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The Sky This Week
U.S. Naval Observatory's Weekly Blog
Wading Into Lunar Seas
by Geoff Chester, USNO Public Affairs
23 March 2021
Annotated Gibbous Moon, imaged 2019 October 10 from Shoestring Observatory, Alexandria, VA
with a 10.2-cm (4-inch) f/6.5 Explore Scientific AR102 refractor, TeleVue 2X Barlow lens, and a Canon EOS Rebel SL2 DSLR
The Moon descends from her high declination this week as she brightens the sky surrounding the springtime constellations. Full Moon occurs on the 28th at 2:48 pm Eastern Daylight Time. The March Full Moon is popularly known as the Worm Moon, Crow Moon, or Sap Moon, as all of these mark the warming of the ground as winter loses its grip on the Northern Hemisphere. It is also the Paschal Moon for Christians, setting the date for their most important feast day, Easter. For Jews, the Full Moon marks the date of 15 Nisan in the Hebrew calendar, the traditional beginning of the weeklong observance of Passover. Look for the Moon near the bright star Regulus on the evening of the 25th. She rises with the star Spice on the 28th and 29th.
The first few nights of the week offer yet more good views of the Moon through the telescope. As Luna waxes through her gibbous phases, the vast lava plain known as Oceanus Procellarum, the “Ocean of Storms”, gradually reveals itself as the terminator line slowly creeps westward from night to night. This feature, the only “ocean” on the Moon’s near side, is the largest formation of its kind on Luna’s surface, covering about 10 percent of her entire surface area. Unlike the other lunar “seas”, it does not have a defined circular shape, and its origins remain something of a mystery. It is noted for many “ghost” craters that have been flooded by lava over the eons, but it also has some of the most distinctive craters to be found on our natural satellite. The prominent crater Copernicus is well placed on the evening of the 23rd, just to the south of the broken mountain ring that defines the edge of Mare Imbrium, the “Sea of Rains”. This crater would have been the landing site for Apollo 20 had that mission not been cancelled in 1972. Apollos 12 and 14 landed in Oceanus Procellarum, touching down in a small “bay” that had also been visited by the American Ranger 7 and Surveyor 3 missions several years earlier. All of this activity has caused this area of the Moon to be officially named Mare Cognitum, the “Known Sea”. On the evening of the 25th look for a small but very bright crater at about the “10 o’clock” position on the terminator. This is the crater Aristarchus, one of the brightest and youngest features on the lunar near side. “Young” in this case is a relative term as the crater is believed to have formed some 400 million years ago! Just to the west of Aristarchus is the flat-bottomed crater Herodotus and the curious feature known as the “Cobra Head”, believed to be a collapsed lava tube. Careful scrutiny of this area on a night of steady air will show several more such “rilles” and the half-flooded crater Prinz.
As winter’s stars heel over to the west, the bright beacon of the star Arcturus becomes more prominent in the east. To me this star has always been symbolic of spring with its rose-tinted glow. You won’t have any trouble finding it despite the bright light of the Moon, but tracing out the rest of the star’s parent constellation is a bit more tricky. Arcturus leads a group called Boötes, the Herdsman and is supposed to represent a shepherd with two dogs on leashes guiding the Great Bear and Little Bear around the pole star. This takes a dark sky and considerable imagination to make out, but I find it easier to recognize as an ice-cream cone, with Arcturus marking the tip, a much more seasonal asterism!
Mars continues his easterly trek among the stars of Taurus, the Bull. This week the red planet passes north of the star Aldebaran, and by the end of the week he forms a line with Aldebaran and the star El Nath, which marks the tip of the Bull’s northern horn.
Jupiter and Saturn should now be pretty easy to find low in the southeastern sky about an hour before sunrise. The two giant planets have moved into the obscure constellation of Capricornus, and will become more prominent in the evening sky as summer approaches.
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