If you are a moon-watcher, you know that the moon has been getting brighter lately and tonight it is full, meaning it is directly opposite the sun and appears perfectly round. It is brightest when it is full because we then see its entire sunlit surface. At all other times, it is only partially illuminated from our perspective, so it appears either somewhat crushed or crescentshaped. For about three days every month, the moon is completely lost from view as it moves in alignment with and crosses in front of the sun.

Since the moon makes one circuit of the earth in about 30 days and on average there are 30 days in a month we can expect one full moon each month. However, this month is different and we will have two full moons due to an accident of the calendar.

The moon was full on December 2nd and on December 31st, a second full moon will occur, an event known as a blue moon. This phenomenon (two full moons in a single month) occurs only about 37 times per century. The last blue moon was May 31, 2007 and before that July 31, 2004.

The term blue moon was first used in the 1500s to describe an event that could never happen. However, when seen through smoke or dense haze or dust, the moon does sometimes have a bluish cast.

For example, after the Indonesian volcano Krakatoa erupted in 1883 (the sound heard round the world), wildly colored sunsets and blue-hued moons followed for a period of almost two years. Thus, in common parlance the term once in a blue moon refers to an event whose occurrence is unlikely but not impossible.

We of the Foothills Astronomy Club hope you and your family will take the time to go outdoors tonight, New Years Eve, where a once in a blue moon will be waiting patiently for your attention and ready to usher in a new year.

And you can then decide for yourself if the moon really is blue!~ Skywatch written by Jim Cooper