Come Saturday night and Indians can watch a phenomena that rarely occurs -- a second full moon in a calendar month. The first full moon was on July 2.

It is an occurrence from which the oft-used English saying 'once in a blue moon' is said to have originated.

For Indians wanting to witness the event, the moon will rise slightly after 1800 IST and set at around 0400 IST, M P Birla Planetarium Scientific Director Debiprosad Duari said.

Since there are roughly 29.5 days between full moons, it is unusual for two full moons to 'fit' into a 30 or 31 day month, Duari explained.

The last such occurrence happened in November 2001. In 1999, there were two blue moon occurrences on January 1 and 31 and March 2 and 31. The next blue moon will appear in June 2007.