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The Sky This Week, 2019 August 13 - 20

Scouting the Scorpion
Jupiter and Scorpius, imaged 2019 June 28 at Fishers Island, New York.
Jupiter and Scorpius, imaged 2019 June 28
with a Canon EOS Rebel SL2 DSLR from Fishers Island, New York.
Antares is the bright star just above the left side of the airplane trail.

The Moon leaves the summer constellations behind her this week, wending her way through the rising stars of the dim autumnal constellations.  Along the way she casts her bright light across the landscape as late night turns into early morning.  Full Moon occurs on the 15th at 8:29 am Eastern daylight Time.  August’s Full Moon is variously known as the Grain Moon, Corn Moon, or Sturgeon Moon, indicative of the season when these resources are harvested.

By now most of us are noticing the decrease in daylight from the days of mid-summer just a few weeks ago.  By the end of the week the Sun sets just before 8:00 pm here in Washington, forty minutes later than he did at the time of the latest sunsets in late June.  The total duration of daylight from sunrise to sunset is now 80 minutes shorter that it was at the time of the solstice.  On average we’re losing just over two minutes of daylight per day, and we’ll lose

Shorter days mean longer nights, and for astronomers this is a welcome time.  We are entering the time of year when weather conditions become milder and the air becomes more transparent.  In addition, we still have the bright stars of the summer sky gracing our evenings, and despite the bright moonlight we can enjoy these bright beacons for much of the night.

Evening twilight now ends just after 9:30 pm, and at this time you’ll find the prominent constellation of Scorpius just above the southern horizon.  Scorpius is one of the few constellations that clearly resembles its namesake.  The Scorpion’s “heart” is marked appropriately by the red-tinted star Antares.  The star’s name means “rival of Mars”, and when the two objects are close to each other in the sky you will see why.  It is located about 550 light-years from the solar system, yet despite this enormous distance it is the 15th-brightest star in the sky.  Its red tint tells us that its surface temperature is relatively cool, but it is still over 75,000 times brighter than the Sun.  To produce this prodigious amount of energy its surface area is vast on typical stellar scales.  If Antares were to replace the Sun in our planetary system, Mars would orbit inside its outer layers!  Antares is flanked by two stars, Tau and Sigma Scorpii, the latter of which is called Alniyat.  If you point your telescope between Antares and Alniyat you will see a fuzzy ball of light that betrays the globular star cluster Messier 4.  A four-inch telescope is sufficient to resolve the cluster into a swarm of stars.  The “head” of the Scorpion is defined by three second-magnitude stars in a vertical row that lie about eight degrees west of Antares.  The rest of the creature’s “body” forms a fish hook shaped asterism to the southeast of Antares.  Many of these stars have a distinctive blue tint and share a common origin along with dozens of stars in the southern constellations of Centaurus and Crux.

Moving from Scorpius up to the zenith, the three bright stars of the Summer Triangle are in prime position between 10:00 pm and midnight.  Vega, Deneb, and Altair are among the twenty brightest stars in the sky, and from dark locations the summer Milky Way runs through the trio with two distinct star clouds.  In the middle of the Triangle is a third-magnitude star that is one of the best objects for viewing from urban areas.  Known as Albireo, it is one of the finest examples of a double star in the sky.  It can be resolved with almost any telescope, and where your naked eye sees one star the telescope reveals two, each of which has a distinctive color.  The brighter component glows with a golden hue, while the fainter one has a distinct blue color.

Jupiter hovers a few degrees northeast of ruddy Antares, shining brightly in the southwestern sky.  Despite his low altitude he offers a fine view for the small telescope.  The four large moons first described by Galileo are visible under almost any atmospheric condition, and in moments when the air is still look for the equatorial cloud belts that girdle the planet itself.  

Saturn is the best planet for “prime time” viewing, crossing the meridian at around 10:30 pm.  This distant world is one of the most iconic sights too be seen through the telescope.  When Galileo first saw it he thought that it was three separate worlds.  It wasn’t until 1655 that its strange appendages were identified as rings by the Dutch astronomer Christiaan Huygens.  Modern telescopes show the planet’s rings quite easily, and on nights of steady air will show subtle differences in color and structure.

Mercury is nearing the end of a very favorable morning apparition.  You will find the elusive planet in morning twilight, about 10 degrees above the east-northeast horizon half an hour before sunrise.  He reaches peak brightness by the week’s end as he starts his plunge toward the Sun.

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