Eclipse of the Moon - Blue Moon

The eclipsed moon, illustrated for Dec. 31, 2009, rides high above the Cathedral of Santa Maria del Fiore in Florence, Italy. Credit: Starry Night Software

Eclipses of the moon occur twice a year, on average. Each eclipse is visible only on the half of the Earth turned towards the moon at the time the Earth's shadow falls on the moon.

There was a partial eclipse of the moon on New Year's Eve, Dec. 31. Because of its timing, it was not visible in North and South America, but was visible over most of Europe, Africa, and Asia.

The event has also marked the second full moon of the month in North America, thereby garnering the title of "blue moon." Unless unusual atmospheric circumstances come into play — such as widespread dust from a volcano — the moon will not be blue, however.

Since it is a partial eclipse, the moon has just brushed past the darkest part of the Earth's shadow, never becoming totally immersed. It was, however, deep enough into the shadow that shading and reddish color was visible.