July 2015 will see two full moons: the first occurred on July 2 and the second will take place on July 31.

This evening's "blue moon" will be the first one since 2012 — and the last until 2018 — is set to be seen in the sky.

However, that does not mean the Earth’s moon will appear blue to anyone that sees the full moon tonight. The history of how the second full moon in a calendar month came to be known as a “blue moon” is rather complex in itself.

Fortunately, Nasa has provided a simple explanation to the terminology in a short video – as well as explaining what it would take for the moon to actually appear blue, rather than its usual hue of pale grey and white.

Blue moons are due to the difference between calendar and lunar months. While a calendar month lasts from 28 to 31 days, a lunar month - the time interval between two full moons - is always 29.53 days long.

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