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The Sky This Week
U.S. Naval Observatory's Weekly Blog
Hazy Nights with the Thunder Moon
by Geoff Chester, USNO Public Affairs
20 July 2021
Mount Wilbur by Moonlight, 2021 June 28, imaged in Glacier National Park, Montana
with a Canon EOS Rebel SL2 DSLR, 18mm f/5.6, 30 seconds @ ISO6400
The Moon skims the southern horizon this week, waxing through her full phase before gradually climbing through the rising autumnal constellations. Full Moon occurs on the 23rd at 10:37 pm Eastern Daylight Time. July’s Full Moon is popularly called the Thunder Moon in some traditions; other names include the Buck Moon and Hay Moon. Look for the Moon near rising Saturn on the evenings of the 3rd and 24th. She rises with bright Jupiter during the late evening of the 25th.
July 20th is a special day for those of us who have grown up in the so-called “Space Age”. On that date in 1969 Neil Armstrong and Edwin “Buzz” Aldrin touched down on the lonely expanse of the Moon’s Sea of Tranquility, fulfilling the age-old dream of walking on the surface of another world. Just five years later a three-legged emissary from Earth successfully landed on a far more distant and rocky shore on the Chryse Planitia region of Mars. As with the Apollo 11 landing, I found myself glued to the TV set in the wee hours of the morning as the Viking 1 lander approached the surface of the red planet. Unlike the Apollo lunar module, however, Viking had no pilots, and it took more than eleven minutes for the success of the landing to reach controllers on Earth. As the first image from the lander slowly scrolled across the TV screen, it was almost impossible to imagine that I was looking at the surface of another world. Our voyages to the bounds of the solar system since those two July days have given all of us an opportunity to experience the thrill of discovery as we peel back the mysteries of our planetary neighbors.
Scattered light from the bright Moon and smoke from wildfires in the American west and Canada will diminish the splendor of the midsummer sky this week. That said, the subdued sky still reveals a few bright targets that you can use to find more subtle sights when conditions improve. Early in the evening we can see two holdovers from the spring in the form of the bright stars Arcturus and Spica. Arcturus, the brightest star in the northern hemisphere sky, may be found high in the west once the sky becomes dark. Spica glows from the southwestern part of the sky. By 10:00 pm you may be able to pick out the seven stars of the Big Dipper asterism. These stars are visible to us year-round, but if you follow their progress across the sky overnight you will see the Dipper skim the northern horizon in the early morning hours. In the east the three stars of the Summer Triangle, Vega, Altair, and Deneb, climb toward the meridian as evening turns into morning. These stars outwardly appear similar, but we now know that the faintest of the trio, Deneb, is a star of almost unimaginable luminance. Vega and Altair are relatively nearby at 25 and 16.7 light-years, respectively, but Deneb is 100 times more distant than Altair. To appear as bright as it does in our sky Deneb must shine with the equivalent light of nearly 200,000 Suns! If we could somehow drag Deneb to Altair’s distance it would shine with nearly the same apparent brightness as a full Moon. Night would be a very different experience if that were the case.
Go out about half an hour after sunset and look for the bright glow of Venus about 10 degrees above the western horizon. If you have a pair of binoculars, look near the planet’s dazzling glow for the fainter light of the star Regulus. Over the first few nights of the week Venus is close to the star, with a minimum one degree separation at dusk on the 21st.
Mars follows Venus, reaching the proximity of Regulus by the week’s end. Mars is also much dimmer than Venus, and is actually dimmer than the star. You will most likely need binoculars to pull the red planet out of the horizon haze.
Saturn now rises before 9:00 pm, and by midnight he should be well-placed for viewing through the telescope. The ringed planet is just under two weeks from opposition, when he will shine in the sky all night long.
The much brighter giant planet Jupiter rises one hour after his more distant companion, and by midnight he, too should be a good sight in the telescope. Try to find a spot to observe these two worlds that has a flat horizon with no buildings. Their southerly declination makes viewing them more susceptible to atmospheric currents on these muggy summer nights.
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