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The Sky This Week, 2012 June 5 - 12

Chasing Venus across the Sun.

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The historic 1874 Alvan Clark & Sons Transit of Venus telescope
awaiting the next hole in the clouds over USNO, 2012 June 5


The Moon wanes in the morning skies this week, reaching the Last Quarter phase on the 11th at 6:41 am Eastern Daylight Time. Luna spends the week drifting through the star-poor regions of the so-called "water signs" of the Zodiac, finding very little in the way of bright companionship.

The much-anticipated Transit of Venus is now behind us. Most of us here in the Washington area suffered with cloudy skies, which only gave up a few very fleeting peeks of the dark disc of Venus edging its way across the face of the Sun. Here at the Observatory we greeted the event with a telescope that has had lots of experience observing Transits. Completed in 1874, the five-inch equatorial refractor was one of eight similar instruments commissioned by Congress for the U.S. Transit of Venus Expedition that were summarily dispatched to far-flung corners of the Eastern Hemisphere to record the event. Our telescope, No. 856, was under the charge of Prof. Asaph Hall (who would discover the moons of Mars a few years later) and successfully observed the transit from Vladivostok, Siberia. Eight years later Prof. Hall observed the second transit of the 19th Century with the same instrument at San Antonio, Texas. In 1928 the telescope was lent to the government of the Panama Canal Zone, which returned it to the Observatory in the mid-1990’s in its original shipping crate! After careful restoration by the USNO Instrument Shop, No. 856 once again saw the back side of Venus on June 8, 2004 from the Observatory’s roof. Would we be lucky enough to go 4-for-4? On the afternoon of June 5th we set up the telescope on the Observatory’s south lawn as tantalizing "sucker holes" opened and closed in the clouds. One allowed me to focus the telescope and adjust the solar filter. Another permitted a quick assessment of the telescope’s performance after nearly 140 years. Finally, the time of the transit approached, and the clouds…thickened! However, a couple of fleeting lucky breaks over the Sun’s eastern limb allowed for a couple of quick glances of Venus silhouetted on the Sun about half an hour after the transit began. Success! We now have one of a very few (if not the only) telescope that has observed four of the seven transits that have been observed in all of recorded history.

By the week’s end you might be able to catch a quick peek at Venus emerging from the solar glare. By the morning of the 12th she pokes above the horizon half an hour before the Sun, so if you have a good eastern horizon start looking for her at that time.

The early evening twilight sky hosts another evening appearance of the fleet planet Mercury, but unfortunately he never really climbs high enough in the west to escape twilight. Look for him below Castor and Pollux, the Gemini Twin Stars, half an hour after sunset towards the week’s end.

Ruddy Mars is now nearly 20 degrees east of the bright star Regulus in the constellation of Leo. He’s easy to spot in deepening twilight and is well-placed in the southwest for viewing once the sky is fully dark. Unfortunately he’s not much to look at anymore unless you have a large telescope. Most amateur instruments now show him as little more than a small gibbous-shaped pink dot.

Saturn is still the best thing going for evening observing. The ringed planet straddles the meridian an hour after sunset, at which time the sky is dark enough for a nice view through the telescope. Saturn will linger in this spot just under five degrees north of the bright star Spica for another few weeks before resuming his plodding course eastward against the stars. As always, the view of Saturn through almost any telescope is one of the most amazing sights in the natural world; while not as rare as a Transit of Venus, it is far more consistently visible. You won’t have to wait until December 8th, 2125 to see it again!

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