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The Sky This Week, 2016 May 10 - 17

Scratching the Moon's surface.
Hyginus_12_160415_01small.jpg
The Moon, detail of Hyginus and Treisnecker region in Mare Vaporum,
imaged with the USNO's 12-inch f/15 Clark/Saegmüller refractor telescope
on 2016 April 15 (south at top)

The Moon waxes in the evening sky this week, reaching the First Quarter phase on the 13th at 1:02 pm Eastern Daylight Time. Luna begins the week passing between the Twin Stars of Gemini, Castor and Pollux, and Procyon, the brightest star in the diminutive constellation of Canis Minor. These last of winter’s constellations give way to Leo, where the Moon closes in on the bright star Regulus on the evening of the 13th. On the following night Luna inches close to Jupiter. By the week’s end she enters the large but faint constellation of Virgo.

After what seems like interminable clouds over the past few weeks we can only hope for a break in the local weather. Even if it’s not completely clear you can still enjoy views of the myriad surface features on our natural satellite, the Moon. This is far and away the easiest celestial target to observe, and just about any optical aid will reveal Luna to be a place as opposed to just an object in the sky. Binoculars will resolve many of the Moon’s largest features, and craters are easy to spot along the terminator, the line dividing the sunlit portion from that in shadow. Small telescopes of 3-inch aperture take the view to another level, showing hundreds of interesting features including craters of every size, isolated mountains, valleys, and sinuous "rilles". Larger telescopes will, of course, reveal finer details, but the Earth’s atmosphere limits the resolution of larger instruments to just under a kilometer, about the size of the famed Meteor Crater near our dark-sky station in Flagstaff, Arizona. There are thousands of named features on the Moon, so it really helps to have an atlas of these features. Fortunately today we have the benefit of the Internet and smart phones, and many such atlases may be found there. One of my favorite areas of the Moon lies near the center of the disc, in an area that’s cut by many long, fault-like formations. Best seen on the evening of the 13th, the region is notable for a crooked gash in the lunar surface with a shallow, sunken crater located at the bend in the cleft. The crater, known as Hyginus, seems to be related to the formation of the rille and is one of the few large craters on Luna’s surface that may be volcanic in origin. This region of the Moon is most subject to tidal flexing, and you’ll notice that there are many surface cracks in the vicinity. Just to the south of Hyginus is an impact crater, Treisnecker, which is surrounded by a number of intricate rilles in the otherwise smooth plain known as Mare Vaporum. Each passing evening brings new lunar landscapes and formations into view, and you can also see the previous night’s targets under different illumination. I often say that the Moon is "looked over, then overlooked" by most amateur astronomers, but it is nonetheless the easiest celestial object for the beginner and the advanced observer to explore.

As the Moon brightens the sky the fainter stars of spring’s constellations begin to get washed out. One star, though remains easily visible no matter how bright the sky. Arcturus is now high in the east as twilight fades, shining with a distinctive amber tint. At a distance of just under 37 light-years it is the closest "orange giant" star to the Sun and gives us a glimpse into Old Sol’s probable fate. Arcturus has exhausted the hydrogen in its core and is now fusing hydrogen in a shell around an inert helium core. Its girth has expanded to some 30 million kilometers, so its surface is cooler than that of the Sun. However, its surface area is much larger, so it shines with 170 times the Sun’s energy. It is the fourth-brightest star in the sky and is the only one I’ve seen with the naked eye in daylight, albeit from the summit of Mauna Kea in Hawai’i!

It’s been two weeks since I last saw Jupiter, and in that time the sky has shifted the giant planet to the west. He’s now near the meridian as evening twilight fades, and once the sky is dark he begins his journey toward the western horizon. You still have plenty of time to enjoy him in the telescope, but his disc is beginning to shrink as Earth puts more distance between us. Look for the famous Great Red Spot near the center of Jupiter’s disc on the evening of the 14th.

Mars is now retrograding through the stars at the "head" of Scorpius. This week he closes in on the star Dschubba, the middle star in the "head" asterism. By week’s end he’ll be just a degree from the star. Mars is still best seen in the early morning hours, but by the end of May the red planet will reach opposition and will be visible all night long.

Saturn lags behind Mars and still forms an attractive triangle with the red planet and the bright star Antares. Despite his low altitude, take a peek at him to marvel at his wide-open ring system. It is still one of the most amazing sights you’ll see in a telescope!

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