mid-autumn festival
Britain is set for its first blue moon since 2011 on 31 JulyReuters

Britain is set for its first blue moon since 2011 on 31 July. The rare phenomenon occurs when two full moons appear within the space of a calendar month. We had the first of our double whammy on 1 July, with the second coming about on the final day.

It is an occurrence that only comes once every three years or so, with the next one coming in 2018. Full moons are separated by 29 days, so it is possible to fit two into one month.

The wait that lunar lovers and werewolves have to go through to see a "blue moon" takes some time, which is where the phrase "once in a blue moon" comes from.

However, if you want to see a slightly discoloured moon, you are out of luck. The name does not apply to the colour in these circumstances, but rather a corruption of the term "belewe moon", or "traitor's moon". A slightly blueish moon can occur if the atmospheric conditions are right but it is unlikely to be the case on 31 July.

The moon can take on a blue appearance but this is caused by smoke or dust particles in the atmosphere, giving it the illusion of a blueish tinge. This is often caused by huge volcanic eruptions, with the last literal blue moon coming in 1883 after the eruption of Indonesian volcano Krakatoa.