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The Sky This Week, 2017 July 11 - 25

Meandering along the Milky Way
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The Summer Milky Way
Imaged on 2013 August 12 from Fishers Island, NY

We’ll be away on vacation until July 25, so this edition covers the next two weeks.

The Moon moves northward through the autumn constellations as she wanes in the morning sky over the next two weeks. Last Quarter occurs on the 16th at 3:26 pm Eastern Daylight Time. She disappears into the glare of the Sun as she passes through New Moon on the 23rd at 5:46 am. Look for Luna close to dazzling Venus in the pre-dawn sky on the morning of the 20th. The next New Moon, on August 21st, will cross the ascending node of its orbit directly between the Earth and the Sun, creating a total solar eclipse that will be the first one to cross the U.S. from coast-to-coast since 1918. You can find lots of information on this rare event on our website at http://aa.usno.navy.mil/data/docs/Eclipse2017.php

As the Moon moves into the morning sky we can turn our attention to the showpiece of the summer sky, the shimmering star clouds of the Milky Way. Evening astronomical twilight ends at around 10:30 pm, and at this time the bright stars of the Summer Triangle are high in the eastern sky. Vega, Deneb, and Altair are bisected by the Milky Way, and as you scan toward the south the galaxy’s bands become brighter. Not only does the Milky Way bisect the Summer Triangle, it splits into two distinct parts in this part of the sky. The "Great Rift" can be traced down to the southern horizon. This dark band is caused by cool clouds of gas and dust that absorb the light of more distant stars and contains the building blocks of new stars and planetary systems. The most prominent of these dark spaces are best seen from equatorial sites and regions in the Southern Hemisphere, where they pass overhead at this time of the year. They are so prominent from these areas that the ancient Inca made up their sky lore around the shapes of the dark clouds. Exploring the Milky Way is one of my favorite activities when on vacation, and I am planning to spend some quality time under dark skies with a pair of binoculars and a small telescope to enjoy some of the Milky Way’s features. Scanning the Milky Way with binoculars will begin to resolve some of the brighter patches of the Milky Way into clouds of countless stars interspersed with isolated knots of nebulous light. Applying a low-power telescope to many of these patches will resolve them into clusters of stars and clouds of glowing gas. By midnight the center of the Milky Way crosses the meridian, just west of the "spout" of the "Teapot" asterism of Sagittarius.

Jupiter continues to week watch over the evening hours from the southwestern sky. The giant planet is best seen in evening twilight, offering a couple of hours of good telescopic viewing before he dips toward the horizon where he starts to wallow in denser air. You should be able to see the nightly configuration of his four bright Galilean moons through any telescope as long as the planet is visible, but to see any detail on his distant cloud tops requires very steady air and a modest-aperture instrument.

Saturn gains prominence as Jupiter exits the night sky. The ringed planet crosses the meridian at around 11:00 pm, and lies on the western edge of a dense Milky Way star cloud. From a dark site the view of Saturn, surrounded by a bevy of his moons and a myriad of background stars hints not only of the huge distance to the planet but also the sheer number of stars beyond the solar system’s outer bounds.

Look for dazzling Venus hovering in the northeastern sky as morning twilight gathers. She will get a visit from the Moon on the morning of the 20th, when she will be only three degrees from the waning crescent.

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