'Once in a blue moon': why today's the day

A rare lunar event will appear in the sky tonight, on the day of Neil Armstrong's memorial

A blue moon over QuitoReuters

The appearance of a rare "blue moon" tonight could not be more fitting, as the memorial for astronaut Neil Armstrong, who died last Saturday aged 82, takes place 43 years after he first stepped on its surface in 1969.

The lunar event happens when there are two full moons in one month. As a lunar cycle lasts only 29.5 days, blue moons come rarely - on average seven times every 19 years.

The next will take place on 2 July 2015. The rarity of the event gave rise to the phrase: "once in a blue moon".

A full moon occurs as the moon reflects the maximum amount of sunlight from Earth.

The first full moon this month peaked on 2 August and will be at its fullest again at 2.59pm today, though it will have dipped below the horizon by then. It will appear again tonight, when viewers will be able to view a blue moon for the first time since 2010.

There may be some disappointment, though, for the blue moon won't actually be blue.

The reasons we call it a blue moon are unsure, though it probably dates back to the 18th century when the clergy needed to differentiate between the Easter moon and a false one called the "betrayer moon" (the old English word for "betray" being "belewe").

The occurrence of blue moons was also noted in the Farmers' Almanac.

A visibly blue moon only occurs after volcanic activity or forest fires. So, if we're fortunate, the moon will appear as white as usual.

When the Indonesian volcano Krakatoa erupted in 1883, the explosion, likened by scientists to a 100 megaton bomb going off, sent plumes of ash up into the top layers of the atmosphere. In many places, for up to two years afterwards, the moon appeared to be blue.

Krakatoa's ash is the reason. Some of the ash-clouds were filled with particles about one micron (one millionth of a metre) wide - the right size to scatter red light, while allowing other colours to pass. White moonbeams shining through the clouds emerged blue, and sometimes green.

The ash caused "such vivid red sunsets that fire engines were called out in New York, Poughkeepsie, and New Haven to quench the apparent conflagration," according to volcanologist Scott Rowland at the University of Hawaii.

Sadly, chances of seeing the event tonight have dipped as the Met Office shows that tonight will be very cloudy - not such a rare occurence!