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April
24, 2009: If you're reading this at the end of the
day on Sunday, April 26th—stop! You're supposed to be outside
looking at the sunset.
On
Sunday evening, the crescent Moon, Mercury and the Pleiades
star cluster will gather for a three-way conjunction in the
western sky. It's a must-see event.
The
show begins before the sky fades to black. The Moon pops out
of the twilight first, an exquisitely slender 5% crescent
surrounded by cobalt blue. The horns of the crescent cradle
a softly-glowing image of the full Moon. That is Earthshine—dark
lunar terrain illuminated by sunlight reflected from Earth.
If the show ended then and there, you'd be satisfied.
Right:
A crescent Moon with Earthshine over Manassas, Virginia, on
Dec. 30, 2008. Photo credit: Karen Schmeets. [Larger
image]
But there's more.
Shortly
after the Moon appears, Mercury materializes just below it.
The innermost planet has emerged from the glare of the sun
for its best apparition of the year in late April—perfect
timing for a sunset encounter with the Moon. To the naked
eye, Mercury looks like a pink 1st-magnitude star. The planet
itself is not pink; it only looks that way because it has
to shine through dusty lower layers of Earth’s atmosphere.
A backyard telescope pointed at Mercury reveals a tiny fat
crescent. The innermost planet has phases like the Moon!
Next,
do nothing. Spend some quiet moments absorbing the view. As
the twilight deepens, your eyes will dark-adapt and—voilà!
There are the Pleiades.
Also
known as the Seven Sisters, the Pleiades are a cluster of
young stars about a hundred light years from Earth. They form
a miniature Little Dipper located, on this particular evening,
halfway between Mercury and the Moon. The brightest stars
of the cluster are only 2nd magnitude, not terrifically bright.
Nevertheless, the Pleiades are compelling in disproportion
to their luminosity. Every ancient culture--Greek, Maya, Aztec,
Aborigine, Māori and others—put the cluster in its myths and
legends. On April 26th you may discover why, even if you cannot
articulate your findings.
The
Pleiades, Moon and Mercury are all visible to the naked eye
even from light-polluted cities. Nevertheless, if you have
binoculars, use them. A quick scan of the threesome reveals
a rugged moonscape in startling detail, the rich pink hue
of Mercury, and many more than seven sisters (there are hundreds
of stars in the cluster).
Still
reading? Stop! Twilight awaits.
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Author: Dr.
Tony Phillips | Credit: Science@NASA
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