'Blue moon' to be visible above London as rare phenomenon set to occur on Friday night

Londoners are set to catch a glimpse of a rare 'blue moon' above the capital on Friday night.

The unusual phenomenon is when a second full moon appears within a calendar month - though the moon will not appear a different colour.

Blue moons occur because although 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.

They are not quite as rare as other lunar events, but are still fairly unusual as they only occur every two to three years.

The lunar event taking place on Friday night will be the first blue moon to occur since September 2012 - and the next one will not happen until January 31, 2018.

Astronomer Dr Radmila Topalovic, from the Royal Observatory Greenwich, said blue moons occur because lunar months do not match calendar months exactly.

She said: "A blue moon is an extra full moon. You expect to see one full moon in a month, but you have blue moon when you have 13 full moons instead of 12 in a year.

"The phrase once in a blue moon refers to the fact that something is rare, but it's confusing because it's not blue - it looks the same as any other full moon."

She said that there are varying explanations as to how the phrase 'blue moon' came about, but it could have stemmed from historic volcanic eruptions, the dust from which can cause the moon to appear blue.

Dr Topalovic said the best time to view the moon would be at around midnight.

She said: "It looks like it's going to be clear, so we should be able to see it, the full moon will be at its highest at around midnight.

"It only happens every two or three years - you have to wait a while until you end up with an extra moon in your year.

"It doesn't look blue, that's the important point, but you get a second chance to see a full moon in a month, so it's pretty cool."