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The Sky This Week, 2019 September 3 - 10

A fall reprieve for summer's stars
The Summer Triangle rising, imaged 2019 July 5 at Mollusk, Virginia.
The Summer Triangle rising, 2019 July 5
imaged made with a Canon EOS Rebel SL2 DSLR from Mollusk, Virginia.
The bright stars Vega (upper left), Altair (lower right) and Deneb (lower left)
are embedded in the Milky Way.

The Moon spends the week skirting the southern horizon, waxing from crescent to gibbous phase as she passes through the southern summer constellations.  First Quarter occurs at 11:10 pm Eastern Daylight Time.  Luna may be found northeast of bright Jupiter on the evening of the 5th.  She may near golden Saturn on the nights of the 7th and 8th.  By the end of the week she sets out across the dim autumnal constellations. 

If you’re following the progress of the Sun, you’ve probably noticed that the times of sunrise and sunset are changing quite rapidly.  As we approach the autumnal equinox, the rate at which these quantities change reaches its maximum, and the overall length of day now decreases by up to three minutes per day.  As September opens the length of day is about 13 hours, and by the month’s end it will be 70 minutes shorter.  I particularly notice this in the mornings, as I now find that my alarm goes off well before the Sun comes up.  

As the time of sunset moves earlier in the evening it offers a bit of a reprieve for the summer constellations.  As the Earth orbits the Sun we see the constellations change incrementally throughout the year.  A given star will rise four minutes earlier on each successive evening, which means that it will set four minutes earlier as well.  However, with the time of sunset occurring almost two minutes earlier each evening the summer stars seem to linger, setting only two minutes later with respect to Old Sol.

The summer Milky Way gradually erodes as the waxing Moon casts more light as the week progresses.  Fortunately you can trace the course of the galaxy through the sky by looking for the bright stars scattered from the southwest to the northeast along its path.  Just below and to the right of bright Jupiter is the ruddy star Antares, the “heart” of Scorpius, the Scorpion.  If you have a good view of the southwest horizon look for the other stars that delineate this prominent constellation.  The Scorpion’s “tail” is marked by a naked-eye pair of stars, Lesath and Shaula.  Above and to the left or these is a grouping of stars that resemble a teapot with the “spout” tilted toward the pair.  This is the most recognizable part of the constellation of Sagittarius, the Archer.  The planet Saturn lies just to the left of the teapot’s “top”.  

High overhead you’ll find the stars of another large asterism, the Summer Triangle.  Each of these stars belongs to its own constellation.  Vega, brightest and westernmost of the Triangle, leads a small parallelogram grouping that represents the Lyre of Orpheus in Greek mythology.  Vega is one of the closer stars to the solar system, shining from a distance of 25 light-years.  Just east of Vega binoculars will show a closely-spaced pair of stars, Epsilon 1 and Epsilon 2 Lyrae.  This pair is a good test of resolving power for small telescopes; a good 3-inch telescope will resolve each component into a close pair.

The southernmost star in the Triangle is Altair, which is the central star in the constellation of Aquila, the Eagle, which carried thunderbolts for Zeus in mythology.  Altair is just under 17 light-years distant, and is one of the closest bright stars to the Sun.

The Triangle’s third and faintest star is Deneb, which marks the “tail” of Cygnus, the Swan.  Although it outwardly appears that Deneb is comparable to its companions, it is altogether a different kind of star.  Both Vega and Altair are bright because of their relative proximity in space, but Deneb is bright because it is one of the most luminous stars in the galaxy.  Its distance is about 2500 light years, so in order to appear as bright as it does over that vast stretch of space it shines with the radiance of over 200,000 Suns!

Jupiter is still well-placed for evening perusal.  You’ll see the giant planet appear just west of the meridian about 20 minutes after sunset, and for the next couple of hours he dominates the southwestern sky.  By 8:30 pm you should have a clear view of him in the telescope, and you should have well over an hour to enjoy the view before he begins to sink into the horizon haze.

Saturn follows Jupiter, crossing the meridian at around 9:00 pm.  Take some time to give Saturn a good look through the telescope.  On evenings when the atmosphere is steady, a modest instrument should show a nice view of the structure in the planet’s mysterious rings.

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