Wire service reports from Boston, quoting associate editor of Sky and Telescope magazine, Roger Sinnott, said a 53-year-old error had crept into the calculations making astronomers misinterpret it as a `blue moon'.The reports said a full moon seen for the second time in a single month of a calendar year was not a `blue moon', according to an almanac, which says that when a season contains four full moons, the third is considered blue."However, the almanac has stopped publication for over five decades now and even when it existed, it hardly influenced astronomical theories," planetarium director R Subramanium told PTI.

Sinnott said on Friday that an amateur astronomer James Pruett had wrongly mentioned in a 1946 write-up for the magazine that the second full moon of a month was a`blue moon'. "He never lived to see his mistake corrected, but his definition endured. Nothing we can do is going to put the genie back in the bottle."

Subramanium said this confirmed that the terminology had come to stay in astronomy, adding the `season' mentioned in the reports was vague since it did not define the time unit considered for such calculations.

Subramanium said a season could mean three months of a calendar year taken together, or an entire year. "Since these calculations were based on lunar calendars, they contradicted new world astronomical data based on the Gregorian calendar," he explained.