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The Sky This Week, 2017 July 5 - 11

Slogging through summer.
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Happy Birthday, America!
The view from the U.S. Naval Observatory, 2017 July 4

The Moon skirts the southern horizon this week, waxing to the Full phase on the 9th at 12:07 am Eastern Daylight Time. July’s Full Moon is popularly known as the Thunder Moon, Buck Moon, and Hay Moon. Look for Luna just under 10 degrees north of the ruddy star Antares in the constellation Scorpius on the evening of the 5th. On the following night she’s just two degrees above Saturn.

Earth reached its farthest point from the Sun on the 3rd at 4:11 pm EDT. This year, aphelion found us some 152,093,000 kilometers (94,526,000 miles) from the Day-star. Since Earth is moving at its slowest velocity in its orbit at this time, this makes Northern Hemisphere summer the longest of the astronomically defined seasons. By the same token our winter is the shortest. This situation is gradually changing, though, since the "anomalistic year", defined as the time between successive perihelion passages, is some 25 minutes longer than the "tropical year", which is the interval between successive spring equinoxes. Over the course of about 21,000 years the dates of aphelion and perihelion will precess through the tropical year, so in about 11,000 year boreal winter will be our longest season. For the most part, though, few of us would notice, since the departure of earth’s orbit from a perfect circle is minimal. This year the variation is around 5 million kilometers (just over 3 million miles).

The bright light of the waxing Moon washes out the delicate glow of the summer Milky Way, but that doesn’t diminish the stargazing potential of these short summer nights. As evening twilight gathers you can still catch the departing brighter stars of spring. The seven stars that make up the "Big Dipper" asterism hang high in the northwestern sky. Use the "arc" formed by the Dipper’s "handle" to find your way to Arcturus, the brightest star in the northern sky. Arcturus shines with a cheery warm glow than cuts through the atmospheric haze common to these more sultry July nights. Farther to the southwest you’ll find the blue star Spica in the sprawling constellation of Virgo. The nearby bright planet Jupiter will help you locate Spica on muggy nights. Once twilight fully ends after 10:30 pm the bright stars of the Summer Triangle advance toward the meridian, dominating the eastern sky. The brightest of these stars, Vega, leads the small constellation of Lyra, the Lyre, and has a striking blue tint. Vega is one of the closer first-magnitude stars to Earth, at a distance of 25 light-years. It has been used as a "standard" calibration star for over a century, but new research indicates that there are some very interesting things going on there. Its rotation has recently been found to be very rapid, and its equatorial diameter may be up to 20 percent larger than its polar diameter. It has also been found to be surrounded by a large disc of dusty material, so some astronomers now question about how "standard" it really is.

Jupiter appears in the southwestern sky soon after sunset. He still dominates the early evening twilight hours and remains visible until he sets shortly after midnight. Old Jove is still the most satisfying planet to observe with the small telescope, and you can always see some combination of his four bright Galilean moons. On nights of steady air you should also be able to spot the planet’s two equatorial cloud belts. These dark parallel stripes are low-pressure streams in Jupiter’s atmosphere that reveal clouds that are deeper than the bright surrounding zones. Look for the planet’s famous Great Red Spot on the evening of the 10th. On this date the Juno space probe will skim above this long-lasting storm in Jupiter’s atmosphere and return the highest resolution images we have ever had of this strange feature.

Saturn will soon become the sky’s best planet for telescopic viewing. You can spot the ringed planet in the southeastern sky at the end of evening twilight, and he crosses the meridian at 11:30 pm. You can spot his rings in almost any telescope, but you’ll need a night of very steady air and a telescope of at least four inches aperture to make out some of the more subtle details in the rings and on the planet’s disc.

Venus continues to grace the pre-dawn sky, shining down with a bright glow as morning twilight begins to brighten the sky. She maintains a nearly constant altitude at the same time each morning, but her azimuth gradually creeps northward over the next few weeks.

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