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The Sky This Week, 2012 June 19 - 26

The Moon and Mercury Dance at Dusk.
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Saturn, imaged 2012 June 17, 02:08 UT


The Moon returns to the evening sky this week, waxing to the First Quarter phase on the 26th at 11:30 pm Eastern Daylight Time. Luna passes about seven degrees south of elusive Mercury on the evening of the 21st. To find the fleet planet on that evening, locate the Moon in a pair of binoculars shortly after sunset, then look to the right and slightly above the crescent’s upper cusp for a dusky star-like object. Mercury should pop out of the twilight half an hour to 45 minutes after the Sun goes down. On the nights of the 25th and 26th the Moon passes below the red planet Mars.

The astronomical season of summer officially begins with the occurrence of the summer solstice, which falls on the 20th at 7:09 pm EDT. At this time the Sun will be located directly over the Tropic of Cancer about halfway between Hawai’i and Midway Island. For residents of the Northern Hemisphere this will be the longest day of the year. Here in Washington we’ll experience 14 hours and 54 minutes of daylight. That’s a difference of nearly 5.5 hours over the length of day at the winter solstice! For several days the Sun will seem to pause in his north/south excursion, then gradually begin inching toward the celestial equator, which he will cross on the autumnal equinox. However, thanks to the Earth’s elliptical orbit around Old Sol, the date of the year’s latest sunset won’t occur for another week, so those of you who enjoy late afternoon or early evening activities will still have plenty of daylight to play in. The solstice is widely observed in many cultures, both ancient and modern. Perhaps the most famous solstice markers are the megaliths of Stonehenge, which have borne silent witness to the passing of the seasons for thousands of years. Similar alignments have been found in structures all over Europe, the Middle East, and even the American desert southwest. Tracking the yearly meanderings of the Sun was an important tool for the earliest civilizations, and the ability to harness the power of time gave rise to some of the most enduring cultures in the historical record.

The planet Mercury lurks in the evening twilight, never quite gaining enough altitude to be seen against a dark sky. As I mentioned earlier, the Moon provides a convenient guidepost to the fleet planet on the evening of the 21st. If the western sky is very clear on the evenings of the 23rd through the 25th you’ll find Mercury in line with Castor and Pollux, the Gemini twins. The best time to look for them will be about an hour after sunset when they will all be about 10 degrees above the west-northwest horizon.

Mars continues to move eastward among the stars of the constellation Leo. By the end of the week he passes some 12 degrees south of Denebola, the star that marks the mythical lion’s tail. At this time he will also receive a visit from the waxing Moon. Mars moves into the constellation of Virgo this week, where he will overtake and pass the planet Saturn in mid-August.

Saturn reaches the second stationary point in this year’s apparition on the 26th. He will cease his barely-perceptible westward progress at this time before resuming his slow march to the east. He will remain just under five degrees north of the bright star Spica for a few weeks before you’ll be able to notice any perceptible motion. The ringed planet is located on the meridian at sunset and should be easily spotted in the telescope half an hour later. You’ll have several hours to take in the sight of his rings and swarming moons before he drifts into the horizon haze.

The giant planet Jupiter should now be quite easy to spot in the pre-dawn sky. Old Jove rises about two hours before the Sun and is 10 degrees above the horizon at 5:00 am. If you have a clear eastern view try to spot the bright glow of Venus about halfway between Jupiter and the brightening skyline.

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