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

Dog Days and Summer's Haze

by Geoff Chester, USNO Public Affairs
03 August 2021

The Milky Way, imaged 2017 August 21 from Smith's Ferry, Idaho
with a Canon EOS Rebel SL2 DSLR.

The Moon begins the week as a slender waning crescent in the pre-dawn sky.  New Moon occurs on the 8th at 9:50 am Eastern Daylight Time.  Early risers can watch Luna pass through the rising stars of the Great Winter Circle.  She returns to the evening sky on the evening of the 10th, when you will find her near the dazzling planet Venus.

The August campaign for the Globe at Night citizen-science program is underway and runs through the evening of the 8th.  So far this year the program has received over 20,000 reports from observers around the world who are recording the faintest stars that they can see in their skies.  The procedure is simple; find the target constellation, compare your view with reference charts on the Globe at Night website, and report your results.  If you are away from your computer, the program has apps for smart phones that can assist you in finding your limiting stellar magnitude and reporting your results.  The aim of the program is to measure the spread of “light pollution” that is affecting more and more of the night sky.  Bright nighttime lighting not only robs us of the view of our place in space, it has a number of dire effects on wildlife and humans, upsetting circadian rhythms and navigation for migrating species.  It is one environmental issue that has a relatively easy and inexpensive fix, and the energy savings from sensible lighting will also lower carbon emissions that contribute to climate change. 
 
Most of us associate early August with the heat and humidity of the so-called “Dog Days” of popular lore.  The origin of this expression is, of course, astronomical, and it dates back to very early times.  The “Dies Canicularum” of the ancient Romans corresponded with the first appearance of the bright star Sirius rising just before the Sun.  Sirius is the brightest star in the sky after the Sun, and it is the hallmark of the constellation Canis Major, the Greater Dog in Roman skylore.  The star’s name derives from the ancient Greek word for “scorcher”, and it was thought that the light of the star augmented that of the Sun and led to the hot, sultry days of the season.  This “heliacal rising” of Sirius has roots which pre-date the Romans by two millennia.  Ancient Egyptians noticed and recorded the phenomenon around 3000 BCE and saw that it corresponded to the annual life-giving flood of the Nile River.  This became the most important date in the Egyptian agricultural calendar and formed the basis for their civilization’s success.

August is the month to get out and enjoy the splendors of the summer sky.  Many of us take vacations at this time of year, and any escape from the light-domes of urban areas should include time to look up and see the stars.  Whether you’re staying at the beach or camping in the mountains, plan to spend some time taking in a view of our wonderful universe.  There are many apps for your phones and tablets that can help you navigate the night sky, but even without these you can still find wonders.  By 10:00 pm the sky is fully dark, and the ghostly glow of the Milky Way rises from the southern horizon.  The Milky Way’s path is sprinkled with bright stars that form familiar patterns like the Summer Triangle, Scorpius, and the “Teapot”.  Start your interstellar adventure by identifying these patterns, then see how many more you can find.

The bright glow of Venus climbs a little bit higher above the western horizon each night.  You should have no trouble spotting her piercing white light shortly after sunset.  By the end of the week she is joined by the Moon, whose slender crescent is nearby on the evenings of the 10th and 11th. 

As Venus settles in the west, Saturn climbs higher into the southeastern sky.  Saturn’s glow is much more subdued than that of Venus, but he is still easy to spot among the dim stars of the constellation of Capricornus.  The planet’s famous rings can be seen in small telescopes, but they really show off in apertures of six inches or more.  On nights with steady air and good transparency look for several of the ringed planets many moons, looking like tiny fireflies around the planet’s light.

Jupiter follows Saturn’s path into the southeastern sky.  You won’t have any trouble finding the giant planet by 10:00 pm, but you should wait another hour before training the telescope on him.  His cream-colored disc is crossed by two prominent dark stripes in small telescopes, which will also easily show his four Galilean moons.  Larger instruments reveal more detail in his turbulent atmosphere especially when the air is steady.
 
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