Some facts about the moon

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Blue Moon?

 

 

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. 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 colour, 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 colour as it rises or sets.

New Moon - New Moon
The Moon's unilluminated side is facing the Earth. The Moon is not visible (except during a solar eclipse).

Waxing Crescent -
The Moon appears to be partly but less than one-half illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.

First Quarter half moon-
One-half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.

Waxing Gibbous - moon
The Moon appears to be more than one-half but not fully illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.

Full Moon - Full Moon
The Moon's illuminated side is facing the Earth. The Moon appears to be completely illuminated by direct sunlight.

Waning Gibbous - 3 q moon
The Moon appears to be more than one-half but not fully illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.

Last Quarter - half moon
One-half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.

Waning Crescent - The Moon appears to be partly but less than one-half illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.
Following waning crescent is New Moon, beginning a repetition of the complete phase cycle of 29.5 days average duration. The time in days counted from the time of New Moon is called the Moon's "age". Each complete cycle of phases is called a "lunation".

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