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The Sky This Week, 2013 May 21 - June 4

A planetary traffic jam!
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 Venus and Mercury over the Air Force Memorial, 2007 FEB 7


"The Sky This Week" will be visiting some of our "sister" institutions in London and Edinburgh next week, so we now present an extended look at things that will be happening in the sky until our return.

The Moon brightens the overnight hours this week, diving southward along the ecliptic as she passes from the springtime constellations into the rising summer stars. Full Moon occurs on the 25th at 12:25 am Eastern Daylight Time. May’s Full Moon is variously known as the Milk Moon, Flower Moon, or Corn Planting Moon. Look for Luna this week near the bright star Spica on the evening of the 21st. On the following night she sits about five degrees below the golden glimmer of Saturn. On the 24th try to spot the second-magnitude star Graffias less than a degree from Luna’s bright northern limb. The Moon moves into the morning sky and joins the dim constellations of autumn as May passes into June. Last Quarter occurs on May 31st at 2:58 pm EDT.

The main attraction for the next few weeks is the gathering of planets in the evening twilight sky. Mercury, Venus, and Jupiter will be playing a drawn-out game of "celestial tag" this week, then the two inner planets will abandon Jupiter to the encroaching Sun as June opens. Mercury and Venus steadily march up from the west-northwest horizon, edging closer to Jupiter into Memorial Day weekend. On the evening of the 26th they form a close equilateral triangle less than two degrees on a side. Just three degrees to the north of this triangle is the second-magnitude star El Nath, the northern "horn" of Taurus, the Bull. Over the course of the next several evenings Mercury speeds past Jupiter and begins to stretch the triangle into a line. On the 28th Venus passes a mere one degree north of Jupiter, and the two bright planets will be about three degrees below fainter Mercury. By June 1st Mercury will have moved eight degrees east of Jupiter, and Venus will be parked halfway between the fleet planet and Old Jove. By this time Jupiter will become quite difficult to observe, so be sure to bring binoculars to your favorite sighting spot. The best time to look for this planetary trio will be at around 9:00 pm EDT. Twilight will still be fairly bright, but both Venus and Jupiter should be bright enough to see with the naked eye. Mercury will be a bit fainter, so use binoculars to locate him if you need to. This is one of the best chances you’ll ever have to catch a glimpse of the elusive Mercury, which never strays very far from the Sun. Not only will the fleet planet put on his best evening apparition for the year, he’ll be in the company of easy-to-find Venus for most of the time he’s visible. It has been said that the great 16th Century astronomer Copernicus never saw Mercury owing to the persistent mists of the Vistula River; his account of it in his epic work "De Revolutionibus…" was based on observations by other sources! Not only will you have a chance to see him for yourself, but if you have a digital camera you can try to capture images of this spectacular event. I’m hoping to spot it from the old Royal Observatory at Greenwich!

The twilight planetary gathering will upstage Saturn for the next week or so, but rest assured that the ringed planet isn’t going anywhere for awhile. He will patiently await your perusal as darkness falls, becoming prominent in the southeast at the end of evening twilight. By June 1st he crosses the meridian at around 10:45 pm, which should be prime time for telescopic perusal. By this time Saturn has moved far enough away from opposition to exhibit a tiny phase effect. You can see this as a distinct dark line where the rings go behind the planet on the east side of the disc. It should be readily apparent in a four-inch telescope on nights when the atmospheric "seeing" is very steady.