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The Sky This Week, 2015 July 21 - 28

Reflections on a bygone crescent.
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The Moon, with Venus (above) & Jupiter (far right)
Imaged from Alexandria, VA on the evening of 2015 July 18

The Moon waxes in the evening sky this week as she skirts the southern horizon. First Quarter occurs on the 24th at 12:04 am Eastern Daylight Time. Luna can be found on either side of the bright star Spica on the evenings of the 22nd and 23rd. On the evening of the 25th the Moon lies about four degrees west of golden Saturn. On the following night she forms one apex of a triangle with Saturn and the ruddy star Antares, the heart of Scorpius, the Scorpion.

Forty-six years ago the crew of Apollo 11 completed the first human mission to the Moon. If you look at the Moon on the evening of the 21st you’ll see almost exactly the same phase that greeted Neil Armstrong and Buzz Aldrin as they descended to the Sea of Tranquility on July 20, 1969. In the following three-and-a-half years ten more astronauts would sample different areas of the Moon’s surface, unlocking many of its secrets as well as many more regarding the origin of the Earth. And then we left. Today I often spend evenings at the telescope admiring the stark landscapes of the Moon’s battered surface and think about those days of first-hand exploration. While the Moon hasn’t changed much over the course of 46 years, there’s something just a bit different about it when I look at it now as opposed to the views I had before the Apollo era. It is now more of a place that we’ve visited as opposed to a distant light in the nighttime sky. Just over 10 days after Apollo 11 lifted off from Luna’s surface, Mariner 6 flew by Mars followed a week later by its twin, Mariner 7. Since that time we have had preliminary looks at all of the planets, two dwarf planets, several asteroids and comets, and extensively mapped the surfaces of Mercury, Venus, Mars, Jupiter, Saturn, and several dozen of their respective satellites. The recent flyby of Pluto by New Horizons has closed out this initial era of exploration. Right now the telescope is once again the only means to look at several of these far-flung places.

While reaching Pluto in less than a decade is an enormous achievement, it’s a sobering thought that at the speed of light we could make that trip in just over four hours. Our summer sky sports a number of bright stars which are considered to be relatively close, but at the same time their remoteness is staggering. Consider the star Arcturus, the brightest star in the northern sky. Now located high in the west after sunset, at the speed of light it would take 37 years to get there. At the speed of New Horizons it would take about 788,000 years!

We’re coming down to the wire on the evening apparitions of Venus and Jupiter, which can now only be seen in the hour after sunset low in the western sky. Venus should be easy to spot owing to her brightness, but Jupiter will require the sky to darken a bit before he pops into view. A pair of binoculars should now show Venus as a slender crescent, and as the days pass her crescent will become more slender as the distance between her cusps grows. By the end of the week both objects will set at around 9:30 pm EDT.

Saturn is now the sole planet available for telescopic viewing. The ringed planet crosses the meridian at sunset, and as twilight deepens you can find him just over 30 degrees above the southern horizon. Saturn’s rings are tipped a generous 24 degrees to our line of sight, and on a night with steady air you should have no trouble spotting some structure in them with a three-inch telescope. In particular look for a thin black line that separates the dusky outer "A" ring from the inner, whiter "B" ring. This gap, known as Cassini’s Division, is a void in the rings that’s about the equivalent to the diameter of Mars in its width. It is caused by a gravitational resonance with the four innermost large Saturnian moons that effectively sweeps stray ring particles from the area. However, some particles do exist quite happily in the gap for reasons which we still don’t understand. It’s gratifying to know that there are still some riddles to ponder within our own solar system!

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