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The Thunder Moon and Summer's Stars

by Geoff Chester, USNO Public Affairs | 09 July 2024

by Geoff Chester, USNO Public Affairs | 09 July 2024


The Summer Triangle rising, 2019 July 5, Mollusk, Virginia.

The Moon waxes in the evening sky, diving southward along the ecliptic to spend some time among the southern summer constellations.  First Quarter occurs on the 13th at 6:49 pm Eastern Daylight Time, with Full Moon following on the 21st at 6:17 am EDT.  July’s Full Moon is popularly known as the Buck Moon, since this is the time of year when buck deer begin to grow new antlers.  Another popular name is the Thunder Moon due to the active weather that July’s heat can spawn.

Look for the Moon to occult the bright star Spica on the evening of the 13th.  This event will be visible from much of the U.S., but here in Washington we will only see the star’s disappearance behind Luna’s dark limb.  Watch the Moon creep closer to the star as evening twilight fades to darkness.  At 11:26 pm the star will abruptly wink out.  On the evening of the 17th, Luna may be found near the ruddy star Antares in Scorpius, the Scorpion. 

Sunset is gradually beginning to occur later with each successive evening.  Over the course of the next two weeks Old Sol will set earlier, shaving 10 minutes off the time of local sunset here in Washington.  Astronomical twilight, defined as the time when the Sun reaches 18 degrees below the horizon, also occurs earlier, but even by the end of the month still occurs after 10:00 pm.  For most of us the sky is dark enough by then to enjoy the bright stars that are now visible once darkness falls.

We still have spring’s brightest star, Arcturus, coursing high in the western sky as twilight fades, but a look to the east will show the stars of the Summer Triangle asterism.  The brightest of these is Vega, lead star in the diminutive constellation of Lyra, the Harp.  Vega is almost equal to Arcturus in brightness, but it has a distinctively blue tint compared to the rosy hue of Arcturus.  Like its rosy counterpart, Vega is relatively close to us, shining across some 25 light years of space.

Even closer to us is the southernmost member of the triangle, Altair.  It lies a mere 16 light years away from us.  Both Vega and Altair rotate very rapidly, spinning in a few hours as opposed to the Sun’s period of about 27 days.  

The northernmost star in the Triangle is Deneb, brightest star in Cygnus, the Swan.  While at first glance Deneb appears similar to its bright cohorts, it is a very different kind of star.  Deneb lies at a distance of well over 2000 light years, which means it must be incredibly luminous.  Indeed, it’s energy output is the equivalent of some 200,000 Suns!  It is a type of star known as a Blue Supergiant, and its lifetime will be very short by cosmic standards.  While the Sun will evolve over tens of billions of years, Deneb will end its life as a supernova in a few million.  Enjoy it while you can.

Low in the south at the end of evening twilight is the ruddy star Antares, which marks the heart of Scorpius, the Scorpion.  Antares is a red supergiant star some 550 light years from us.  It is nearing the end of its life, and gives us a kind of snapshot of what the future holds for Deneb.  Antares will likely become a supernova in about one million years.

Several planets are beginning to make their way toward the evening sky.  Saturn rises at around 11:30 pm, but he is still best observed shortly before morning twilight begins.  If you enjoyed your view of his rings last year, you’re in for a bit of a surprise.  They are tipped just two degrees to our line of sight, looking like a couple of stilettos poking out from the planet’s yellow disc.

Mars is next up, rising shortly after 2:00 am. He is also best seen before dawn.  He is moving steadily eastward into the stars of Taurus, and by the 21st and 22nd he will be about five degrees south of the rising Pleiades star cluster.

Jupiter also enters the picture, rising about an hour after Mars.  Old Jove is awaiting the arrival of the red planet as he slowly plods eastward about five degrees north of Aldebaran in Taurus.  Mars will pass the giant planet in mid-August.

 
 

Commander, Naval Meteorology and Oceanography Command | 1100 Balch Blvd. | Stennis Space Center, Mississippi 39529

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