The Sky This Week
U.S. Naval Observatory's Weekly Blog

Squaring Off in the Autumnal Sky

by Geoff Chester, USNO Public Affairs
30 November 2021

Jupiter and three moons, 2021 November 9
imaged from the U.S. Naval Observatory, Washington, DC
with the 30.5-cm (12-inch) f/15 Clark/Saegmüller refractor
and a ZWO Optical ASI183MC CMOS color imager.

The Moon begins the week as a waning crescent in the pre-dawn sky, then flirts with dazzling Venus as the week wraps up. New Moon occurs on the 4th at 2:43 am Eastern Standard Time.  Early risers on the 2nd should look for the thin crescent less than a degree north of the second-magnitude star Zubenelgenubi, the southern “balance pan” in the constellation of Libra, the Scales.  By 6:30 am you should be able to see the ruddy glimmer of Mars halfway between the Moon and the horizon.  Luna will be just three degrees from Venus as evening twilight falls on the evening of the 6th.

When New Moon occurs, a lucky few people along with hordes of penguins and seals will be treated to a total solar eclipse.  The path of totality is restricted to parts of West Antarctica and adjacent waters in the Amundsen and Weddell Seas.  

As we transition into the final month of 2021 we enter the period of the year when we experience our longest nights.  Throughout December we have less than ten hours of daylight, and the early sunsets mean that astronomers can get several hours of observing time before going to bed at a decent hour.  In fact, December 7th is the date of the year’s earliest sunset in most locales in the United States.  Here in the Washington, DC area Old Sol slips below the horizon at 4:46 pm EST for several evenings before and after the 7th.  However, the year’s latest sunrise won’t occur until early January 2022, so the shortest day falls on the 21st, the date of the winter solstice.

If you venture out at around 8:00 pm local time you will find yourself “squarely” under the autumnal constellations.  High in the south is the large “Great Square” asterism formed by the brightest stars of Pegasus, the legendary Flying Horse.  This grouping serves as a kind of celestial signpost to help locate some of the season’s other obscure constellations.  If you draw an imaginary line southward from the stars that form the Square’s western side your gaze will cross the dim constellation of Aquarius, the Water Bearer.  Continue southward and you will find the first-magnitude star Fomalhaut, the southernmost bright star visible from most of the U.S.  Draw a similar line through the stars on the eastern side of the Square and you will run into the second-magnitude star Diphda, brightest star in the constellation of Cetus, the Whale.  Draw another line from the lower right star in the Square through the star Alpheratz in the Square’s upper left corner and your gaze will parallel the diverging “chains” of stars that delineate Andromeda.  Under dark skies look just above the second star in Andromeda’s fainter “chain” for a small misty patch of light that resembles a small, detached portion of the Milky Way.  This amorphous glow is the light of some 200 billion stars that make up the Great Andromeda Galaxy, the Milky Way’s largest companion in a small galaxy cluster known as the Local Group.  Through binoculars or small telescopes the Andromeda Galaxy shines with a soft glow that brightens to a near pinpoint at its center.  An early observer likened its glow to “the light of a candle shining through horn”.  It is located about 2.5 million light years away from us, making it the most distant object that you can see with the naked eye.

Venus is now putting on her best show for the current evening apparition.  She reaches her brightest appearance on the 4th, when she also sets at her latest time of the year.  If you are under dark skies at around 6:30 pm, try to spot your shadow cast by Venus’ brilliant glow.

Saturn hangs between his brighter companions Venus and Jupiter and should be easy to find shortly after sunset.  The ringed planet is slowly drifting eastward against the stars of Capricornus, but his plodding motion is no match for the oncoming Sun.  You’re best views of him will be during twilight hours, and by the week’s end he sets well before 9:00 pm.

Jupiter gives owners of small telescopes something to look at in the twilight and early evening hours.  The giant planet is also moving eastward through Capricornus, but his time in the sky is limited as well.  He’ll hang around through the end of the year, but as January opens he will be fading fast.

 
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