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The Sky This Week, 2018 May 15 - 22

Another week to explore the Moon.
The Moon, 2018 April 21, 01:02 UT,
imaged with the USNO's 30.5-cm (12-inch) f/15 Clark/Saegmüller refractor
and a Canon EOS Rebel T2i DSLR

The Moon returns to the evening sky this week, waxing through her crescent phases toward First Quarter, which falls on the 21st at 11:49 pm Eastern Daylight Time.  Look for the Moon near brilliant Venus in the twilight of the evening of the 17th.  She passes less than a degree north of the bright star Regulus on the evening of the 21st.

Take advantage of the Moon’s northerly declination this week to explore her stark barren landscapes.  The nights around the First Quarter phase are my favorites for lunar exploration with the telescope, and you can start enjoying the view long before the onset of astronomical darkness.  No matter what size instrument you use, Luna never disappoints, showing off her smooth lava plains that bear the romanticized names of hypothetical “seas” and the rugged, battered terrain of her “highlands”.  The vast lava plains are the remnants of collisions with large asteroids during the first billion years after the Moon’s formation.  The huge craters carved by these impacts gradually filled with lava to form these relatively smooth areas.  The last of these great impacts occurred about 3.5 billion years ago, so any features that you see here are considered to be “young” by lunar standards.  The brighter terrain bears testament to the violence of the early solar system’s formation.  Impacts of countless “planetessimals” have left a landscape of shoulder-to-shoulder craters ranging in size from hundreds of kilometers to pits at the limit of terrestrial resolution, a few hundred meters across.  To perhaps convey a sense of scale, the famous Meteor Crater in Arizona, which is very impressive to look at standing on its rim, would be invisible to all but the largest terrestrial telescopes at the Moon’s distance!  A typical lunar crater seen through a small telescope spans a distance across its rim that’s comparable to the distance between Washington, DC and Baltimore.  One of my favorite formations is the crater Theophilus, which is well place west of the center of the terminator line on the evening of the 20th.  It is large, over 100 kilometers across, has a prominent central peak, and looks “fresh”, overlaying a similar-sized crater named Cyrillus.  Indeed, it is one of the more recently excavated impact craters, but with no air or water on the Moon to promote erosion its formation probably occurred over a billion years ago, around the time that the first multi-celled organisms appeared on Earth.  There are a handful of other craters of somewhat more recent date, but the era of large impactors in our neck of the solar system has waned.  The last large one we know of on Earth occurred some 60 million years ago.

The first half of the week still offers chances to glimpse the distant galaxies of the Virgo Cluster before scattered light from the Moon overwhelms their distant glows.  These distant collections of billions of stars remain visible most of the overnight hours, but by the early morning the summer Milky Way begins to rise in the east.  By 3:00 am the stars of the summer constellation of Scorpius cross the meridian to the south, and the bright stars of the Summer Triangle are ascending in the east.

Venus continues to move eastward through the setting winter constellations.  She will continue to set about 45 minutes after the end of evening twilight.  She will maintain a nearly constant separation from the Sun for the next couple of months before gradually beginning to fall back toward the horizon by late July.

Jupiter continues to become better placed in the evening sky, and is well up in the southeast by 10:00 pm.  This is prime observing time for the giant planet, which will dominate the evenings until late summer.  All four of his Galilean moons will be near their greatest elongations on the night of the 18t/19th.

Saturn now rises before midnight along with the stars of the “teapot” asterism of Sagittarius.  The ringed planet is still best seen in the early morning hours, crossing the meridian low in the south at around 4:00 am.

Mars continues to brighten as he drifts eastward into the faint stars of Capricornus. He starts the week in the same telescopic field as the small globular star cluster Messier 75, which you’ll need to spot before the beginning of astronomical twilight. Otherwise the best time to give Mars a telescopic gander is at around 5:00 am as morning twilight begins to creep into the eastern sky.

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