Blue Moon

The month of July 2004 has two full moons, which means one of them is a Blue Moon. But will it really be blue? Believe it or not, scientists say blue-coloured moons are real.

When you hear someone say "Once in a Blue Moon…" you know what they mean: Rare. Seldom. Maybe even absurd. After all, when was the last time you saw the moon turn blue?

On July 31st, you should look, because there's going to be a Blue Moon.

According to modern folklore, 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. 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.

July has already had one full moon on July 2nd. The next, on July 31st, is by definition a Blue Moon.

But will it really be blue? Probably not. The date of a full moon, all by itself, doesn't affect the moon's color. The moon on July 31st will be pearly-gray, as usual. Unless....

There was a time, not long ago, when people saw blue moons almost every night. Full moons, half moons, crescent moons--they were all blue, except some nights when they were green.

One way to make a blue moon: use a blue filter. That's what Kostian Iftica did on July 2nd when he photographed this full moon rising over Brighton, Mass.

The time was 1883, the year an Indonesian volcano named Krakatoa exploded. Scientists liken the blast to a 100-megaton nuclear bomb. Fully 600 km away, people heard the noise as loud as a cannon shot. Plumes of ash rose to the very top of Earth's atmosphere. And the moon turned blue.

Krakatoa's ash is the reason. Some of the ash-clouds were filled with particles about 1 micron (one millionth of a meter) wide--the right size to strongly scatter red light, while allowing other colors to pass. White moonbeams shining through the clouds emerged blue, and sometimes green.

Blue moons persisted for years after the eruption. People also saw lavender suns and, for the first time, noctilucent clouds. The ash caused "such vivid red sunsets that fire engines were called out in New York, Poughkeepsie, and New Haven to quench the apparent conflagration," according to volcanologist Scott Rowland at the University of Hawaii.