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The Sky This Week, 2013 June 25 - July 2

The year's latest sunsets, and what to see despite them.

 Saturn, 2013 June 22, 01:58 UT

The Moon wanes in the morning sky this week, tracing a lonely path through the rising dim stars of the autumn sky. Last Quarter occurs on the 30th at 12:54 am Eastern Daylight Time. The only object of first magnitude brightness that she shares the sky with will be the star Fomalhaut in the obscure constellation of Pisces Austrinus, the Southern Fish. Luna will pass some 25 degrees north of the star in the pre-dawn hours of the 27th and 28th.

We are now currently experiencing the latest sunsets of the year for mid-northern latitudes. Here in Washington Old Sol dips below the west-northwest horizon at 8:38 pm EDT and will continue to do so for the rest of the week. By the week’s end he will start to slowly drift south along the horizon, setting at 8:37 pm on July 2nd. At the same time sunrise is beginning to occur a bit later each day, paring three minutes off the total length of day as the week progresses. If you like long summer evenings, though, don’t fret; the Sun won’t set before 8:00 pm until mid-August.

The fading light of evening twilight betrays only one object that’s easily visible to the naked eye. The planet Venus may be found shortly after sunset low in the western sky. Half an hour after sundown she should be an easy object to spot some 10 degrees up. As the sky gets darker, sweep the area just to the right of the planet with a pair of binoculars on the evenings of the 25th and 26th. If you have a clear evening and a good low horizon you should be able to pick out the Twin Stars of Gemini, Castor and Pollux, before they set. Venus will pull away from the stars over the course of the week, gradually gaining altitude as she drifts southward above the skyline.

By about 9:15 pm you should be able to pick out the planet Saturn near the meridian, about halfway between the horizon and the zenith in the southern sky. This is the best time to observe the ringed planet in a telescope, since he’ll be at his highest altitude for the night. The planet’s magnificent ring system is currently tipped about 20 degrees to our line of sight, and they can be glimpsed with only slight optical aid. Owners of four-inch or larger-aperture scopes should be able to pick out some of the subtle features of the rings, including the famous Cassini Division. This dark line about one-third the distance in from the rings’ outer edge is a true gap in their structure caused by multiple gravitational resonances with the planet’s innermost large moons. In addition to the division itself, look at the differences in brightness and color of the rings on either side of the gap. The same four-inch telescope at a dark location should be able to reveal the planet’s brighter moons, Titan, Rhea, and Iapetus. Move up to an 8-inch scope and the moons Tethys and Dione should be easy to see. Our 12-inch telescope here at the Observatory will often show the tiny but geologically active moon Enceladus when the air is steady and clear. The "classical" moons Mimas, Hyperion, and Phoebe need larger instruments at dark-sky sites to track down. These nine satellites were all discovered before the "space age", but since the development of sophisticated ground-based imaging techniques and the visits by the Voyager and Cassini spacecraft the total number of Saturnian moons is now 62!

The absence of the Moon toward the end of the week opens up the "deep sky" for night owls and insomniacs. From dark-sky locations you can enjoy the misty band of light that defines the summer Milky Way passing through the rising stars of the Summer Triangle, Vega, Deneb, and Altair. The band runs down to the southern horizon, where you’ll find the distinctive fish-hook shape of the constellation Scorpius crossing the meridian at around midnight. This area of the sky is one of the most rewarding to scan with binoculars. Clouds of uncountable stars, mysterious dark nebulae, and bright knots of star clusters and glowing gas clouds dominate the view. This is where a small telescope at low magnification can really perform; I will spend hours scanning this part of the Milky Way with my little 3-inch refractor over the course of the summer months.

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