Image: Sky map showing crescent moon, Venus and Jupiter
Starry Night Software
A triple conjunction frames the crescent moon with the two brightest planets, Venus and Jupiter on Mon., August 13, 2012.
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updated 8/12/2012 4:34:29 PM ET 2012-08-12T20:34:29

Before the sun rises over North America on Monday, Aug. 13, Venus will shine just below and to the left of the thin waning crescent moon in the eastern sky. 

If you watch for a while you may notice the moon creeping closer toward Venus as they fade from view in the brightening sky, weather is permitting.

But if you follow Venus and the moon into the sunlight with binoculars or a telescope, or pick them up later in the day, you may be able to see the moon actually cover Venus in what astronomers call an "occultation."

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The view from out west
The occultation of Venus by the moon will be visible from much of North America. The farther west you are, the better the view. [ August's Major Night Sky Events (Sky Map Gallery) ]

Along the U.S. Pacific Coast, the moon will stand about halfway up in the southwest sky when it passes in front of Venus between roughly 1:05 and 1:45 p.m. Pacific Daylight Time. When Venus emerges between 2:20 and 2:50 p.m. PDT, the moon will be about one-third up from the west-southwest horizon. 

In contrast, along the Atlantic Seaboard, the moon will be very low — less than 5 degrees above the west-northwest horizon — when Venus vanishes between 4:35 and 4:55 p.m. EDT. The reappearance will occur after the moon has set. Five degrees of the sky is about half the width of your closed fist at the end of an outstretched arm.

Draw a line from Caribou, Maine to Huntington, West Virginia, and then down to Vermilion Bay in Louisiana. Anyplace to the right of that line will miss out on the reappearance since the moon will have already set.

For a map showing the region of visibility of this “Venus eclipse” as well as Universal Times of its disappearance and reappearance for over 700 cities, click here. The list not only includes places in North America, but Eastern Asia where the occultation will take place before sunrise — a spectacular sight indeed!

Seeking out the moon
Before the occultation however, give yourself plenty of time to find the moon because its low surface brightness against the night sky may make it hard to see. Finding the moon in the daytime may be easy or difficult depending on the clarity of the air. If the sky is clear and blue with little or no haze you should have little problem, but a hazy sky will make the search problematic.

Although Venus appears much smaller (1/75th as large as the moon), it has a much higher surface brightness and may be easier to spot in a bright sky. 

If your sky is clear, make a clenched fist and hold it out at arm’s length.  Both the moon and Venus, will be lower than the sun in the sky. Scan that region of the sky about 4-1/2 fists to the lower right of the sun.  If you spot the moon, point a binocular or small telescope at it, focus carefully and you should see Venus nearby.

Telescopes allow the best view
Perhaps the most satisfying way of enjoying this spectacle will be to watch in a telescope at very low power so that the images of the moon and Venus are both distinct and complete. In contrast to the moon’s delicate crescent shape, Venus will appear half illuminated. The sunlit portions of both objects will, of course, be facing the same direction toward the sun.

Viewed behind the eyepiece of a telescope, Venus will not disappear suddenly but will take about 25 seconds to fade out, since it’s not a pinpoint of light like a star but it has an appreciable angular size. The cratered, 16 percent sunlit lunar crescent will drift eastward toward the brilliant sunlit portion of Venus’ disk. The moon’s rough edge will bite into Venus and rapidly devour it. 

Then, just a little over an hour later it will reappear, literally from “out of the blue” from behind the moon’s opposite, unseen limb, again taking about 25 seconds to swell back to full brightness.  

When the planet emerges, it will first come suddenly into view against the blue sky as a starlike point that quickly enlarges into its “half moon” phase. Those who are blessed with very sharp eyes might possibly watch the entire event with no optical aid at all, although most people will need at least binoculars to watch for the moon pass in front of Venus.

And on the following morning, at the break of dawn, note how the moon has shifted well below and to the left of Venus in the eastern sky. 

Editor's note: If you snap an amazing photo Venus, the moon, Jupiter or any other night sky event, that you'd like to share for a possible story or image gallery, send images and comments (including name and location) to managing editor Tariq Malik at tmalik@space.com.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.

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