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The Sky This Week, 2021 April 13 - 20

Cracks on the Moon.
Lunar craters Hyginus (upper center) and Treisnecker, imaged 2021 March 22 from Alexandria, Virginia
Lunar craters Hyginus (upper center) and Triesnecker, imaged 2021 March 22 from Alexandria, Virginia
with a Celestron 23.5-cm (9.25-inch) f/10 Schmidt-Cassegrain telescope, 1.6X Barlow lens, and a ZWO ASI224MC imager.
Note the craterlets in Hyginus Rille and the stress fractures around Triesnecker.

The Moon returns to the evening sky this week, waxing through her crescent phases as she moves along the northern extremities of the ecliptic.  First Quarter occurs on the 20th at 2:59 am Eastern Daylight Time.  Luna passes five degrees north of the star Aldebaran in Taurus, the Bull on the evening of the 15th.  Over the following two nights she brackets the fading ruddy glow of Mars.  She ends the week among the faint stars of Cancer, the Crab, just over three degrees northeast of the binocular star cluster Messier 44, commonly called The Praesepe or The Beehive.

This should be another fine week to check out the surface of our nearest neighbor in space.  The Moon’s high northerly declination means that her light passes through a minimal portion of Earth’s atmosphere, and thus Northern Hemisphere observers can glean lots of detail through their telescopes.  Milder evening temperatures also mean that a telescope’s optics reach thermal equilibrium more rapidly, further enhancing performance.  This is the best time to look for subtle details under relatively high magnification.  One of my favorite areas of Luna’s surface will be ideally placed for viewing on the evening of the 19th.  Near the center of the Moon’s disc, just west of the terminator, look for what will look like a hairline crack with a crater near the middle.  The crater, which has no upraised rim and sports a flat bottom, is named for the Roman astronomer Gaius Julius Hyginus, who wrote one of the first astronomical works to be printed with moveable type.  The manuscript for his Poeticon Astronomicon was probably written in the first century BCE and describes the lore behind many of the “classical” Greek constellations.  One of the treasures of USNO’s Library is a 1482 edition of this book.

The area around Hyginus crater is subjected to strong tidal forces caused by the Earth that stress Luna’s crust, so the area is criss-crossed with a number of these so-called “rilles”.  Most of these look like cracks on an egg shell, but on nights of steady “seeing” you will notice that the Hyginus Rille is lined with a series of small craters that appear to be volcanic in origin.  This is one of the few areas on the Moon where volcanism played an important role in shaping the local topography.  

The winter constellations continue to slide westward as evening twilight creeps later each evening.  By the late evening the springtime constellations are in firm control of the night.  High in the south is the crouching figure of Leo, the Lion, led by the bright star Regulus.  Turn to face the north and the seven stars that form the “Big Dipper” asterism are reaching their upper culmination.  To the east is the rose-hued glow of Arcturus, which will dominate the sky for the next couple of months.  While the outlines of Leo and the Big Dipper shouldn’t be too hard to trace out, the constellation associated with Arcturus is another matter.  Gaius Hyginus would have you believe that these stars represent the figure of a herdsman holding two dogs on leashes as he guides the Great Bear and Little Bear around the celestial pole.  To put it mi8ldly, I have trouble with this.  However, the stars that make up Boötes do outline an acceptable ice-cream sugar cone, with Arcturus at the tip.

Mars gets a visit from the Moon on the 16th and 17th, but other than that he doesn’t have many friends from the solar system to pal around with.  He continues to drift eastward from the “horns” of Taurus, and next week he will enter the constellation of Gemini, the Twins.

Both Jupiter and Saturn should now be easy to spot in the pre-dawn sky.  Saturn rises first, coming over the horizon at around 3:30 am EDT, with Old Jove following some 45 minutes later.  Jupiter is slowly moving east of Saturn, and will continue to do so until he starts his retrograde loop around the time of the summer solstice.