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The Sky This Week, 2018 September 4 - 11

Explore the celestial swan.
Mars, 2018 August 24, 03:09 UT
Imaged with the 30.5-cm (12-inch) f/15 Clark/Saegmüller refractor
at the U.S. Naval Observatory.

The Moon spends most of the week as a slender crescent in the twilight hours before sunrise and after sunset, centered by the occurrence of the New Moon, which falls on the 9th at 2:0 pm Eastern Daylight Time.  Early in the week you’ll find her moving through the bright stars of the Great Winter Circle.  By the week’s end she re-appears in the evening sky, closing in on dazzling Venus in the deepening twilight after sunset. 

The rapidly lengthening nights of September offer a great view of summer’s signature constellations.  This week one of these, Cygnus, the Swan, is our focus for the September “Globe at Night” citizen-science observing campaign.  Cygnus is easy to find, passing directly overhead in temperate northern latitudes between 9:00 and 10:00 pm local time.  The Swan’s brightest star is Deneb, which is the faintest and easternmost star in the large asterism popularly known as the “Summer Triangle”.  “Deneb” translates as “the tail”, and it marks the tail of the celestial swan.  If you have a reasonably dark sky you can easily trace the swan’s neck into the center of the Summer Triangle, where its head is marked by the third-magnitude star Albireo.  Crossing the imaginary line between Deneb and Albireo is another line made up of second- and third-magnitude stars; the brightest of these is the central star in the constellation, Sadr.  With a little imagination you can visualize a swan in flight along the star clouds of the Milky Way.  Other cultures have different interpretations; one of my favorites comes from Inuit mythology which describe the image of a man sitting in a kayak paddling along the “Pebbly River”, their description of the Milky Way.  The star Albireo is a wonderful target for small telescopes, one of the most colorful double stars in the sky.  The components are easily split with a 2-inch aperture, and the colors, blue and gold-yellow, give rise to my nickname for the pair, the “Navy Double”.  During the course of this week, locate Cygnus in the sky overhead and compare the number of stars that you see with the star charts on the Globe at Night website.  Try to make multiple observations on different nights from different locations.  Your report will help to monitor the global distribution of “light pollution” that is encroaching on our nighttime skies.

The first hours of evening darkness showcase the splendors of the summer sky.  The softly glowing clouds of the summer Milky Way will dominate the view from dark sky locations.  You’ll notice that the brightest parts of the Milky Way will be in the southern part of the sky, just to the right of the “teapot” asterism of the constellation of Sagittarius, the Archer.  When we look in this direction we are gazing toward the center of our Galaxy, located some 30,000 light-years away.  We can’t actually see the galactic center, but the huge bulge of stars surrounding the center make the region stand out nicely.  Sweep your low-power telescope up along the Milky Way for one of the most spectacular parts of the sky above us.

Bright Venus may be found in the west-southwest during the first hour after sunset.  She hangs less than 10 degrees above the horizon at sunset and is just five degrees up an hour after the Sun goes down.  Fortunately she is reaching her brightest stage of the current apparition right now, so you shouldn’t have much difficulty locating her.  Act quickly, since she sets before 9:00 pm local time. 

Jupiter also hangs in the twilight sky to the southwest, and although he’s much fainter than Venus he should still be easy to pick out as darkness settles.  Old Jove stays around a little bit longer than Venus does, but he too exits the scene early in the evening, setting by 10:00 pm.  You’ll have to get him in the eyepiece shortly after he appears in twilight to get a good view of him before he settles into the horizon haze.

Saturn still enjoys several hours of good viewing time, crossing the meridian just after 8:00 pm.  The ringed planet reaches the second stationary point in this year’s apparition on the 6th, and gradually resumes direct west-to-east motion over the next few weeks.  He is located against a background of one of the Milky Way’s dense star clouds, so picking out his fainter moons is complicated by the stellar backdrop.

Mars continues to beam down brightly from his perch among the faint stars of Capricornus.  The red planet has also resumed direct motion against the stars, and over the course of the next several months he’ll zip through the autumnal constellations while gradually fading.  He will remain visible in the evening sky until well into 2019, but he will be a much dimmer object by then.  He still shows a relatively large telescopic disc, but his size will shrink by about 25 percent over the course of the next month.

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