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The Sky This Week
U.S. Naval Observatory's Weekly Blog
Chasing Comet Bits and Distant Galaxies
by Geoff Chester, USNO Public Affairs
04 May 2021
Markarian's Chain, the heart of the Virgo Galaxy Cluster
imaged 2020 May 24, 03:20 UT from Mollusk, Virginia with an Explore Scientific AR102 10.2-cm (4-inch) f/6.5 refractor telescope
and a Canon EOS Rebel T2i DSLR.
The waning Moon makes her way eastward through the dim rising constellations of autumn. New Moon occurs on the 11th at 3:00 pm Eastern Daylight Time. You will find Luna southeast of bright Jupiter on the mornings of the 5th and 6th.
This week we get a visit of sorts from one of the most famous members of the solar system, Halley’s Comet. We won’t actually see the comet itself; that last happened in 1986 and won’t happen again until 2061, but tiny pieces of the comet are now interacting with the Earth in the annual Eta Aquariids meteor shower. This display can produce a meteor every minute or two, and is best seen from the southern United States. The radiant is located near a small asterism known as the “Water Jar” that consists of a small triangle of 4th and 5th magnitude stars surrounding a central 3rd magnitude star. Observers at the latitude of Washington, DC have about two hours to get a good view before twilight begins to brighten the sky. If you miss this shower the Orionids of October are associated with another meteoroid stream from Halley’s Comet.
With the Moon’s absence from the evening sky it is time for the May campaign for the Globe at Night citizen science project. This month’s featured constellation is Boötes, the Herdsman, whose brightest star, Arcturus, is visible from any location in the Northern Hemisphere with an open view of the sky. Although Arcturus is easy pickings, the rest of the constellation is more obscure. The rest of the Herdsman is located to the east of the “handle” of the Big Dipper and resembles a kite or an ice cream cone. Most of the stars in the constellation are third magnitude and should be visible from the outer suburbs of major metropolitan areas. Making an observation is simple; just go to the Globe at Night web app and follow the instructions. So far this year the project has collected over 14,000 observations, well on the way to their goal of 20,000.
The short nights of late spring find a transition between the bright stars of winter and the rising stars of summer. Although the spring constellations don’t have the “pizzazz” of the brighter star patterns of spring and summer, it’s what’s behind them that makes this time of year special to dedicated skywatchers. The brighter stars are typically associated with constellations along the Milky Way, but at this time of year our home galaxy hugs the horizons. As we look upwards we are looking out of the galactic plane through a relatively thin distribution of stars. If you were to travel toward the center of the triangle bounded by Arcturus, the bright star Spica, and the “tail” of Leo, the Lion you would pass through our spiral arm of the Galaxy for about 1000 light years, then encounter a void of unimaginable dimensions. After a journey of some 35 to 40 million light years you would run into the outer members of a vast cluster of galaxies known as the Virgo Cluster. There are thousands of galaxies in this part of the sky, and many of them are visible in small telescopes. Under dark skies they float like ghostly wisps of light in the eyepiece, each one consisting of hundreds of billions of stars. The brightest members are gigantic elliptical systems sporting trillions of stars and enormous black holes in their cores. Their gravity is so powerful that our own Milky Way feels their influence! I have spent many pleasant spring nights chasing down hundreds of these wisps of light, and I still have several hundred more to find.
Back in our own cosmic back yard, planets are beginning to converge in the night. During evening twilight the elusive planet Mercury continues to shine low in the west-northwest as he climbs toward greatest elongation from the Sun at mid-month. Half an hour after sunset you should be able to spot him about 8 degrees above the horizon as the week begins, and he will climb a few more degrees before he starts to retrograde back toward the Sun.
Dazzling Venus should be visible to folks with flat horizons about five degrees below Mercury. She is set to gradually return to the evening sky, but her progress will be slow. She won’t set after the end of evening twilight until the end of August.
You’ll find ruddy Mars drifting eastward through the stars of Gemini, the Twins. The red planet isn’t very bright these days, but you should be able to find him in the west between the bright stars Procyon and Capella, under Castor and Pollux, the Gemini “twin stars”.
Jupiter and Saturn still grace the pre-dawn sky, beaming down from the faint constellation of Capricornus in the southeastern sky. Both planets should be easy to find if you happen to be up and about at 5:00 am.
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