Once in a blue moon: Halloween treat as lunar phenomenon lights up night sky

The second full moon of the month occurs only every two-and-a-half years and fell on October 31 for the first time in nearly 20 years

The blue moon appears beside a mosque in Chennai, India. AFP

A full moon or Halloween Blue Moon is seen in Alcala de Henares, Madrid, Spain. EPA

The "blue moon", the second full moon of a calendar month, is seen over the Valle shantytown in Caracas, Venezuela. AFP

The "blue moon", the second full moon of a calendar month, is seen over the La Plata River, near a crane, in Buenos Aires, Argentina. AFP

The full moon rises over the outskirts of Frankfurt, Germany. AP Photo

The full blue moon sets beyond an American flag in Kansas City, Missouri. AP Photo

A rare Halloween blue moon moon rises over a bank of lights at Falcon Stadium in the second half of an NCAA college football game between Air Force and Boise State, Saturday, Oct. 31, 2020, at Air Force Academy, Colorado. AP Photo

It is a lunar phenomenon so rare it inspired the phrase 'once in a blue moon' - and it proved even more of a special treat for Halloween stargazers on Saturday.

People across the world had good reason to be over the moon as the Earth's satellite shimmered in the night sky.

A blue moon, is in fact, a full moon and is very often not blue at all.

It captures the imagination as it is not just any full moon, but the second of a calendar month.

Typically, there is but one full moon a month but every few years two can be viewed in the same calendar month.

This was the case on Saturday as a stellar display was enjoyed from Argentina to the United Kingdom, the United States to the UAE, and beyond.

There was even more reason to marvel at the bonus blue moon as it lit up the skies on Halloween for the first time since 2001.

It will not be seen on October 31 again until 2039.

But the extra special thing about this full moon was that it could be seen across all time zones in the world – something that has not happened since 1944, and will not happen again until 2039. Making this blue moon sighting a truly global event.

Why is it called a blue moon?

It is an expression to describe an event that is not exactly rare, but not common either – exactly like a blue moon, which happen every two and a half years or so.

However, the moon can on occasion appear blue when there is dust or smoke high in the Earth’s atmosphere.

That happened almost every night in the late 1800s, when Krakatoa, a volcano, exploded in Indonesia.

According to Nasa, some of the ash-clouds were filled with particles about one millionth of a meter wide, changing the colour of moonbeams shining through the clouds that emerged as blue, and sometimes green.

Blue coloured moons – and lavender suns – persisted for years after the eruption due to the phenomenon, said the space agency.