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The Sky This Week, 2018 February 20 - 27

Check out the waxing Moon and play a game of "Birthday Stars".
Crescent Moon, 2009 November 22
imaged from Alexandria, Virginia with a 50mm (2-inch) f/10 "Galileoscope".
The craters Theophilus, Cyrillus, and Catherina are indicated.

The Moon climbs along the northern reaches of the ecliptic this week, arcing through the bright stars of the Great Winter Circle.  First Quarter occurs on the 23rd at 3:09 am Eastern Standard Time.  This is one of the best times to observe the Moon from the northern hemisphere since she’s at her highest altitude while going through her most eye-catching phases.  Watch the terminator line slowly advance from night to night, revealing a succession of interesting terrains and formations.  On the 20th look for three prominent craters near the terminator.  Theophilus is the most complete and is partly superposed on the similar-sized Cyrillus.  Both of these craters sit just to the north of another large crater, Catharina.  You can get a sense of the evolution of lunar features by looking at this area.  Theophilus is obviously “younger” than Cyrillus, and Catharina is older still.  Theophilus was formed by the impact of a modest-sized asteroid over one billion years ago, which is considered to be “recent” in Luna’s geological time-scale.  At 100 kilometers (61 miles) across and over 4000 meters (13,300 feet) deep, it is one of the most striking features on the Moon.  Its terraced walls and complex central peak are its main features, but also notice how its southeastern wall impinges on 3.5 billion year-old Cyrillus.  The astronauts of Apollo 16 sampled some of the ejecta from Theophilus, finding some of the oldest rocks blasted from the more ancient lunar crust.

Luna’s brightening orb scatters more and more light as the week advances, but you can still enjoy the bright stars of the Great Winter Circle.  Orion stands just below the Moon on the evening of the evening of the 24th.  This should be a fine night to use your small telescope to not only explore the Moon’s jagged features but to also look at the Winter Circle’s bright stars.  Start with Betelgeuse, the reddish-hued star that marks the hunter’s right shoulder, then move southwest to icy-blue Rigel.  Both stars are hundreds of light-years away, which means that they are thousands of times more luminous than the Sun.  Moving counter-clockwise from Rigel, you’ll pass orange-tinted Aldebaran, golden Capella, the close double-star pair of Castor, Pollux, whitish Procyon, and ending up with the blazing blue glow of Sirius.  Compared to Orion’s stars, the rest of the stars in the Winter Circle are all relatively nearby.  Castor is just over 50 light-years distant, while Sirius is a mere 8.5 light-years away.  You can use these stars to play an interesting game that I call “Birthday Stars”.  If you’re 8.5 years old the light from Sirius originated there when you were born.  Procyon’s light is about 11.5 years old, and so on.  I can guarantee that nobody on the planet today has one of Orion’s bright stars as their “Birthday Star” unless they are a giant redwood or a Joshua tree!

You should start looking for Venus in earnest in the twilight after sunset.  You should be able to pick her out just above the western horizon about half an hour after sunset.  She will climb higher in the west as springtime approaches.

Jupiter, Mars, and Saturn are now well-arrayed for pre-dawn skywatchers.  You’ll find Old Jove near the meridian an hour before sunrise.  By the end of next week he’ll rise at around midnight, but the best time to give him a look is still in the wee hours.  Ruddy Mars courses eastward between the stars of Scorpius and Sagittarius.  He is gradually brightening as Earth slowly starts to catch up to him on our faster inner solar orbit.  His disc is gradually getting larger as a result, and owners of large telescopes should start monitoring his surface details.  Saturn is low in the southeast, hanging out just above the top of the “Teapot” asterism in Sagittarius.  All of these planets will be best viewed in the spring and summer months.

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