Is there such a thing as a blue moon

A.� Yes, in fact there are two definitions of a blue moon. In the first, large amounts of dust, the kind that volcanic eruptions eject into the upper atmosphere causes sunsets to turn green and the moon to appear blue.

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Q.� Why does the dust result in a blue glow

A.� When light reflected from the Moon passes through the atmosphere, it interacts with the volcanic materials resulting in a blue casting for Earth bound observers.

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Q.� So it's a rare event

A.� Yes, giving rise to the phrase 'Once in a blue moon'.

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Q.� So the phrase blue moon is relatively new

A.� No, it's been around since at least the sixteenth century. But it wasn't until the eruption of Krakatau in Indonesia 1883, that the relationship between the two natural events was established.

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Q.� But maybe the phrase 'once in a blue moon' relates to the second definition of a blue moon

A.� Unlikely. The second definition of a blue moon is when two full moons occur in the same month, which happens around every 3 years, not very rare at all.

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Look out for the next one in November of this year.

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Q.� And what about green sunsets, how are they created

A.� Sometimes, just after sunset, there is a green glimmer of light on the horizon, known as 'green flash'. Pollutants in the atmosphere cause it to act as a prism, splitting the image of the sun into different colours, and as the sun finally sets the last bit of light appears as a green flash.

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Want to know about a natural phenomenon Click here to ask.

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by Lisa Cardy