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February
26, 2009: Be careful, this sort of thing can cause
an accident.
On
Friday evening, Feb. 27th, the 10% crescent Moon will glide
by Venus, forming a gorgeous and mesmerizing pair of lights
in the sunset sky. Moon-Venus conjunctions are not unusual,
but this conjunction has some special qualities:
(1)
Venus is at maximum brightness: magnitude -4.6. The planet
is twenty times brighter than Sirius, the brightest star in
the sky. It is so luminous that it can actually shine through
thin clouds and cast subtle shadows on the ground.
Right:
A Moon-Venus conjunction in Dec. 2008 photographed by Tamas
Ladanyi of Mönichkirchen, Austria. This month's conjunction
will be even tighter and brighter. [larger
image]
(2)
As seen from North America, the Moon-Venus separation is only
a little more than 1o. Stick up your thumb and
hold it out at arm's length. Venus and the Moon will fit comfortably
behind the thumb-tip. Tight conjunctions like this are the
most beautiful of all.
(3)
Not only is the Moon a crescent, but so is Venus. A small telescope
pointed at the glittering planet will reveal a slender 20%-illuminated
disk.
Add
it all together and you've got a major distraction. Evening
drivers should pull to the verge. Staring at Venus and the
Moon could be riskier than texting!
Venus
is a crescent because, like the Moon, it has phases. The planet
can be be full, gibbous, new, or anything in between. The
illuminated fraction we see on any given date depends on how
much of Venus' nightside is turned toward Earth.
It
might seem odd that Venus is brightest now when it is a crescent.
That reverses our commonsense experience with the Moon, which
is brightest when it is full. A 6-month animation of Venus
created by Hong Kong astrophotographer "Wah!" solves
the mystery at a glance:
The crescent phase of Venus occurs when Venus is close to
Earth, very big and bright. The full phase of Venus, on the
other hand, occurs when Venus is on the opposite side of the
Sun, far away and relatively dim.
Crescent
Venus is so bright, you can see it in broad daylight. During
the day on Friday, scan the sky for the crescent Moon. Hint:
Stand in the shadow of a tall building to block the glare
of the Sun. At noon, the Moon will be due east of the Sun's
position. Got it? Look a few thumb-widths around the Moon
and—voilà!—Venus pops out of the blue. The planet is surprisingly
easy to see when you know where to look.
Once
daytime Venus has been located, you might feel tempted to
examine the planet with binoculars or a telescope. Don't.
The nearby Sun can damage your eyes if you accidentally point
your optics in that direction.
Wait
until the Sun sets and behold the pair framed by deepening
twilight blue, first with your unaided eyes, then with a small
telescope. On the Moon, you will see mountains, craters, and
a vast expanse of nighttime lunar terrain gently illuminated
by Earthshine. On Venus, you will see a delicate little crescent
of impenetrable clouds.
It's a nice way to end the day.
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Author: Dr.
Tony Phillips | Credit: Science@NASA
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