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The Sky This Week, 2018 March 20 - 27

The Moon returns while winter's stars depart.
The Moon, Venus, & Mercury, imaged 2018 march 18 from Alexandria, Virginia.
The Moon, Venus, & Mercury, imaged 2018 March 18, 20:05 EDT from Alexandria, Virginia
with a Canon EOS Rebel T2i DSLR, 100mm @ f/6.3, 1/30s, ISO 3200.

The first week of spring finds the waxing crescent Moon climbing along the ecliptic to a perch among the retiring stars of the winter sky.  First Quarter occurs on the 24th at 11:35 am Eastern Daylight Time.  Luna opens the week in the western sky above the bright glow of Venus.  On the 22nd she passes just to the north of the bright star Aldebaran in the constellation of Taurus, the Bull.  By the end of the week she enters the springtime sky, closing in on Regulus, lead star of the constellation of Leo, the Lion.

The Moon’s path through the sky places her at a high altitude during the phases that reveal some of her most fascinating features.  This makes it a great time to explore her many and varied landscapes.  You can use something as simple as a pair of binoculars to see the changes along the Moon’s terminator from night to night.  It was such a view that I had as a youngster that inspired me to further explore the sky.  Small telescopes offer a wonderful view at relatively low magnifications.  On nights when the atmosphere is steady you can expect to see details to a scale of about a kilometer, roughly the size of Meteor Crater east of Flagstaff, Arizona.  This may hopefully give you a sense of the scale of the features on Luna’s face.  Most of the craters you’ll see are dozens of kilometers across!

The days surrounding the equinox are the times of the year when the Sun’s apparent motion along the ecliptic reaches its maximum change in the rate of change of declination.  This means that right now we add about three minutes of daylight to that of the previous day.  Most of us really notice this change more in the spring than the fall. Since we are coming out of the long dark nights of winter.  It’s a time when I notice, in particular, the demise of the bright winter constellations.  By the time evening twilight ends you’ll see that all of the stars in the Great Winter Circle are west of the meridian, and Orion, winter’s signature star pattern, is well on his way to setting by midnight.  Winter’s bright stars are now being replaced by fainter constellations that make up the springtime sky.  There is only one star in the spring that rivals the bright beacons of Orion and his cohorts, and that star is Arcturus.  You’ll find it in the northeastern sky where it starts to climb to prominence by 10:00 pm.  Arcturus is the fourth-brightest star in the sky and the brightest in the celestial northern hemisphere.  Although it is part of the larger constellation of Boötes, the Herdsman, you’ll be hard-pressed to see much of the star pattern from urban skies.  In darker locations look for something resembling an ice-cream cone to the north of Arcturus.

The easiest springtime constellations to see under brighter skies are Leo, the Lion, and the seven-star asterism we call the Big Dipper.  You’ll find them near the meridian at local midnight.  Leo has a single first-magnitude star, Regulus, which you’ll find close to the Moon on the evenings of the 27th and 28th.  The Big Dipper doesn’t have any first-magnitude luminaries, but its distinctive outline of second-magnitude stars makes it one of the most recognized asterisms in the sky.

Venus continues to gradually climb higher in the evening twilight sky.  As the week opens you’ll find the fainter planet Mercury a few degrees to the right of the dazzling Venus.  Mercury then takes an abrupt dive toward the horizon and rapidly fades to third magnitude, leaving him almost invisible in the evening twilight.

The bright planet Jupiter now rises in the southeast shortly before midnight.  He lords over the morning sky and is high in the southwest at the onset of morning twilight.  At this time you’ll notice the summer constellation Scorpius crossing the meridian, and to the east the ruddy glow of Mars shares the limelight with Saturn.  Watch Mars close the gap with Saturn.  He’ll pass the ringed planet next week. 

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