guess, then, that February has no Full Moon in years when January and March have two Full Moons each. This happens, on average, about three times each century.

There any other situations when February misses out on a Full Moon.
Years in which February has no Full Moon coincide exactly with those years in which there are double Blue Moons. These may be January-March doubles (the most common) or January-April or January-May doubles.

They may also be December-March doubles, such as 1933/1934. Both December 1933 and March 1934 had a Blue Moon, and February 1934 had no Full Moon.

The frequency of double Blue Moons is about 4.5 per century, so this is also the frequency with which February has no Full Moons.

Blue Moons in 2006
There were no Blue Moons during this year.

Blue Moons in 2007
All dates and times are given in Greenwich Mean Time (GMT).
Year Month First Full Moon Blue Moon
2007 June 1st at 01:02 30th at 13:48

Blue Moons in 2026
All dates and times are given in Greenwich Mean Time (GMT).
Year Month First Full Moon Blue Moon
2026 May 1st at 17:23 31st at 08:44

reference: http://aa.usno.navy.mil/faq/docs/moon_phases.html

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