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The Sky This Week, 2011 May 3 - 10

Bright stars of spring (all two of them), and a dawn planet traffic jam.

Saturn, 2011 May 1, 03:25 UT
Imaged from Alexandria, VA, USA

The Moon waxes through her crescent phases in the early evening skies this week. First Quarter occurs on May 10th at 4:33 pm Eastern Daylight Time. Luna spends most of the week ambling up through the setting winter constellations. Look for her slender crescent about eight degrees above the bright star Aldebaran in the gathering twilight of the 5th. On the 8th she lies about midway between the bright star Procyon in Canis Minor and the Gemini Twins, Castor and Pollux. By the week’s end she’s closing in on Regulus, brightest star in the constellation of Leo, the Lion.

May opens with the last of the winter constellations exiting the sky at the end of evening twilight. By 10:00 pm Orion and most of his attendant stars have slipped below the skyline, leaving only Procyon, the Gemini Twins, and the bright star Capella hanging over the western horizon. Just a few short months ago the long, cold nights were lit by what seemed to be a sky full of bright stars, but now as the nights are lengthening the bright stars are fewer and farther between. While the last of winter’s beacons exit the scent, only two bright stars are rising in the east. Arcturus will be the one to catch your eye. It is not only the brightest star in the northern hemisphere of the sky, it’s the fourth-brightest of all the stars, period. Arcturus has a distinctive rosy tint that tells us that it is an evolved star entering the twilight of its life, quite probably a good model of what the Sun will look like in a few billion years. Arcturus is bright because it is intrinsically some 200 times more luminous than Old Sol, and it is also comparatively nearby at a distance of just under 37 light years. If you stand and face Arcturus as it rises, you’ll find the seven stars of the Big Dipper off to your left. Follow the arc of the Dipper’s "handle" to Arcturus, then continue this line to the right. You’ll be "speeding on to Spica", the other bright springtime star. Spica displays a characteristic blue tinge that tells us that its surface is hotter than that of Arcturus or the Sun. It’s about seven times more distant as well, so its intrinsic brightness is over 10,000 times that of the Sun!

There’s a third bright object in the eastern sky as well, but this one is just a stone’s throw from us on the cosmic scale. Forming a triangle with rosy Arcturus and icy Spica is a yellow-hued object of similar brightness. This is Saturn, the outermost of the classical planets known to the ancients. Named for the Roman god of Time, Saturn takes almost 30 years to complete an orbit around the Sun. It took the invention of the telescope to reveal the true beauty of this distant world, which is surrounded by its famous system of rings. First described in the mid-17th Century, these strange adornments have been fascinating astronomers ever since. I never get tired of looking at them. I just wish we had better weather this spring so I can enjoy them more frequently!

Alas, Saturn is now the only planet visible in the evening sky. However, if you’re willing to get up before the Sun, this week and next offer the chance to see the rest of the naked-eye planets as they gather for a rare pre-dawn display. You’ll need binoculars, a very clear east-facing horizon, and absolutely clear skies to pull this off, but with some patience you will certainly be rewarded with a good show. The first thing to look for is Venus, which should be easy to spot half an hour before sunrise. Once you’ve found her bright glow, look just below her for the red-tinted glimmer of Mercury. Now look just over three degrees to the left of Mercury and you should spot the brighter gold tint of giant Jupiter. If it’s exceptionally clear look for ruddy Mars a bit less than two degrees to the left of Jupiter early in the week. By the week’s end Venus approaches to within half a degree of Jupiter for the closest planetary appulse of the year!

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