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The Sky This Week, 2011 March 29 - April 5

Evening planets: Two down, one to go.


 Perigee Moon, 2011 March 19
Imaged with an Antares Sentinel 80-mm f/6
refractor telescope and a Canon PowerShot A95
digital camera from Alexandria, VA

The Moon greets early risers as the week opens, then returns to the evening sky as a graceful slender crescent as it ends. New Moon occurs on April 3rd at 10:32 am Eastern Daylight Time. Look for the Moon about five degrees north of brilliant Venus in the brightening twilight on the morning of March 31st. If you have a flat eastern horizon you should still be able to catch the last waning lunar crescent about 13 degrees to the left of the dazzling planet at about the same altitude

The absence of the Moon from the evening sky gives you another opportunity to participate in the annual "Globe At Night" citizen-science campaign. Last month we used the stars in the constellation Orion to gauge the local sky brightness, but Orion is heeling rapidly to the west, so this month the focus is on the constellation of Leo, the Lion. To participate in the program go to the project’s website, download the finder charts for Leo, and simply count the number of stars that you see. You can report your results via the website or your smart phone. The program hopes to gather some 15,000 observations by the time the campaign ends, and so far they have received more than 10,500 reports from around the world. Leo may be found near the meridian at around 10:30 pm EDT, and while the Lion lacks the flash and color of Orion, his distinctive feline profile should make him easy to locate from your yard.

The early evening sky is now quite bereft of planets. Mercury, which reached his greatest elongation in the evening sky last week, fades rapidly as he plunges toward the horizon and conjunction with the Sun in another 10 days. Fortunately, NASA’s MESSENGER orbiter should begin sending back its first close-up images of Mercury’s surface beginning on March 29th. It will spend at least a full Earth year orbiting the fleet planet, mapping its entire surface to a resolution of a few hundred meters.

Giant Jupiter passes behind Old Sol on the 6th, so he is essentially invisible for the next several weeks. Jupiter will emerge from the Sun’s glare by early May and will once again lord over the evening skies in the fall and winter.

Saturn is now the only planet easily visible during the overnight hours. The ringed planet reaches opposition on April 3rd, rising at sunset and setting at sunrise. Pay extra attention to him on the nights just before and after opposition to see if you can spot the interesting "Seeliger Effect", in which the planet’s rings and small moons gain about half a magnitude in brightness for a day or so around opposition itself. The effect is caused by a very high phase angle with the Sun, similar to the extra brightness we see in the hours around a Full Moon here on Earth. The rings in particular become brighter than most features on the planet’s disc. Normally they are slightly dimmer than the bright equatorial cloud belt.

Venus still hangs in the brightening morning twilight sky. She plays host to the Moon on the last mornings in March, and she will linger in the "twilight zone" until well into the summer months.

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