The event, which inspired the phrase `once in a blue moon' about four hundred years ago, was last observed in 1961.

M P Birla planetarium director R Subramanium told PTI here that a full moon had lit up the last night of January this year, after appearing on the first night of that month.

``We shall witness the same phenomenon in March, as the full moon will reappear on March 31 after being seen on March 2, marking a once-in-19-year happening,'' he said.

A full moon normally appears twice a month in every two and half years, but only in more than 19 years does the event occur in two months of the same calendar year.

However, this moon will be significant since the event failed to occur in 1980, Subramanium said. The blue moon phenomenon was last recorded in the months of January and April,1961.

``Since the moon takes around 29.5 days to complete one lunar cycle, it is rare to spot a full moon twice a month and rarer still for it to appear in two months,'' Subramanium explained, adding that it is seen in its usual hue and not blue as the saying might seem to suggest.

Subramanium said though technically, the event might occur on April 1 in some parts of the globe due to time differences, according to universal GMT it would be recorded on March 31.

On what led to the coining of the adage `once in a blue moon', he said there were hundreds of schools of thought on this issue.

Legend has it that in 1883, an Indonesian volcano erupted, coinciding with the time the full moon occurred for a second time in a particular month. The smoke and dust emanating from the lava made the moon appear to be blue, he said.

Recent scholars believe that during a late Indian monsoon in 1927, the full moon was spotted for the second time in a month against a cloudy night sky giving it a bluish tinge and hencethe adage.

``There are also stories of a forest fire in Newfoundland, lending its smoky colour to such a moon,'' Subramanium added.