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The Sky This Week
U.S. Naval Observatory's Weekly Blog
Crater Hopping on the Moon
by Geoff Chester, USNO Public Affairs
16 March 2021
The Moon, imaged 2018 February 20, 23:30 UT from the U.S. Naval Observatory, Washington, DC
with the 30.5-cm (12-inch) f/15 Clark/Saegmüller refractor and a Canon EOS Rebel SL2 DSLR.
The crater Theophilus is arrowed.
The Moon brightens the evening skies this week, waxing through her crescent phases as she climbs northward along the ecliptic. First Quarter falls on the 21st at 10:40 am Eastern Daylight Time. Look for Mars about two degrees northwest of the Moon on the evening of the 19th. By the end of the week she beams down from the vicinity of the Gemini twins, Castor and Pollux.
This is the best time of the year to get to know our nearest natural satellite. For Northern Hemisphere observers the Moon reaches her highest declination as she approaches first quarter, which means that we won’t be looking through as much of our atmosphere during her most favorable phases for viewing. Exploring Luna’s surface is one of my favorite activities which I can enjoy from the confines of my suburban front yard. There’s no need to load up the telescope and drive to a remote site away from the city; the Moon sheds ample light for exploration from almost anywhere. During this week the terminator line slowly advances from night to night, revealing new landscapes to delight the viewer. On the evening of the 18th the terminator crosses the large impact crater Theophilus, which lies just south of the Moon’s equator. This feature has a very prominent central peak, which will cast a long, opaque shadow over the crater floor. This feature was formed by the impact of a modest-sized asteroid some 2 billion years ago that left a crater just over 100 kilometers (60 miles) across and some 4500 meters (13,500 feet) deep. That’s almost as big as the island of Hawai’i and as deep as that island’s Mauna Kea is tall. As the terminator advances it reveals that Theophilus lies atop an older, similar sized crater, Cyrillus. Both of these craters lie along the eastern edge of Mare Nectaris, the Sea of Nectar, which is one of the smaller lunar “seas” on the Earth-facing side. If the air is very steady look for the dozens of small craterlets that dot the Mare’s surface; these are “secondary” craters formed when material blasted from the impact that formed Theophilus subsequently fell back and impacted the hardened lava plain.
As the groundhog predicted way back in February, the vernal equinox arrives on the 20th at 5:37 am EDT. At this moment the center of the Sun’s disc stands directly over the Equator about halfway between Africa and South America. More importantly, it defines the beginning of astronomical spring. That said, the date when we actually have “equal night” occurs on the 17th since the Sun subtends a tangible disc. From that day until September 25th the duration of daylight will be longer than that of night.
Few people actually like the change from Standard to Daylight Time, but one good effect that it has is that it seemingly prolongs the appearance of Orion and his cohort of bright winter stars for the backyard astronomer. I can set up my telescope after dinner now and still have time to peruse these constellations and enjoy some of their interesting treasures. The Orion Nebula is always a treat to see, even from urban yards, and there are a wealth of fine star clusters to look for if you sweep from Capella to Sirius on the eastern side of Orion. These star clusters become less prominent in the later spring skies as the Milky Way sets to the west. They will be replaced by distant galaxies as we gaze out of the Milky Way’s plane.
Lonely Mars continues his eastward trek through the stars of Taurus. dimmer than the nearby star Aldebaran. During the course of the week Mars passes about seven degrees north of the star. The pair are joined by the nearly first quarter Moon on the evening of the 21st.
Jupiter and Saturn are now becoming easier to see in the glow of morning twilight. Saturn rises at around 5:00 am by the end of the week, with Jupiter following half an hour later. Look for both planets low in the southeast about 45 minutes before sunrise.
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