Today (December 31) is the “once in a blue moon” day. But don’t expect to see a blue colored moon in the sky. Blue Moon is a celestial phenomenon, which is named such not for the color, but because it is the second full moon in a calendar month. The first full moon this month came December 2. A blue moon happens about once every 2 or 3 years.
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, the one-to-one correspondence breaks resulting in two full moon days in a month. December 2009 is such a month. The first full moon in December appeared on 2nd while the second will appear in the sky on December 31st.
The last blue moon that occurred on New Year’s Eve was in 1990. In 1999, there were two blue moons very close together. One on January 31st (after the full moon on Jan. 2nd) and the other on March 31st (after the full moon on March 2nd).
This New Year Eve’s blue moon is rare because it will have an eclipse, an event that has not occurred in the last 5-6 decades. This celestial rarity will be witnessed on the New Year’ eve again only in 2028 and 2066.
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 shadow of the earth can be divided into umbra and penumbra. Within umbra there is no direct solar radiation. However, as a result of the sun’s large angular size, solar illumination is only partially blocked in the outer portion of the earth’s shadow, which is given the name penumbra.
“The eclipse is partial and will begin at 10.45pm on Thursday and continue till 2.59am on Friday, the first day of 2010. The eclipse would be visible from naked eye and India is one of the best places to see it,” says Anil Yadav of Indira Gandhi Planetarium. [via]