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The Sky This Week, 2020 September 15 - 22

Smoke gets in your skies...
Map of Jupiter, compiled  2020 September 6 fron the U.S. Naval Observatory, Washington, DC.
Strip map of Jupiter, compiled from observations made 2020 September 6 at the U.S. Naval Observatory
with the 30.5-cm (12-inch) f/15 Clark/Saegmüller refractor and a ZWO ASI224MC color imager

The Moon returns to the evening sky late in the week, waxing through her slender crescent phases as she skirts the southwest horizon.  Look for her near the third-magnitude star Zubenelgenubi on the evening of the 20th.  On the following night she will lies just west of the star Acrab, the northernmost star in the “head” of Scorpius.  First Quarter occurs on the 23rd at 9:55 pm Eastern Daylight Time.

The astronomical season of autumn begins on the 22nd at 9:31 am EDT.  This is the moment when the Sun’s disc reaches an ecliptic longitude of 180 degrees and crosses from the northern hemisphere of the sky into the southern hemisphere.  While the term “equinox” means “equal night”, a glance at a sunrise/sunset table reveals that the difference between sunrise and sunset on this date isn’t exactly 12 hours as one might expect.  Since the Sun subtends a tangible disc and we measure sunrise and sunset by the first and last appearances of the solar limbs, the duration of daylight is 12 hours and 8 minutes on the 22nd here in Washington.  The actual date when day and night are equal falls on the 26th.  From then until March 16th next year our nights will be longer than our days.

Autumn usually brings crisp, cool nights for us to enjoy the splendors of the night sky, but this year those clear skies have become quite elusive.  My observing logbooks, which date back to 1975, are full of entries for September and October, traditionally our best months for good viewing conditions.  This year, though, you may have noticed a persistent haze hanging high in the sky on what should be days of azure blue sunshine and clear, transparent nights.  The cause lies thousands of kilometers away from us in the unprecedented raging wildfires in the American west coast states.  It is the smoke from these fires that is obscuring our skies right now.  The intensity of these conflagrations is so great that they send rapidly-rising columns of smoke high into the atmosphere where they get swept up in the continental jet streams that steer our weather systems across the country.  Unfortunately it looks as if these plumes may be with us for awhile as the western fires show no signs of abating.

The smoky aerial haze serves to remind us that we are actually quite fortunate to see the night sky at all.  The mix of nitrogen and oxygen that gives us life is also relatively transparent, affording us the luxury of gazing into outer space at night.  Despite its relative clarity our atmosphere is quite dense, with currents that we feel in the form of wind, and these currents can affect our outward view.  Astronomers rate the atmosphere in terms of two parameters, transparency and “seeing” and it is a rare night indeed when you get the best of both conditions.  Transparency is just what it sounds like, and is generally best when a cold front brings in a shot of dry, cool air.  “Seeing” is the apparent stability of the atmosphere.  With air currents in the form of surface winds and the high-speed currents that define the jet streams thousands of meters overhead, light from distant objects gets distorted as it crosses air layers of different density and relative speed.  Nights with very good transparency usually come with poor seeing.  These are nights to look for faint deep-sky objects from dark locations.  Sultry nights, when the air just seems to hang, are usually the best nights for looking at the Moon and planets since different layers of air at different altitudes waft slowly relative to each other.

These smoky nights favor objects such as Jupiter since the atmosphere is more stratified and Old Jove’s brightness is subdued.  These are the nights to look for details in the giant planet’s own atmosphere.  It is dominated by clouds of ammonia and methane which move past each other in well-defined latitudinal dark belts and bright zones.  This regimented order results from Jupiter’s small axial tilt of just three degrees.  Careful scrutiny of the planet’s darker belts will show countless spots and eddies, the signatures of storm systems that are the size of earthly continents!

Nights of good seeing won’t reveal much of the weather on Saturn since the planet’s atmosphere is topped by a layer of methane “smog”.  Good seeing will reveal subtle detail in the planet’s famous rings.  A modest telescope will reveal a thin dark gap near the rings’ outer edge, and you may notice a subtle color difference on either side of this gap.  A good six-inch telescope should show the dusky inner ring in the space between the planet’s disc and the inner part of the bright “B-ring”.  On nights of exceptionally steady seeing, large telescopes reveal a much narrower gap in the outermost ring, which I have glimpsed a handful of times.

Mars doesn’t hide its surface beneath dense layers of clouds.  It is the only place other than the Moon and Mercury where we are looking at a solid surface, so the dark features set in the brighter orange-pink areas are actual topographic features.  For centuries these features have tantalized astronomers, but today we have space probes exploring its surface and mapping it from orbit.  Despite these robotic emissaries from Earth, there is still something quite special about teasing out visual details of the planet through the telescope eyepiece.

Venus owes its brightness to globe-girdling clouds that perpetually hide her surface.  You will find the dazzling planet passing through the faint stars of the constellation of Cancer, the Crab in the pre-dawn sky.