Blue Moon
© Astroprof
When you hear someone say "Once in a Blue Moon" you know what they mean. They're usually talking about something rare, silly, and even absurd. After all, when was the last time you saw the Moon turn blue? Well, rare or not, we're having one this week, and according to astronomer David Reneke writer and publicist for Australasian Science magazine, a Blue Moon is slated for the last day of this month, Friday, August 31.

It's not at all clear where the term 'Blue Moon' comes from. According to modern folklore it dates back at least 400 years. A Blue Moon is the second Full Moon in a calendar month. "Usually months have only one Full Moon, but occasionally a second one sneaks in, David said. "Ancient cultures around the world considered the second Full Moon to be spiritually significant."

Full Moons are separated by 29 days, while most months are 30 or 31 days long, so it is possible to fit two Full Moons in a single month. This happens every two and a half years, on average. By the way, February is the only month that can never have a Blue Moon by this definition. We had one Full Moon on August 2 this year and the second will be Friday night.

Does the Blue Moon actually turn blue? No. Physically colored Blue Moons are rare, and that's where the phrase comes from, "Once in a Blue Moon". There are occasions though when pollution in the Earth's atmosphere can make the Moon appear to look blue in color. The extra dust scatters blue light. For example, the Moon appeared bluish green across the entire Earth for about 2 years after the eruption of Krakatoa in 1883.

There were also reports of a blue-green coloured Moon caused by Mt. St. Helens in 1980 and Mount Pinatubo in 1991. "So in a sneaky sort of way, it could be true," Dave said with a grin. Look up at the night sky on August 31 and see for yourself. Everywhere in the world the full Moon rises in the east just as the Sun is setting in the west.

"Blue Moons don't have any real significance scientifically but they're fun to look at," David said. "Anytime you can get people out to look at the real sky to me is a great plus, enjoy it while you can this Friday night and while you're looking moonward, think of Neil Armstrong, OK?"