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The Sky This Week, 2017 June 27 - July 3

Looking through an ocean of air.
Jupiter, with the Great Red Spot and four moons
imaged 2017 June 29, 01:54 UT in poord seeing from Alexandria, Virginia

The Moon waxes in the evening sky this week, passing bright Jupiter on the evenings of June 30th and July 1st. First Quarter occurs on the 30th at 8:51 pm Eastern Daylight Time. As Luna waxes, she also moves southward along the ecliptic, ending the week among the stars of Scorpius.

The first full week of summer finds Washingtonians enjoying the latest sunsets of the year. From now through sunset on July 1st Old Sol dips below the west-northwest horizon at 8:38 pm EDT. As July opens, the time of sunset gradually becomes earlier, and by the end of the month sunset will occur 18 minutes sooner than it does now. Despite this, the overall length of day has already begun to shorten. At the solstice we saw 14 hours, 54 minutes of daylight. On July 1st that length is three minutes shorter. We’ll lose another 20 minutes of overall daylight buy the end of July.

This is another good week to explore the Moon with a small telescope. In fact, the Moon’s diminishing declination favors the smaller instrument over larger ones thanks to the phenomenon known as "astronomical seeing". This is a qualitative measurement of the overall stability of the air between your telescope and a celestial target. The lower an object is in the sky, the more of our atmosphere you must look through. Looking through more air means that you’re more likely to look through more cells of moving air, and these cells cause distortion of fine details on objects like the Moon. Larger telescopes look through more of these "seeing" cells, smearing out the fine details that would otherwise be resolved by the larger aperture. The best instruments for viewing objects like the Moon at low altitude will be telescopes in the three- to five-inch aperture, so dust off that small telescope and enjoy our only natural satellite.

Jupiter is still the dominant planet in the evening sky, but by 10:00 pm he shares the limelight with Saturn. Old Jove is well west of the meridian as evening twilight fades, and the best viewing will be during the fading twilight hour. This is actually my favorite time to view the giant planet and his four bright moons, which stand out nicely against the dark blue of the twilight sky. Like the Moon, Jupiter is subject to the vagaries of seeing. If you see the nearby star Spica twinkling rapidly, the air is unstable and a small telescope will be your preferred viewing choice. However, prolonged hot and humid nights often lead to stagnant air with few distorting currents of moving air columns. These can lead to some of the best views of the Moon and planets through larger aperture telescopes. On these nights it’s worth the sweat and mosquitoes for a great look at the solar system’s largest planet.

Saturn follows Jupiter into the sky, with both planets easily visible by 10:00 pm. Saturn transits the meridian just after midnight, but he’s just 30 degrees above the southern horizon when he does so. Like the Moon and Jupiter, his low altitude means the view of this giant distant world is greatly affected by our relatively thin atmosphere. Look to the nearby bright star Antares to judge the steadiness of the air.

You’ll find Venus in the pre-dawn sky, hovering at about the same altitude each morning at 5:00 am. If you follow the planet’s progress you’ll probably notice her motion to the north along the eastern horizon. This will continue until early August, when she begins to move toward the south again.

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