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The Sky This Week
U.S. Naval Observatory's Weekly Blog
Chasing Myths Around the Sky
by Geoff Chester, USNO Public Affairs
20 April 2021
Lunar composite, imaged 2021 April 20 01:31 UT from Alexandria, Virginia
with an Explore Scientific AR102 10.2-cm (4-inch) f/6.5 refractor telescope, 1.6X Barlow lens,
and a ZWO ASI224MC CMOS imager. 15 subframes stitched using AutoStitch software.
The Moon brightens the evening sky this week, waxing through her gibbous phases towards Full Moon, which will occur on the 26th at 11:31 pm Eastern Daylight Time. April’s Full Moon has a number of popular names associated with it, all indicators of the rejuvenation of the spring season. The most popular name is the Pink Moon, a tribute to the outburst of spring wildflowers throughout north temperate climes. Other names include the Hare Moon, Sprouting Grass Moon, Fish Moon, and Egg Moon. Look for the Moon nestled among the stars that form the “head” of Leo, the Lion, on the evening of the 21st. On the 25th you will find her just over five degrees north of the bright star Spica.
Fans of annual meteor showers have had to wait for several months to enjoy a decent display. After the Quadrantids peak in early January, the next major shower to look forward to is the Lyrids, which peak on the morning of the 22nd. These meteors appear to originate from a point in the sky near the small constellation of Lyra, the Harp, whose brightest star, Vega, forms one apex of the Summer Triangle asterism. You will find Vega rising in the northeast during the late evening hours, and under moonless conditions up to 20 meteors should be visible per hour beginning at around local midnight. Unfortunately, the waxing gibbous Moon throws a lot of light around the sky, so viewing the Lyrids this year will probably something of a bust. Take heart, though, meteor lovers; the next good shower, the Eta Aquarids, should be active in early May, and the Moon shouldn’t be a factor.
As the end of evening twilight slips later into the night the last of winter’s constellations slip inexorably toward the western horizon. This is the time of year when we can witness an ancient myth that has been re-enacted for millennia. The signature constellation of winter is Orion, who can be seen low in the west as twilight deepens. One by one his brighter stars wink out as they set, and the last one to do so is the distinctive red-tinted star Betelgeuse. In mythology, Orion was not only a masterful hunter, he was also something of a braggart. A son of Poseidon by the Gorgon Euryale, he boasted to Gaia, goddess of the Earth, that he could kill any animal that came his way. To punish him, Gaia dispatched a lowly scorpion that stung the Hunter on his foot. Eventually both were placed in the sky, but in such a way that they would never be seen together. Indeed, the star Betelgeuse sets about 15 minutes after Antares, the ruddy heart of Scorpius, rises. It is interesting to note that these two stars are physically very similar. Each is a highly evolved “red supergiant” that is nearing the end of its cosmic life. These stars fuse elements in a series of shells that surround the core. The byproduct of fusion of the innermost of these layers is iron, which has the most stable nucleus of all of the elements. No matter if you try to fuse it or split it, you need to put more energy into the reaction than you get out of it, so the star’s core effectively becomes a large heat sink. Some time in the next few hundred thousand years these stars will implode on their iron cores, liberating a fantastic amount of energy in a supernova explosion. Which one will go first? It’s anybody’s guess.
By the end of the week we welcome the return of two planets to the evening sky. The elusive Mercury and the effusive Venus can be glimpsed just above the west-northwest horizon about half an hour after sunset. You will need a good flat horizon to see them, but they should be fairly easy to spot. Venus will be the brighter of the two, while Mercury will be just to the northwest of Venus. Mercury will continue to be visible for the next several weeks as he enters his best evening apparition for the year.
Mars continues to diligently forge eastward along the ecliptic. This week he moves into the constellation of Gemini, the Twins. On the evening of the 26th he may be found half a degree north of the galactic star cluster Messier 35. Despite the light of the Full Moon, owners of small telescopes should get a good view of this interesting cosmic meet-up.
You will still find Jupiter and Saturn lurking in the southeastern pre-dawn sky. Both planets are now ensconced in the dim constellation of Capricornus, the Sea-goat, so you shouldn’t have any problems locating them if you are up before the Sun.
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