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The Sky This Week, 2016 July 5 - 12

Summer of the Scorpion.
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The Heart of the Scorpion: the star Antares (left)
and the globular cluster Messier 4

imaged from Morattico, Virginia, 2014 July 5
with an 80mm (3.1-inch) f/6 Antares Sentinel refractor
and a Canon EOS T2i DSLR

The Moon returns to the evening sky this week, waxing as she passes through the late spring and early summer constellations. First Quarter occurs on the 11th at 8:52 pm Eastern Daylight Time. Luna may be found just two degrees southeast of the bright star Regulus on the evening of the 7th. The Moon is also near bright Jupiter over the next two evenings. She will end the week in the vicinity of the bright star Spica.

In addition to being the 240th anniversary of American independence, July 4th found the Earth at aphelion, its farthest point from the Sun. At that time our fair home planet was just over 152 million kilometers (94,513,000 miles) from our daytime star. The annual excursion between our closest and most distant points from Old Sol amounts to just under 5 million kilometers (3.1 million miles), which makes our orbit almost circular. This is a very good thing for us, since this small excursion means our general climate is quite constant over the eons. There are various long-term changes in the eccentricity of the Earth’s orbit that take place on time-scales of hundreds of thousands of years which are primarily caused by the gravitational effects of the planet Jupiter. These forces, known as Milankovich cycles, also include a slow change in the tilt of Earth’s axis known as the Obliquity of the Ecliptic, which plays out over about 41,000 years, and Precession, which averages around 25,000 years. All of these cycles cause long-term effects on the planet’s climate and the occurrence of the seasons. Fortunately for us these cycles are very long and the Earth has time to adjust to the long-term changes, otherwise we probably wouldn’t be here to measure their properties! We do indeed occupy a very special little place in space.

The late evening hours now show off one of my favorite constellations of the summer sky, but you’ll need a clear sky and a good southern horizon the appreciate it. Fortunately there are two bright planets, Mars and Saturn, which will draw your attention to the pattern. Scorpius is now on the meridian at around 11:00 pm, and its distinctive shape doesn’t require the application of too much imagination to draw out. Ruddy Mars lies just to the west of a vertical group of three blue-tinted stars that mark the Scorpion’s head, while the ruddy star Antares, just a few degrees below yellowish Saturn, marks its heart. The Scorpion’s body loops down toward the horizon east of Antares, then curls up to a pair of close second-magnitude stars that mark the beast’s stinger. With a bright red "supergiant" star set in a moving group of hot blue stars, this pattern bears a physical resemblance to another familiar constellation that defines the winter sky, and interestingly the two are connected in sky lore. Orion, the well-known winter group, was dispatched by a scorpion in ancient Greek mythology, so when the pair were placed into the sky they were set on opposite sides so they would not share the same space at the same time.

Jupiter has welcomed a new satellite to its fold of 67 other known moons. Late on the evening of the 4th NASA’s Juno spacecraft executed a perfect orbital insertion rocket firing to allow it to be captured by the giant planet’s gravity. Unlike the previous Galileo orbiter, Juno will circle the planet over its poles, peering at the bright auroral displays and sampling the lethal radiation environment. It is expected to succumb to this radiation in about two years. It is interesting to note that all of the planets visible to the naked eye now have emissaries from earth in orbit around them.

Mars is now starting to move eastward toward the head of Scorpius. In another month he will pass just south of Dschubba, the middle star in the head. Right now he still presents a decent target for the modest telescope, although you’ll need a night of very steady air to make out much detail on his far-flung desert surface.

Saturn is on the meridian at 11:00 pm, so you still have ample time to take a look at his mysterious rings. Data from the Cassini space probe, which has been orbiting Saturn since July, 2004, indicate that the rings are a transient phenomenon. In a few million years they will fall into the planet’s atmosphere, so spend some time enjoying them now!

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