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The Sky This Week, 2018 August 28 - September 4

Changing seasons
Saturn, 2018 August 24, 01:45 UT
Imaged with the 30.5-cm (12-inch) f/15 Clark/Saegmüller refractor
at the U.S. Naval Observatory.

The Moon climbs through the dim stars of the autumnal constellations this week, gliding northward along the ecliptic before ending the week among the stars of the Great Winter Circle.  Last Quarter occurs on September 2nd at 10:37 pm Eastern daylight Time.  Luna forms an attractive triangle with the bright star Aldebaran and the Pleiades star cluster during the pre-dawn hours of the 2nd.  Onn the following morning you’ll find the Moon just east of Aldebaran.

You’ve probably noticed by now that the days are getting shorter as summer winds down through its last few weeks.  During the month of August the time of sunrise has moved some 25 minutes later and sunset now occurs 30 minutes earlier over the course of the month, but it seems that this has been particularly notable over the past two weeks.  We’re now entering the time when the apparent position of the Sun changes at its most rapid rate, and on average sunset occurs about two minutes earlier each night at the latitude of Washington.  While this means that we lose an hour’s worth of outdoor time in the evenings, it’s a welcome respite for skywatchers like me who like to turn in for the night at a decent hour.  September finds the most radical change in the duration of daylight over the course of the month, losing one hour and 12 minutes to the encroaching hours of twilight.  

The longer nights provide something of a respite for the stars of the summer sky.  As the Moon slips into the morning sky the evening hours provide a “ringside” seat to the showpieces of the summer sky.  By the end of evening twilight you’ll find the heart of the summer Milky Way crossing the meridian, with the brightest star-clouds hovering directly over the southern horizon.  By 11:00 pm the Galaxy bisects the sky, providing a wonderful hunting ground for all kinds of celestial targets.  Binoculars will reveal luminous clumps of star clusters and glowing gaseous nebulae, and under good dark-sky conditions you might also be able to trace out seeming “voids” in the star clouds.  The most prominent of these is the so-called “Great Rift”, which splits the Milky Way into two distinctive streams from near the bright star Deneb in the Summer Triangle asterism to the scattered stars of the constellation Ophiuchus, the Serpent-bearer.  These “empty” spaces are actually concentrations of non-luminous gas and dust, the very stuff that stars and planets are made of, that obscure the light of more distant stars in the Galaxy’s inner spiral arms.  Small low-power wide-field telescopes are the best instruments to explore the Milky Way with, offering enough aperture to resolve stars in the billowing clouds and highlighting the dark voids.

The early evening sky offers an exquisite parade of bright planets that span the southern sky.  From west to east you’ll see Venus, Jupiter, Saturn, and Mars arrayed like bright beads on an invisible string.  This will give you a good impression of the location of the ecliptic, the imaginary line that defies the apparent path of the Sun through the constellations of the Zodiac. 

Start looking for Venus in the southwestern sky shortly after sunset.  She should pop into view almost immediately, and if the sky is very clear you may even spot her while the Sun is still up.  She is brightening rapidly now as she begins her swing between the Earth and the Sun.  If you point a telescope at her you’ll see a dazzling fat crescent shape.  Over the course of the month her disc will grow larger as her crescent grows thinner.  Onn the evenings of the 31st and the 1st you’ll find her just over a degree south of the bright star Spica.

Jupiter sis slowly creeping eastward from the second-magnitude star Zubenelgenubi in the constellation of Libra, the Scales.  He should also become visible during evening twilight and should be prominent in the southwest as Venus dips into the horizon haze.  Old Jove is still a treat to view in any telescope.  You can follow the motions of his four bright Galilean moons with just about any telescope, and if the conditions are good a modest instrument will show the shadow of the innermost moon, Io, exiting the planet’s disc as twilight deepens.

Saturn stands on the meridian as twilight ends and is located in one of the densest starfields of the Milky Way.  If you have binoculars look just to the right of the ringed planet; you’ll see a pair of glowing knots of light.  These are two signature deep-sky targets of summer stargazers, popularly known as the Lagoon Nebula and the Trifid Nebula.  If you can tear yourself away from a telescopic view of Saturn, you’ll find each of these glowing gas clouds and star clusters well worth more than a casual look.

Mars brings up the rear of the planet parade, and despite his extremely low declination you’ll have no trouble spotting him.  He’s currently the second-brightest planet after Venus, and his ruddy tint ensures that you won’t mistake him for anything else.  Surface features are once again becoming visible as the global dust storm subsides, and you should have no trouble spotting his south polar ice cap in modest telescopes. 

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