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The Sky This Week, 2020 June 9 - 16

The summer stars are rising.
The Summer Triangle rising, imaged 2018 June 20, Desolation Canyon, Utah.
The Summer Triangle rising, imaged 2018 June 20, Desolation Canyon, Utah
with a Canon EOS Rebel T2i DSLR and 18-55mm Canon EFS zoom lens @ 18mm, 30s, f/5.6, ISO 6400.

The Moon wanes in the morning sky this week, climbing through the rising autumnal constellations as she reaches Last Quarter, which occurs on the 13th at 2:24 am Eastern Daylight Time.  Early risers on the 13th will find the Moon close to ruddy Mars, which is currently the brightest object in this part of the sky.

This week we begin the two-week “season” of phenomena connected with the summer solstice.  On the morning of the 13th we will experience the earliest sunrise of the year.  Here in Washington that means that Old Sol crests the horizon at 5:42 am EDT.  Sunrise will be a minute later by the time we get to the solstice itself on the 20th, but the time of latest sunset will still be gradually increasing by then.  We won’t see our latest sunset until the 27th, when the Sun will go below the horizon at 8:38 pm.  In between we will see the longest duration of daylight fall on the date of the solstice itself, when we will experience 14 hours and 54 minutes under the Sun.  The reason for this seeming discrepancy is a combination of the Earth’s elliptical orbit around Old Sol and our precise way of reckoning time in standard time zones.  If our solar orbit was perfectly circular then all of these phenomena would “line up” on the day of the solstice, but Nature has a way of making things a bit more complicated.  Earth’s orbit is slightly elliptical, so its instantaneous velocity along this path is not constant.  It moves faster when it is closer to the Sun and slower when it is farthest.  However, its rotation speed is essentially constant, so the apparent motion of the Sun through the sky varies slightly from day to day.  This wasn’t an issue when folks kept time by sundials; noon always occurred when the Sun crossed their local meridian.  The advent of mechanical clocks changed that.  Such devices keep a regular 24-hour time scale, and sometimes the “apparent” Sun would cross the meridian a few minutes before of after noon as indicated on the clock.  This effect is most noticeable around the times of the solstices, which occur within weeks of Earth’s perihelion in early January and aphelion in early July.  

As we approach the year’s shortest nights, we begin to bid farewell to the springtime constellations and welcome the brighter stars of the summer sky.  By the end of evening twilight, which now occurs at around 10:30 pm local time, the stars of Leo, the Lion are well on their way to setting in the west as three bright stars ascend in the east.  The brightest of these is Vega, which is prominent in the northeast at this time.  Vega is part of the diminutive constellation of Lyra and is the brightest star in a larger asterism known as the Summer Triangle.  You will find the other two stars in the triangle somewhat lower toward the horizon, but they are easily seen from even inner-city skies.  Below Vega is the star Deneb, which marks the “tail” of Cygnus, the Swan.  Farther toward the south is Altair, the brightest star in Aquila, the Eagle.  All three stars have a bluish-white tint that contrasts nicely with the rosy glow of Arcturus, which sits high in the south at this time.  Finally, glowing like a distant hot coal in the southeastern sky, you will find Antares, the “rival of Mars” and the heart of Scorpius, the Scorpion.  

By midnight the triangle will be higher in the sky and Antares will be near the meridian.  From a good dark-sky location you should be able to see the billowing star clouds of the Milky Way bisecting the triangle and wafting down toward the tail of Scorpius.  These seeming clouds are the disc of our home galaxy, composed of billions of remote stars that blend their light into an amorphous haze.  Binoculars or a small, rich-field telescope will reveal their true nature and provide some of the most stunning views of our place in the cosmos.

The elusive planet Mercury may still be glimpsed in the west as evening twilight falls.  Half an hour after sunset he may be found about five degrees above the horizon and about ten degrees below Castor and Pollux, the Twin Stars of Gemini.  He continues to fade, however, so binoculars will be a big help pulling him out of the sunset afterglow.

Jupiter rises in the southeast shortly before 11:00 pm, followed a few minutes later by Saturn.  The two gas giants are still best seen in the wee hours of the morning, but they will soon be fixtures in the evening sky.  

Mars continues to march through the dim stars of Aquarius, but he is unrivalled in this part of the sky.  He will get a visit from the Moon on the morning of the 13th, but you shouldn’t have any trouble spotting his cheery ruddy glow as twilight begins to brighten the sky.

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