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The Sky This Week, 2011 May 10 - 17

Crater-hopping on the Moon, and call a traffic cop for the morning sky!


Lunar craters Tycho (top center) and Clavius
imaged from Alexandria, VA on 2011 FEB 13

The Moon brightens the springtime sky this week, waxing through her gibbous phases among the stars of Leo, Virgo, and Libra as she grows to Full Moon, which occurs on the 17th at 7:09 am Daylight Time. May’s Full Moon is variously known as the Flower Moon, Milk Moon, or Corn Planting Moon, depending on whose mythology you prefer. Look for Luna near the star Regulus on the evenings of the 10th and 11th. On the 13th she can be found nine degrees southwest of Saturn, then on the following night she is within four degrees of the bright star Spica. The Full Moon rises to the northwest of the ruddy star Antares in the constellation of Scorpius at sunset on the 17th.

The scattered stars of spring become washed out as the Moon waxes toward her brightest phase, but this shouldn’t discourage you from getting the telescope out for a look. As the terminator line creeps slowly across Luna’s face, some of the Moon’s best-known formations come into view. As each night passes you can see the interesting interplay of light and shadow as new features are revealed and older ones fall under higher Sun-angles. The vast smooth plain of the Imbrium basin, the lava-filled scar from the impact of a large asteroid some 3.8 billion years ago, is a stark contrast to the shoulder-to-shoulder craters of the southern "highlands" which bear mute testimony to the ferocity of bombardment the Moon suffered in its earliest years of formation. The prominent crater Tycho becomes visible in the jumbled crater fields of the south on the evening of the 11th. As the evenings pass and the Sun climbs higher over the formation bright "rays" become apparent; these are blankets of ejecta thrown out by the impact that formed Tycho over 100 million years ago. On the 12th, look to the south of Tycho for the very large eroded crater Clavius. This formation is very distinctive due to the arc of secondary craters that span its floor. These craters form an almost perfect progression from large to small. How they came to be in this particular arrangement is still a mystery.

Sharing the sky with the Moon this week is another object that’s well worth your attention. The ringed planet Saturn is now perfectly placed for evening observation, and on the 13th he lies about nine degrees to the upper left of the Moon, making for an easy telescopic "two-fer". Saturn’s distinctive yellow hue contrasts nicely with the silvery-grey of the Moon and the pale blue glimmer of the bright star Spica nearby. Through the telescope Saturn will probably be something of a disappointment after looking at the Moon, but once you realize the distance to his cloud-shrouded face (some 900 million miles vs. a mere 238,000 to the Moon) he suddenly becomes much more interesting. You may notice a faint star-like object in Saturn’s tow. That’s his largest moon Titan, which is nearly twice the size of our natural satellite!

The real action this week occurs low in the eastern sky just before dawn. There is a veritable traffic jam of planets that rise about an hour before the Sun, but your best chance of glimpsing them will be about 5:40 am local time. Mercury, Venus, and Jupiter start the week off in a tight scrum with all three objects separated by just over one degree. On the morning of the 11th Venus passes just half a degree below Jupiter in the closest planetary appulse of the year. Mercury may be found about a degree below Venus, and the two objects keep pace with one another through the rest of the week as they leave Jupiter behind. By the week’s end they are catching up to ruddy Mars. You’ll need binoculars, clear skies, and a very low eastern horizon to catch this morning dance, but it will be worth getting up for. Groupings such as this one are comparatively rare!

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