Once in a blue moon event for sky gazers on New Year's eve

When a second full moon in a calendar month appears in the night sky, the occurrence is known as a blue moon. There are 12 full moons most years but every two or three years there is an extra full moon which is called a blue moon.

"We've already had a full moon on December 2 and the next one is on New Year’s Eve that is on December 31," Science Popularisation Association of Communicators and Educators (SPACE) President C B Devgun told PTI.

On rare occasions the moon actually had a blue appearance - often caused by smoke from large-scale fires or excessive dust particles in the atmosphere. Explaining the phenomena of bue moons, Devgun said the moon cycle takes about 29.53 days to complete that is almost a month. That is why there's one full moon once a month, at least on most years.

"Because the lunar cycle does not reach a full 30 days, the full moon happens about a half a day earlier each month. So over time, this small difference eventually pushes the date of the full moon until there can be two full moons in one month. When this happens, the second full moon in that month is called the blue moon," Devgun said.

This comes about quite rarely, only about seven times in 19 years, which is equal to one blue moon every 2.7 years more or less. The next blue moon will occur on November 21, 2010. After that, planet watchers will have to wait until August 21, 2013, to see another blue moon.