Rare Blue Moon set to rise this weekend

A 'Blue Moon' will light up our night sky on 31 May this year.

Occurring on average every two or three years, its a fairly rare spectacle when we see 13 full Moons within a year.

The Moon will be officially "full" at 09:45 BST on Sunday morning. In the UK it will be below the horizon at that time so the preceding Saturday night or the night of Sunday 31 will give you two chances to view the Blue Moon.

However, keep a close eye on the forecast as the weather is all change this weekend with cloudier conditions moving in from the west.

The most well-known definition of a Blue Moon is the second full Moon to occur in a calendar month.

The lunar cycle is 29.5 days long where as most months are 30 or 31 days long.

Eventually these cycles catch each other up to produce two full Moons in a month -usually on either of the first couple of days, followed by another at the very end.

This definition - also known as a "monthly Blue Moon" - is actually a fairly modern interpretation dating back just 80 years.

According to astronomers at the Royal Observatory Greenwich, external, it is actually an astronomically incorrect definition, first printed in a 1946 edition of Sky & Telescope magazine and perpetuated by the game Trivial Pursuit in the 1980s.

The more accurate definition is a "seasonal Blue Moon" which occurs when there are four full Moons in an astronomical season instead of the usual three. The third Moon of the season is known as a "Blue Moon".

The term "Blue Moon" has nothing to do with its actual colour.

However, there have been a few recorded times in history when the Moon has had a blue tinge to it.

The most notable have been following large volcanic eruptions such as the 1883 eruption of Krakatoa in modern-day Indonesia. The volume of ash ejected into the atmosphere cooled the earth by around 0.6C and also had the effect of making the Moon appear slightly blue. This was due to the ash particles absorbing long-wavelength red light whilst allowing more - shorter wavelength - blue light to reach the Earth.

Because of its rare occurrence it's often thought to be the origins of the phrase "Once in a Blue Moon" meaning something that happens only occasionally.

The arrival of the Blue Moon comes at a time when the weather pattern is changing.

After a sunny and warmer day on Saturday for England and Wales, the cloud and patchy rain in Scotland and Northern Ireland will be heading across more of the UK overnight. Clear skies are expected to last longer over East Anglia, south-east England and the east Midlands.

Cooler air will follow on Sunday with a good deal of cloud and some showers. During Sunday night the cloud should break up for eastern parts of the UK with showers continuing in the west.

So, generally speaking, east is best.