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?

Yet there was a time, not long ago, when Blue Moons were commonplace.
Crescent moons, half moons, full moons: they were all blue\--that is,
except on some days when they were green.

The time was 1883, the year Krakatoa, an island volcano in Indonesia,
exploded. There was nothing special about Krakatoa until, one day, ocean
water spilled into its magma chamber; hot expanding steam blasted the
island apart like a 100-megaton nuclear bomb. Tsunamis swamped nearby
islands (thirty-six thousand people died) and plumes of ash rose 50
miles high.

Krakatoa\'s ash is what made the moon turn blue. Some of the ash-clouds
were filled with tiny particles about 1 micron (one millionth of a
meter) across\--just the right size to scatter red light, while allowing
other colors to pass. Normally white moonbeams shining through the
clouds emerged blue, and sometimes green.

Ash quickly drifted around the world, and people everywhere were seeing
blue moons. They 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.

Those were the days; Blue Moons were really blue.

The modern definition of a Blue Moon is different: It\'s \"the second
full moon in a calendar month.\" Because the interval between full moons
is only 29 days, two full moons can fit in a 30- or 31-day month. This
happens approximately once every 2.5 years; the next one is due this
month, on July 31^st^.

\"Calendar Blue Moons\" are not rare. They happen more often than, say,
US presidential elections or Olympic games. In 1999, there were two
calendar Blue Moons in a three-month span! Nor are they blue. Calendars
don\'t affect the color of the moon, so a typical Blue Moon is
pearly-gray.

What happened? How did Blue Moons get their modern definition?

It\'s a long story involving old almanacs, Sky & Telescope magazine, the
radio program StarDate and the board game Trivial Pursuit. You can read
a folklorist\'s accounts here and here.

In short, \"Blue Moons\" are an offshoot of the custom, widespread in
many cultures, of naming full moons. June\'s full moon, for instance, is
called the \"Strawberry Moon.\" July\'s full Moon is the \"Buck Moon.\"
August has the \"Sturgeon Moon,\" September the \"Harvest Moon,\" and so
on. The Farmer\'s Almanac lists twelve moon names, one for each month of
the year. But what do you do when a month has *two* full moons? People
started calling the extra moon a \"Blue Moon.\"

When the Blue Moon rises on July 31^st^, it will probably look
pearly-gray, as usual. But you never know... Genuine blue moons are
still possible. Volcanic eruptions can cause them, and it doesn\'t have
to be a blast as big as Krakatoa. Sahara dust storms have turned some
moons blue, as have forest fires.

\"On September 23, 1950,\" writes physics professor Sue Ann Bowling
(University of Alaska), \"several muskeg fires that had been quietly
smoldering for several years in Alberta suddenly blew up into major \--
and very smoky \-- fires. Winds carried the smoke eastward and southward
with unusual speed, and the conditions of the fire produced large
quantities of oily droplets of just the right size (about 1 micron in
diameter) to scatter red and yellow light. Wherever the smoke cleared
enough so that the sun was visible, it was lavender or blue. Ontario and
much of the east coast of the U.S. were affected by the following day,
but the smoke kept going. Two days later, observers in England reported
an indigo sun in smoke-dimmed skies, followed by an equally blue moon
that evening.\"

Wildfire season in the western US will be near on July 31^st^. If any
fires produce ash or oily-smoke containing lots of 1 micron particles,
the Blue Moon there could be blue.

More likely, it\'ll be red. Ash and dust clouds thrown into the
atmosphere by fires and storms normally contain particles with a wide
range of sizes, mostly smaller than 1 micron. The tiny particles scatter
blue light, and let red light pass. As a result, red Blue Moons are lots
more common than blue Blue Moons.

Absurd? Yes, but that\'s what a Blue Moon is all about. Step outside at
sunset on July 31^st^, look east, and see for yourself.