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

A Planetary Dance in Twilight, and Beware of the Scorpion!

by Geoff Chester, USNO Public Affairs
06 July 2021

Scorpius (Antares at center right) and the summer Milky Way, imaged 2020 July 26 from Mollusk, Virginia
with a Canon EOS Rebel SL2 DSLR and and an Omegon LX2 mechanical star tracker

The Moon returns to the evening sky by the latter part of the week, with New Moon occurring on the 9th at 9:17 pm Eastern Daylight Time.  Luna joins the twilight planets Venus and Mars on the evenings of the 11th and 12th.  With the two planets undergoing a close conjunction, the Moon’s addition should make for a fine photo opportunity.

The time of sunset is now gradually becoming earlier for those of us who enjoy stargazing at a decent hour.  However, we’ll have to wait a few more weeks before most of us will notice a significant change.  Sunset won’t occur before 8:30 pm until the evening of July 20th here in Washington.  

Once the sky does become fully dark, the absence of the Moon’s bright glow allows us to enjoy the splendors of the summer sky.  The last of spring’s constellations are setting, while the bright stars of the Summer Triangle stand out prominently in the east.  By 10:30 pm one of summer’s most prominent constellations reaches prominence on the southern horizon.  Scorpius never rises very high in the sky for those of us in temperate northern latitudes, but it’s worth the time to find a good viewing spot with an open southern sky to look for it.  Its brightest star, Antares, crosses the meridian at around 10:30 pm.  It is notable for its red tint, which is similar to that of Betelgeuse in the winter sky.  To the west of Antares you’ll see a near-vertical line of three blue-hued stars that outline the scorpion’s head, but it’s the arc of stars that sweep down to the horizon, then curve upward to the east of Antares that give the scorpion its distinctive shape.  A close pair of stars indicate the beast’s fearsome stinger. 

Scorpius is one of the few constellations that resembles its namesake, and as such its origins go way back in time.  One of the earliest depictions of the constellation date to the pre-dynastic period of ancient Egypt, where a stone mace-head depicts an early king performing a ritual opening of an irrigation canal.  The scorpion is depicted as his personal glyph, and it is very likely that he associated himself with the scorpion in the sky.  The mace-head has been dated to c. 3200 BCE.

Of course, different cultures don’t necessarily see the same patterns in these stars.  Polynesians see the scorpion’s curving tail as an asterism representing the giant fish hook used by their creator god Maui to dredge islands up from the bottom of the Pacific Ocean.

From a dark sky, look above the two stars in Scorpius’ “stinger” to see the dense star clouds of the summer Milky Way.  When we gaze in this direction we are looking toward the central bulge of our home galaxy.  This is a particularly spectacular region to scan with binoculars or a low-power telescope.  The formless glow of the Milky Way reveals itself to be the combined light of millions of distant stars, and sprinkled among these are clusters of closer stars and the glow of star-forming clouds of gas and dust.

The evening twilight hours offer the most interesting phenomena of the week.  You should be able to spot the bright glow of Venus above the western horizon shortly after sunset, and an hour later ruddy Mars should appear nearby.  Depending on the clarity of the sky you may need to use binoculars to spot the red planet, but by the week’s end Venus will cozy up to within half a degree of the dimmer Mars.  On the evening of the 12th Mars will be just to the left of Venus, and on the following night Mars will be to the right of the dazzling planet.  The crescent Moon is near the pair on the 12th and 13th.

By midnight the solar system’s two giant planets Saturn and Jupiter can be seen in the southeastern sky.  Saturn rises first, and should be a good telescopic target.  Jupiter lies farther to the east, and you’ll probably need to wait another hour to train the telescope on his cloud-streaked surface.  Both planets will become better placed for evening viewing over the next several weeks.
 
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