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The Sky This Week, 2019 June 18 - 25

Scouting the southern summer horizon.
Scorpius and the summer Milky Way, imaged with a Canon EOS Rebel T2i DSLR from Sky Meadows State Park, Paris, Virginia
Scorpius and the summer Milky Way, imaged 2014 June 2 with a Canon EOS Rebel T2i DSLR
from Sky Meadows State Park, Paris, Virginia.

The Moon moves into the morning sky this week, climbing northward along the ecliptic as she wanes to Last Quarter, which occurs on the 25th at 5:46 am Eastern Daylight Time.  She begins the week close to Saturn among the stars of eastern Sagittarius.  Over the course of the week she traverses the dim star fields of the rising autumnal constellations.

The summer solstice occurs on the 21st at 11:54 am EDT.  At this moment the center of the Sun’s disc reaches an ecliptic longitude of 90 degrees.  This is also the moment when Old Sol reaches his most northerly declination for the year.  Here in Washington we’ll experience the longest duration of daylight, clocking 14 hours, 54 minutes between sunrise and sunset.  This was a special time of year for primitive civilizations throughout the Northern Hemisphere, and we find evidence of structures and sacred locations where this event was observed.  The most famous of these is undoubtedly Stonehenge on the Salisbury plain in England.  This site was occupied and worked over for over 1500 years, beginning as an elaborate circle of earthworks and wooden post-holes at around 3100 BCE.  Its axis was aligned to the point on the distant horizon where the Sun rose on the day of the solstice.  The stone circle that we see today was erected circa 2600 to 2400 BCE, and a number of other astronomical alignments were incorporated in its design.  Structures similar to Stonehenge are found throughout megalithic Europe, and they are almost all aligned with the midsummer sunrise.  Here in North America there are also many sites left behind by long-vanished indigenous people.  Throughout the southwestern U.S. there are dozens of so-called “medicine wheels”, pecked rock spiral carvings, and kivas that are markers for this special date.  Today the solstice marks the beginning of the astronomical season of summer, but for many of these ancient cultures the date was observed as mid-summer.  Our longest days will persist until the 25th.  After that date the length of day will gradually begin to shorten as the calendar presses toward the autumnal equinox in September.

As the Moon wanes and moves into the morning sky, it’s a good week to enjoy the few hours of darkness and the rising bright stars of the seasonal sky.  From a dark location at midnight the summer Milky Way cuts through the eastern sky, wafting southward to the distinctive constellation of Scorpius, the Scorpion.  This is one of the few constellations in the sky that gives a passable rendition of its namesake.  It is highlighted by a bright red-tinted star, Antares, which marks the scorpion’s heart.  To the west you’ll see three stars in a vertical arc that outline the beast’s head, but to me its most striking feature is the large “fish hook” asterism that curls down toward the horizon.  To ancient Polynesians that feature was just that: the giant fish hook used by the god Maui to dredge islands from the bottom of the ocean!  Scorpius leads a number of bright stars into the sky as you look northward along the Milky Way.  The brightest of these may be found high in the east in a large asterism known as the Summer Triangle.  The brightest and highest of these stars is Vega, which leads the small constellation of Lyra, the Harp.  It is the fifth-brightest star in the sky, and its blue tint offers a striking contrast to the ruddy hue of Antares.  The southernmost star in the triangle is Altair, the lead star of Aquila, the Eagle.  This is one of the closer stars to the Sun, located just under 17 light-years away.  It has been extensively studied and found to have a very rapid rotation which causes it to be highly flattened at its poles.  The third star in the Triangle is Deneb, the “tail” of Cygnus, the Swan.  While superficially resembling Altair, Deneb is over 150 times more distant than Altair, which means it must be over 150,000 times the luminosity of the Sun to appear as bright as it does in our sky. 

Mars and the elusive planet Mercury may still be found cavorting in the western twilight sky.  The two planets are just over 5 degrees above the west-northwest horizon at around 9:30 pm EDT.  Use binoculars and wait for a good clear night to bag the planetary pair. 

Jupiter is now prominent in the southeastern sky as evening twilight fades to darkness.  The giant planet is now at his best for telescopic observation, although even a pair of binoculars will give you a view of his four bright Galilean moons.  The evenings of the 24th and 25th will be good opportunities to track them down with binoculars, and a small telescope will offer a very pleasing view of each of them at their farthest from the planet on the latter evening.

Saturn follows Jupiter into the southeastern sky, rising with the waning gibbous Moon on the evening of the 18th.  The ringed planet and Jupiter will be with us all summer long, so you’ll have plenty of opportunities to spend evenings with these distant worlds. 

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