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The Sky This Week, 2017 May 2 - 9

Scaling the Moon.
Alphonsus_161107red_02small.jpg
The "Three Walled Plains", lunar craters Arzachel, Alphonsus, and Ptolomaeus
imaged 2016 November 7 with the USNO's 30.5-cm (12-inch) f/15 Clark/Saegmüller refractor,
1.6X Antares Barlow lens, and a Canon EOS Rebel T2i DSLR

The Moon brightens the evening sky as she moves through the springtime constellations this week, plunging to the southern reaches of the ecliptic.  She waxes from First Quarter to Full Moon, which occurs on the 10th at 9:42 pm Eastern Daylight Time.  Luna spends the evening close to the star Regulus on the evening of the 3rd.  On the 7th she may be found within two degrees of the bright planet Jupiter.

This is another fine week to explore the surface of our nearest celestial neighbor.  The Moon is an ideal target to explore with virtually any kind of optical aid.  It was a view of the Moon through my father’s binoculars at the age of seven that whetted my interest in astronomy, and it is still a great way to introduce anyone to the fascination that is inherent in space exploration.  The slightest magnification transforms the Moon from a glowing object in the sky to a place, one that has topography in mountains and craters and their associated shadows.  These land forms are unlike anything that we routinely experience on Earth, and they are further accentuated by the absolute starkness of the shadows which look as if they were painted with India ink.  While the craters may appear to be small in small- to modest-aperture telescopes, their true size is masked by the nearly quarter-million mile distance of Luna’s surface from us.  Even in our venerable 12-inch telescope here at the Naval Observatory the smallest features we can see on the Moon are about the size of Meteor Crater, about 40 miles east of our Flagstaff Station.  That earthly impact feature, about one kilometer across, would barely be visible if our telescope were located on the Moon.  That means that the craters that look impressive in our eyepieces are truly enormous.  Most of the craters in the Moon’s battered southern region have diameters that measure in the tens of kilometers/miles and more.  Look at the picture we’re featuring this week.  The three prominent craters to the right of center will be well-placed for viewing on the evening of the 3rd and following evenings.  Closest to the center is Arzachel, named for a 12th Century Persian astronomer.  It is 97 kilometers (59 miles) across.  Next to it is Alphonsus, named after King Alfonso X of Spain, whose 13th Century planetary tables were widely used until the time of Kepler.  It is 118 kilometers (71 miles) from rim to rim and was the target of the Ranger 9 lunar “hard lander”.  The largest and oldest of the “Three Walled Plains” is Ptolomaeus, named after the Greek astronomer Claudius Ptolemy, author of The Almagest, one of the first epic treatises on astronomy.  It is 154 kilometers (93 miles) across, roughly the distance from Washington to Richmond, Virginia!

Mars continues to loiter in the western sky, but you’ll have to contend with deepening twilight and atmospheric haze to see him.  He is gradually losing his year-long race with the Sun, but he will still be about 10 degrees above the western horizon at 9:00 pm.  This week he glides to the north of the bright star Aldebaran in Taurus.  

Jupiter is well-paced for viewing as evening twilight fades to darkness.  The giant planet crosses the meridian just after 11:00 pm EDT, so you have all of the evening hours to view him through the telescope.  On the evening of the 4th you can watch the moon Io drag its shadow across the planet’s disc between 9:00 and 10:00 pm.  On the evening of the 6th the Great Red Spot will be ideally placed for evening views.  On the 9th you can watch Jupiter’s largest moon, Ganymede, emerge from the planet’s shadow at 10:50 pm, joining its other three companions in an unusually tight formation around the hulking planet.

Saturn rises at around 11:00 pm by the end of the week, but your best view of him will still be before the onset of morning twilight.  Despite the fact that his rings are tipped close to their widest presentation toward us, the planet never gets higher than 30 degrees above the southern horizon.  You’ll need very steady morning air to see their finer details.

Venus greeted me at 5:30 this morning, as she now greets many of my fellow dog walkers.  You’ll be hard-pressed to miss her if you’re up before the Sun.

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