December 2009 ended with a rare blue moon, that was made even more rare by the fact that this blue moon coincided with a partial lunar eclipse and it was on the last day of the International Year of Astronomy.

The moon is not actually blue . . . Arvind Paranjpye, scientific officer of the public outreach programme, IUCAA, said “It is a very rare event and hence it derives its name from the phrase ‘once in a blue moon’. It is basically a calendar event and has no astronomical importance as such.

“It specifically appears once in 2.7 years,” said Mayuresh Prabhune, secretary of Khagol Vishwa, an amateur astronomer’s club.

But this blue moon is special because it is accompanied by an eclipse, an event that has not occurred in the last five to six decades. A lunar eclipse occurs whenever the moon passes through some portion of the earth’s shadow. This can occur only when the sun, earth and moon are aligned exactly or very closely aligned with earth in the middle. Lunar eclipse occurs only on a full moon day. “The full moon has to happen on the first or the second day of the month and the month must be of 31 days for such an event to occur. It has perfectly matched this month and hence the rarity,” Prabhune added.

Most calendar months see just one full moon. The 29.5 day cadence of the lunar cycle matches up almost perfectly with the 28 to 31 day length of calendar months. Sometimes, however, the one-to-one correspondence breaks resulting in two full moon days in one month. December was one such month. The first full moon of last month appeared on December 2.

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