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The Sky This Week, 2020 August 25 - September 1

Anyone for a spot of tea?
Sagittarius and the summer Milky Way, imaged from Smith's Ferry, Idaho, 2017 August 21.
Sagittarius and the summer Milky Way
imaged from Smith's Ferry, Idaho, 2017 August 21

The Moon skims the southern horizon this week, waxing toward her Full phase, which occurs on September 2nd at 1:22 am Eastern Daylight Time.  We usually call September’s Full Moon the Harvest Moon, but I have always thought that this name was best used on the Full Moon that occurs closest to the autumnal equinox.  That Full Moon will fall on October 1st, a time when farmers would be traditionally bringing in their harvested bounty.  Fortunately, some crops are at their peak ripeness now, so an early September Full Moon is often called the Corn Moon, Barley Moon, or Fruit Moon.  Luna passes just south of bright Jupiter on the evening of the 28th.  On the following night she will be just southeast of Saturn.

We are now approaching the time of year when we see the greatest rate of loss in the length of daylight.  Over the course of the week we will lose about 17 minutes of total daylight, with 10 minutes of that coming off our sunset times.  Since the time of the summer solstice more than an hour and a half of daylight has disappeared, and we’ll lose another hour by the time the equinox occurs.

The lengthening nights have a positive side, though, since we can now enjoy stargazing at more reasonable times.  The constant seasonal change of the constellations seems to slow a bit during the late summer and early autumn, giving us more time to take in the splendors of the summer sky.  Even with the brightening Moon moving through the evening sky we can still see the brighter summer constellations.  To the south, the curving arc of the stars of Scorpius hover over the southern horizon, led by the orange-tinted star Antares.  You will find three second-magnitude blue-tinted stars that mark the Scorpion’s “head” to the right of Antares.  The middle star of this trio is called Dschubba, and over the past few years it has shown remarkable variability.  Normally it is comparable to its northern and southern companions, but a couple of times over the past 20 years it has doubled in brightness, second only to Antares as the constellation’s brightest star.  North of Dschubba is the star Acrab, which resolves into a fine double star in small telescopes.  However, there is more than meets the eye here, as each component is itself a triple-star system.  I often wonder what it might be like to live on a planet in orbit around such unusual star systems!

East of Scorpius is the constellation of Sagittarius, the Archer.  To the ancients this represented a Centaur, a mythical beast with the head and torso of a man and the body of a horse.  Here is one of those situations where I have trouble “seeing” what the ancients saw, but fortunately we can look at the constellation’s brighter stars and delineate a fairly respectable “teapot” asterism.  If you have a really clear view of the southern horizon you can even imagine tea being poured out of the teapot’s spout into the tail of Scorpius, which makes an acceptable tea mug!  On moonless nights look for “steam” emanating from the teapot’s spout; that’s the bright Sagittarius star cloud of the Milky Way.  Our Galaxy’s center is about 30,000 light-years off in that direction.

The bright planets Jupiter and Saturn follow Sagittarius across the southern sky.  Jupiter’s retrograde motion has brought the giant planet closer to the “handle” of the teapot, and if you look at Old Jove with a pair of binoculars you will find him under another asterism known as the “teaspoon”.  Jupiter now crosses the meridian at 10:00 pm local time, so he’s in prime viewing position during the evening hours.  Look for the planet’s famous Great Red Spot near the center of his disc on the evening of the 29th.  On the following night you can watch his innermost large moon Io transit his disc with its shadow following behind it.

Saturn is also in prime viewing position during the evening hours.  Detail on this planet is much more subtle than the prominent cloud belts of Jupiter, but that is more than made up for by the planet’s enigmatic ring system.  Thanks to the multi-year observations of the Cassini space probe we now know that the rings are composed of a myriad of chunks of ice that form the flattest structure known in the solar system.  While they span hundreds of thousands of kilometers in diameter, they are less than 100 meters thick!

Mars continues to drift through the faint stars of Pisces and is still best placed for viewing in the wee hours of the morning.  He’s slowing his eastward pace as Earth catches up to him, and he will begin retrograde motion in another two weeks.  He is well within range of modest amateur telescopes, so if you’re up before dawn consider giving him a look.

Venus joins the rising stars of winter, beaming down from a northeasterly perch as morning twilight begins.  This week you will find her plodding eastward between the stars Castor and Pollux in Gemini and the star Procyon in Canis Minor.