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The Sky This Week, 2016 June 21 - 28

Arc to Arcturus...
Mars, 2016 June 15, 02:10 UT
imaged from Alexandria, Virginia

The Moon wanes from the Full phase, moving into the morning sky as she wends her way through the sparse star fields of the autumnal constellations. Last Quarter occurs on the 27th at 2:19 pm Eastern Daylight Time. 

As we experience the year’s shortest nights in the wake of the summer solstice, we will soon see the latest sunsets for the year. Between June 24th and 30th the Sun sets at 8:38 pm EDT here in the Washington metro area. As July begins that time begins to slowly creep a bit earlier. The latest end of evening astronomical twilight falls on the 24th, when the last traces of sky glow from the Sun disappears from dark-sky locations at 10:37 pm! Those of us who like to travel to such locations look forward to the earlier onset of complete darkness since we no longer have to wait half the night for the conditions we need to enjoy the deep sky. Starting with the 24th the total length of daylight also begins to gradually shorten. By July 4th the day will be five minutes shorter than it was at the solstice. That said, the length of daylight won’t fall below 14 hours until the first week of August.

As evening twilight ends there is a great variety of celestial sights to enjoy on these midsummer nights. In addition to the bright planets, the bright star Arcturus is nearly overhead on the meridian, casting a rosy glow from its high perch. Arcturus is the brightest star in the northern hemisphere sky, partly due to its intrinsic brightness and partly due to its relative proximity at a distance of 37 light-years. If you look to the north of Arcturus you’ll notice the seven stars that form the asterism we call the Big Dipper. Notice the three stars that form the Dipper’s "handle". Follow the arc of the curve of the handle and you’ll see that it leads right to Arcturus, leading to one of the most familiar "signposts" in the sky: "follow the arc to Arcturus". Extend the line southward from Arcturus and you’ll "Speed on to Spica", the second brightest of the springtime stars. Look toward the east and you’ll see the rising stars of the Summer Triangle, Vega, Deneb, and Altair. Vega shines with a glow that’s almost as bright as Arcturus, but it has a noticeable blue tint that contrasts with the warmer glow of Arcturus. Vega is another relatively nearby star, located just 25 light-years from us. The southernmost star in the Summer Triangle, Altair, is also quite nearby, just 16 light years distant. Once could therefore naturally assume that the third star in the Triangle, Deneb, is also nearby, but in reality it is almost 100 times more remote than Altair!

Jupiter’s time in the evening sky is gradually dwindling as the giant planet wheels to the west before setting after local midnight. You still have a few hours to enjoy a look at him through the telescope, but plan on catching him as soon as he becomes visible in the twilight glow. He will soon become the focus of much media attention as the Juno spacecraft, which was dispatched to Old Jove in August, 2011, arrives for orbital insertion on July 4th. 

Mars now transits the meridian at the end of evening twilight, so you should have ample time to give him a look if you have a clear southern horizon. The red planet is now well west of the stars that form the "head" of Scorpius, and he shines with almost the same brightness as Jupiter. This is a good week to study him through the telescope in moments of steady air, since his most prominent albedo feature, Syrtis Major, is facing toward us. This feature, first identified by the Dutch astronomer Christiaan Huygens in 1656, should be easy to spot in a four-inch telescope.

Saturn transits about an hour after Mars and should provide some late-night entertainment. Despite his southerly declination, the planet’s famous rings will be visible in almost any telescope. Larger instruments will show more detail up to a point that’s limited by the turbulence of our atmosphere. On nights with very steady air look for Cassini’s Division about two-thirds of the distance from the inner edge to the outer. This gap in the rings is about as wide as the diameter of the Earth!

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