New York, Dec 31 (DPA) Times Square revelers will be treated to a rare “blue moon” on New Year’s Eve Thursday night.

Hundreds of thousands of people are expected to watch the ritual descent of the crystal ball in Times Square to usher in 2010 at midnight. But what many of them may not yet expect is a special full moon above their heads, the second this month.

The astronomical phenomenon known as a blue moon is a second full moon happening within one month and has nothing to do with colour. The last full moon was Dec 2. The full moon can be seen in North and South America, Europe and South Africa.

Astronomers say a full moon occurs every 29.5 days for an average 12 a year, and a blue moon occurs every 2.5 years on average. The event has given rise to the phrase “once in a blue moon” to describe a rare occurrence.

They say blue moons on New Year’s Eve are rare, happening only every 18 years with the next one probably in 2028.