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The Sky This Week, 2010 May 11 - 18

Catch a young Moon and daytime sky delights
22- and 44-degree heliacal arcs
Imaged from the U.S. Naval Observatory on
2010 May 7, ~16:30 UT

The Moon returns to grace the early evening sky this week. New Moon occurs on the 13th at 9:04 pm Eastern Daylight Time. See if you can spot Luna as a very thin hairline crescent shortly after sunset on the evening of the 14th. This is one of the most favorable occasions this year for spotting the first crescent Moon within 24 hours of the New Moon phase. If you don’t see Luna on this evening, you should have no trouble spotting her on the next two nights. On the 15th she is just five degrees below bright Venus, while the next night she may be found seven degrees above the dazzling planet. On these evenings as well as the next two, see if you can see "the Old Moon in the arms of the New Moon", the phenomenon known as "Earthshine". During the Moon’s crescent phases it’s possible to see the ghostly outline of the portion of the Moon’s disc that’s not in direct sunlight. This is light reflected from our home planet off Luna’s dusty grey surface, which gives it a characteristic blue color. Earthshine becomes harder to see as the phase waxes, so try to catch it on these ideal evenings.

The daytime sky is something most of us pay very little attention to, but it is as much a part of Nature as the night. During the springtime months we are often treated to spectacular displays of "atmospheric optics" involving the Sun and various types of ice crystals that form high in the sky. While surface temperatures can often approach those of the summer, high clouds often form in layers of very cold air. This is one of the reasons why spring storms can sometimes be quite severe. Ice crystals blowing off the tops of distant approaching thunderstorms will often form wonderful arcs, halos, and other phenomena depending on their angles to the Sun. In addition, differently shaped crystals will cause different types of displays; sometimes you’ll see a complete ring around the Sun, while at others you may see arcs and "Sun-dogs". The possibilities are almost endless. This time of the year I almost always carry my camera with me to try to capture some of these elusive delights.

Back in the evening sky, Venus continues to slowly ascent in the west after sunset. Her brilliant glow pops out almost immediately after the Sun goes down, and if you know where to look you can spot her well before sunset if you can hide the Sun behind a convenient wall or tree. She sets about 40 minutes after the end of evening twilight, and this is about as late as she’ll set relative to the Sun for the duration of this apparition.

Mars continues to pick up the pace as he closes in on the bright star Regulus, lead star of Leo, the Lion. This week the red planet shaves another three degrees of separation distance from the star. He’ll pass less than a degree north of the star during the first week of June. While some amateur astronomers continue to glean hints of detail from his now tiny disc, most telescope owners will see little more than a pink-shaded dot.

Saturn on the other hand displays a generous pale yellow disc in the small telescope that sports two dagger-like extensions formed by the nearly edge-on rings. From Earth’s perspective the rings are actually closing up to a minimum of less than two degrees’ tilt that will occur in another few weeks before they gradually begin to open to their more familiar oval shape. The Sun, however, is almost five degrees above the ring plane, so a distinct shadow from the rings falls across the planet’s equator. This leads to an interesting illusion in the telescope eyepiece which makes the rings look "warped". This strange appearance should continue for much of the rest of the planet’s apparition.

Jupiter continues to loll in the pre-dawn sky, drifting slowly eastward against the dim stars of the rising constellation Pisces. Old Jove is biding his time right now, but once we’re well into summer he’ll move in to take Saturn’s place as the dominant planet of the season. Right now be content to see him in morning twilight.

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