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

The Sky of High Summer

by Geoff Chester, USNO Public Affairs
28 June 2022

The crescent Moon and Earthshine, HDR image made on 2018 April 19 from Alexandria, Virginia
with an Antares Sentinel 80-mm (3-inch) f/6 refractor and a Canon EOS Rebel SL2 DSLR.

The Moon returns to the evening sky this week, waxing through her crescent phases and providing a cosmic backdrop to fireworks celebrating Independence Day.  First Quarter will occur on July 6th at 10:14 pm Eastern Daylight Time.  As Luna waxes and moves higher into the evening sky, look for the phenomenon known as Earthshine.  If the sky is clear you should be able to see the faint outline of the disc of the Moon that is not under direct solar illumination.  This is caused by sunlight reflecting off the Earth and casting a ghostly light onto Luna’s disc.  This should be particularly prominent on the first few evenings of July.  Look for Luna near the bright star Regulus in Leo on the evenings of the 2nd and 3rd.

Now that we are past the summer solstice the time of sunrise is gradually becoming later each morning.  By the week’s end Old Sol peeks above the horizon about five minutes later than at the week’s start.  However, the time of sunset occurs only a minute earlier over the same time interval.  Most of us probably won’t notice the gradual change in sunset times until the middle of July.

We still need to wait until some two hours after sunset for the end of astronomical twilight, which is when the sky is completely dark.  At this time the stars of spring are setting in the western sky while the stars of mid-summer are climbing in the east.  The constellation of Leo, the Lion, seems to dive headfirst toward the western horizon.  Arcturus, the brightest star in the northern hemisphere in the sky, is now well west of the meridian, and the Big Dipper wheels into the northwestern sky, chasing the last vestiges of twilight.  

If you look to the south, you will see the ruddy glow of the star Antares, the heart of Scorpius, the Scorpion.  If the sky is free of haze you should be able to trace the distinctive outline of Scorpius, which is one of the few constellations that bears a strong resemblance to its namesake.  Antares is a red supergiant star that lies about 550 light years away and is analogous to Betelgeuse in Orion.  In mythology, it was a lowly scorpion that killed Orion, so it is fitting that the two constellations, each marked by these ruddy beacons, are never seen in the sky at the same time.  

The eastern sky is dominated by three bright stars that form an asterism known as the Summer Triangle.  The brightest and highest of these stars is Vega, a dazzling blue-tinted star that leads the diminutive constellation of Lyra, the Harp.  Vega is one of the closest bright stars to us at a distance of just 25 light years.  As such it is one of the most studied stars in the sky.  Its southern cohort in the Triangle is Altair, which is even closer to us at a distance of just 16.7 light years.  Interestingly, both Vega and Altair rotate very rapidly, spinning on their axes with periods measured in hours.  By contrast the mean rotation of the Sun is about 27 days.  The third and faintest member of the Triangle is Deneb, which marks the “tail” of Cygnus, the Swan.  Deneb’s relative faintness compared to Vega and Altair belies its true nature.  It is located over 100 times further from us than Vega which means that it is intrinsically very bright, pumping out the equivalent energy of some 200,000 Suns!  Deneb is a blue supergiant star and represents a small population of extremely hot, young stars that dot the galaxy.  These stars evolve quickly, and in a few million years Deneb will disintegrate in a cataclysmic supernova explosion.  This will seed the surrounding area with the heavy elements that are essential for the formation and sustenance of life as we know it.

If you are looking for the bright planets, they are beginning to make inroads on the evening sky.  Saturn rises in the southeast at around 11:00 pm local time and is the first to beg for attention through the telescope.  Jupiter rises at around 1:00 am, and you will find both giant planets well-placed for viewing before dawn.  Ruddy Mars follows Jupiter by about 45 minutes, and should also be easy to spot before dawn.  Finally, Venus rises at around 4:00 am as morning twilight begins to paint the horizon.  
 
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