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The Sky This Week, 2016 May 17 - 24

A flowery Moon and a red planet.
The Moon, detail of Aristarchus region in Oceanus Procellarum,
imaged with the USNO's 12-inch f/15 Clark/Saegmüller refractor telescope
on 2014 January 13 (south at top)

The Moon dives toward the southern reaches of the Ecliptic this week, wending her way through summer’s rising stars before entering the sparse star fields of autumn. Full Moon occurs on the 21st at 5:14 Eastern Daylight Time. May’s Full Moon is popularly known as the Milk Moon, Flower Moon, or Corn Planting Moon. Look for the Moon on either side of the bright star Spica on the evenings of the 17th and 18th. On the 21st the Full Moon shares the sky with the planets Mars and Saturn along with the bright star Antares. Look for Luna about four degrees east of Saturn on the night of the 22nd.

Observing the Moon during the first part of the week reveals the largest of the large dark lunar plains, the so-called Oceanus Procellarum, or Ocean of Storms. This huge lava plain dominates the phases between First Quarter and Full Moon, and is noted for its relative lack of craters and its unusual "wrinkle ridges". There are a number of isolated craters that slowly reveal themselves as the phase increases, and many of them bear names of famous astronomers and philosophers. Here you’ll find the magnificent crater Copernicus, one of the most isolated of the Moon’s craters. Another smaller crater in the middle of the Procellarum plain is named for Johannes Kepler, which is close to the terminator line on the 17th. On the evening of the 18th a pair of similar-sized craters appear, one of which is shallow with a flat basin and the other a deep, terraced bowl. The latter crater is also one of the brightest features on the Moon and one of the youngest of the Moon’s larger features. These craters, known respectively as Herodotus and Aristarchus, seem to be the origin of one of the Moon’s most prominent sinuous rilles, a feature known as Vallis Schroteri, or Schroter’s Valley. Named for a pioneer lunar observer of the late 18th Century, it is believed to be the collapsed remnant of an ancient lava tube. Under good observing conditions a number of small volcanic domes can be seen in the hummocky terrain surrounding the craters, and the area is one where a number of mysterious faint glows have been detected over the years. These glows may be the last faint outgassing events of lunar volcanism, but their reality has never been fully proven. As Luna approaches the full phase, notice how the detail changes as the terminator line approaches the limb. The view through the telescope is now dominated by different shades of grey, and features are outlined by these differences. It’s a very different view, and one that few people ever bother to appreciate.

Jupiter is now best seen west of the meridian when the sky becomes fully dark. The giant planet still dominates the evening sky, but his time for god viewing through the telescope is beginning to grow shorter. By midnight he’s halfway between the meridian and the western horizon. That said, he’s still the best planet to view in the telescope, with his constantly-shifting bevy of moons and animated cloud belts. Look for the moon Europa to cast its shadow on the planet’s clouds starting at around 9:30 pm EDT on the 17th. On the 21st the Great Red Spot rotates across the disc during prime observing time.

Mars can now be spotted low in the southeastern sky as twilight ends. You shouldn’t have too much trouble spotting him as he is now almost as bright as Jupiter and has a very distinctive ruddy tint. The red planet reaches opposition on the 21st, when earth passes between him and the Sun. This is the best time to give him a look in the telescope. Even though his disc is only half as large as Jupiter’s you should try to give him a look. The faint dusky features you see are actual surface features, not the tops of clouds in an atmosphere.

Saturn follows Mars into the sky and will reach opposition in another two weeks. Saturn is also worth a good look. Even if his low altitude distorts fine details, the planet’s enigmatic rings will still provide some amazing views.

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