The Sky This Week, July 14 – 21, 2015 !

The Sky This Week, 2015 July 14 – 21

Waiting for word from Pluto.
Saturn and six moons, 2015 July 12, 03:04 UT
Imaged with the U.S. Naval Observatory’s 1895 12-inch Clark/Saegmüller refractor

The Moon returns to the evening sky this week, offering up a nice photo opportunity as she returns to grace the southwest horizon. First Quarter occurs on the 24th at 12:04 am Eastern Daylight Time. The photo op takes place on the evening of the 18th, when Luna’s slender crescent joins Venus and Jupiter in evening twilight.

The highlight of the week has already taken place, but we will only begin to understand its magnitude as the week passes. The event is the historic flyby of the distant world Pluto and its satellites by the New Horizon’s space probe on the 14th. For weeks we have seen Pluto grow from a dot in the probe’s long-range cameras to a world with tantalizing surface features that hint at a very complex geologic past. For us here at the USNO it is an especially exciting time, since it was just over 37 years ago, on July 7, 1978, that we announced our discovery of Charon, Pluto’s largest moon. Charon is also showing hints of an extremely interesting surface which may be fundamentally different from that of Pluto, but as with the scientists at mission control, we’re going to have to wait to find out. New Horizons is so far away and it will be so busy during the fleeting hours of close encounter that it won’t be able to return any data in “real time”, so we won’t start getting images from the encounter until later this week. Even then, we’ll just see a trickle of the trove of information recorded in the probe’s computer. It may be well into the fall before all of the information is downlinked to Earth! Interestingly, this flyby comes almost fifty years to the day after Mariner 4 returned the first close-up pictures of Mars. Those first crude images started a torrent of subsequent robotic exploration which has now visited all of the major (and several minor) members of the solar system.

Pluto is currently just past opposition, and can be located in large telescopes among the dense starfields of the Sagittarius Milky Way in a small asterism known as “The Teaspoon”. It is a very difficult observation to record Pluto, as it looks like one of hundreds of thousands of faint stars in the field of view. I have viewed it on a handful of occasions, but the next time I see it I will think of it in an entirely different light.

Much easier to observe is the bright pair of Venus and Jupiter which are now setting in the evening twilight sky. Look to the west shortly after sunset to see Venus first, then wait another few minutes to see Jupiter pop into view a few degrees to the right of the dazzling planet. The two objects are about five degrees apart throughout the week. On the evening of the 18th the waxing crescent Moon passes by the duo, just one degree below Venus. A low-power wide-field telescope trained on the Moon and Venus should show both objects as graceful crescents in the deep blue of the twilight sky.

Saturn spends the evening hours in the western half of the southern sky, but he still puts on a good show for the small telescope. He’s near his highest point above the southern horizon in deepening twilight, and by 9:30 pm he should be ideally placed for viewing. As the sky darkens around Saturn see if you can spy some of the planet’s many moons. The largest, Titan, should be visible in almost any telescope. A four-inch telescope should show Tethys, Dione, Rhea, and the enigmatic Iapetus when the latter is west of the planet. An 8-inch telescope under a dark sky may reveal Enceladus, a small icy sphere that appears to be erupting plumes of liquid water from its south polar region. You’ll need a 16-inch instrument to spy Mimas, the innermost of Saturn’s “classical” moons.


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