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March 19, 2010: When the sun sets on
Saturday, March 20th, a special kind of night will fall
across the Earth. It's an equal night.
Or as an astronomer would say, "it's an equinox." It's the
date when the sun crosses the celestial equator heading
north. Spring begins in one hemisphere, autumn in the other.
The day and night are of approximately equal length.
To celebrate the occasion, Nature is providing a sky show.
It begins as soon as the sky grows dark. The Moon
materializes first, a fat crescent hanging about a third of
the way up
the western sky. Wait until the twilight
blue fades completely black and you will see that the Moon
is not alone. The Pleiades are there as well.
The Moon and the Pleiades are having a close encounter of
rare beauty. There's so little space between the two, the
edge of the Moon will actually cover some of cluster's
lesser stars. According to David Dunham of the International
Occultation Timing Association, this is the best
Moon-Pleiades meeting over the United States until the year
2023.
Right: A similar Moon-Pleiades conjunction
photographed by Marek Nikodem of Szubin, Poland, in July
2009.
The Pleiades are a cluster of young stars some 440 light
years from Earth. They formed from a collapsing cloud of
interstellar gas about 100 million years ago. By the
standards of astronomy, that's really young. The
Earth under your feet is almost 50 times older. Dinosaurs
were roaming our planet long before the Pleiades popped into
being.
Only about seven of the Pleiades are visible to the unaided
eye. The "Seven Sisters" are Sterope, Merope, Electra, Maia,
Taygete, Celaeno and Alcyone, named after daughters of the
mythological Greek god Atlas. Together, they form the shape
of a little dipper, which is why the Pleiades are often
mistaken for the Little Dipper, an asterism of Ursa Minor.
Binoculars are highly recommended for this event.
First, scan the Moon. You'll see craters, mountains and lava
seas. Note that you can see the entire Moon, not just the
brightly-lit crescent. The Moon's dark terrain is illumined
by a ghostly glow called "Earthshine." It is the light of
our own blue planet shining down on the Moon.
Next, scan the sky around the Moon. The Pleiades come into
sharp focus---and they are more than seven. Dozens of faint
"sisters" can be seen through even modest optics.
This night doesn't sound equal. It sounds much better than
that.
Experience the equinox! |
