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The Sky This Week, 2017 September 19 - 26

Tea time in the summer sky, anyone?
Idaho_MW_170821_lined_01small.jpg
The Summer Milky Way, 2017 August 21,
imaged from Smith's Ferry, Idaho by Geoff Chester
with a Canon EOS Rebel T2i DSLR, 30-second unguided exposure.
The "Teapot" asterism is highlighted.

The Moon returns to the evening sky this week, skirting the southwestern horizon as she waxes to First Quarter, which will occur on the 27th at 10:54 pm Eastern Daylight Time. Look for Luna’s thin crescent near Jupiter in the fading twilight of the 21st and 22nd. On the evening of the 26th you’ll find her just three degrees north of Saturn.

The autumnal equinox occurs on the 22nd at 4:02 pm EDT. This is the moment when the center of the Sun’s apparent disc crosses the apparent ecliptic longitude of 180 degrees. This is also the point in the sky where the ecliptic crosses the celestial equator, so Old Sol effectively moves from the northern to the southern hemisphere of the sky. If the Sun was a point source of light and we lived on a perfectly spherical planet with no atmosphere this would be the date on which we experience equal amounts of daylight and night. However, the Sun’s disc subtends an arc of about half a degree of arc and Earth has a substantial atmosphere that bends light near the horizon. This means that here in Washington, the time of "equal night" won’t occur until the 25th. From that time until March 17th next year our nights will be longer than our days.

Light from the waxing crescent Moon shouldn’t be too much of a factor if you’d like to get away from the city and enjoy the bright stars and dense star clouds of the summer Milky Way. Evening astronomical twilight ends at around 8:30 pm, at which time the Summer Triangle passes directly overhead. The center of the Milky Way is well placed in the south, not far from the golden glimmer of the planet Saturn. Here you will find one of my favorite asterisms in the sky, popularly known as The Teapot. It consists of the brighter stars of the constellation of Sagittarius, the Archer. To the ancients Sagittarius represented a Centaur, but a Teapot is far easier to form from the pattern’s brightest stars. From a dark site the brightest Milky Way star cloud looks just like steam emanating from the Teapot’s "spout". If you sweep this area from south to north with binoculars you’ll encounter many bright knots of "fuzzy" light that betray vast glowing clouds of luminous gas, dazzling galactic star clusters, and round hazy globular star clusters. If you have a small telescope, this is the place to spend a few hours taking in these sundry delights of the "deep sky". By midnight, the Milky Way wheels into the western sky and autumn’s dimmer stars take the evening stage. High in the east look for a square asterism of second-magnitude stars. They represent a part of Pegasus, the mythical Flying Horse of ancient Greek Lore.

Jupiter gets his last visit from the Moon for 2017 this week. The giant planet is now only visible during evening twilight, but he’s bright still enough to appear in the glow shortly after sunset. Look for a very slender crescent Moon a few degrees below Old Jove on the evening of the 21st. You’ll have an easier time spotting the Moon just above Jupiter on the following evening.

Saturn gets his own visit from the Moon on the evening of the 26th. The ringed planet is now well west of the meridian as twilight ends, but you’ve still got a couple of hours to enjoy him in the telescope. With the end of the Cassini mission, Saturn now joins Mercury, Venus, Mars, and Jupiter as planets that now contain small traces of material from the Earth.

Most of the planetary action takes place in the pre-dawn sky as Venus, Mars, and Mercury cavort in the gathering twilight. Venus will be the easiest to spot, shining brilliantly about 20 degrees above the eastern horizon at 6:00 am. Use binoculars on the morning of the 20th to spot the bright star Regulus just over half a degree away. Venus will rapidly leave the star in her wake as the week progresses. Ten degrees below Venus you should see the glimmer of Mercury, which should be easy to see in binoculars. The two planets maintain this gap for the next several mornings. Between Mercury and Venus is the much dimmer glow of Mars, which will lie halfway between the two brighter planets on the mornings of the 23rd and 24th. Venus will overtake the red planet and pass him in early October.

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