The bursting stars RS Ophiuchi and Nova Cassiopeiae 2021 continue to hover around magnitude 8½.


■ Before moonlight returns to the evening sky next week, trace out the Milky Way arching from horizon to zenith to horizon after nightfall is complete. Even through a fair amount of light pollution you can see subtle traces of it. The Milky Way runs from the low south-southwest (between Sagittarius and the tail of Scorpius) up through Aquila and then Cygnus high overhead, on down through Cepheus and Cassiopeia to Perseus low in the north-northeast.

Cygnus sports the Cygnus Star Cloud, one of the Milky Way's richest areas. Explore here in depth with binoculars or a small scope using Matt Wedel's "Touring Cygnus with Binoculars" in the September Sky & Telescope starting on page 34.

■ In the dawns of Saturday and Sunday, spot the thin morning Moon waning away as shown below.

The waning crescent Moon steps down in the east-northeast in the dawns of Saturday and Sunday September 4th and 5th. On the 6th, shortly before sunrise, the Moon will almost certainly be too low and slim to detect even with binoculars or a telescope, even if the air is very clear; the Moon will then be only about 16 hours from new for East-Coast crescent hunters, and a mere 13 hours from new in the Pacific time zone.

Want to give this really major challenge a try? And maybe set your thin-Moon lifetime record sighting? Read how in "Seeking Very Old Lunar Crescents" in the September Sky & Telescope starting on page 49.


■ In early twilight, look for little Spica lower left of Venus by 1.8°. It's only 1/100 as bright. Tomorrow they'll be 1.6° apart.


■ Now in early twilight, look for Spica under Venus by 1.6° as shown below. And, can you manage to catch Mercury 15° (about a fist and a half) to their lower right?

Spica, a finger-width below Venus this evening, is only 1% as bright. Well at least it twinkles.

In reality, Spica is nearly 1,000 times larger than Venus in physical diameter. And with its blinding blue-white temperature, it emits 5 trillion times more light than the sunlight that Venus reflects. But at a distance of 260 light-years, Spica is 16 million times farther away. Space is big.

■ As autumn approaches, Fomalhaut, the Autumn Star, makes its appearance above the southeast horizon in mid- to late evening. Its rising time will depend on where you live. But by about 10 p.m. you should have no trouble spotting it low in the southeast; just find an open view in that direction. This season Fomalhaut is easier to locate than usual: Look about two fists below Jupiter and a little left.

■ New Moon (exact at 8:52 p.m. EDT).


■ By 10 p.m. two of the best-known deep-sky objects, the Double Cluster in Perseus and the Great Andromeda Galaxy M31, are in high view in the east. Did you know they're only 22° apart? They're both cataloged as 4th magnitude but to the naked eye they look rather different from each other, the more so the darker your sky. See for yourself; they're plotted on the all-sky constellation map in the center of the September Sky & Telescope, which should be all the map you need to identify their locations. They're below Cassiopeia and farther to Cassiopeia's right, respectively. Sky too bright? Use binoculars!

The two clusters of the Double Cluster (NGC 869 and NGC 884) are at very similar distances about 7,600 light-years away. M31, at 2.5 million light-years, is about 330 times as far.


■ Early in twilight use the low, thin Moon to guide you down toward Mercury, as shown below. They'll be about 5° apart. Binoculars help.

Reentering the evening sky to start a new lunation, the thin Moon hangs upper right of Mercury, then upper right of Venus, low in the west-southwestern twilight. Little Spica switches position with respect to them from the 8th to the 9th. (The Moon is drawn three times its actual apparent size.)


■ Now catch the more obvious crescent Moon paired with white-light Venus low in the west-southwest in twilight, as shown above. They'll be 4° apart for North America. Can you make out Spica below them?


■ The two brightest stars (not planets) of September evenings are Vega high overhead and Arcturus in the west, both magnitude zero.

Draw a line from Vega down to Arcturus. A third of the way down you cross the dim Keystone of Hercules. Two thirds of the way you cross the dim semicircle of Corona Borealis with its one modestly bright star: Alphecca, the gem of the crown.


■ Look left of the thick crescent Moon for the head stars of Scorpius, then orange Antares. Halfway between the Moon and Antares (for the Americas) is the brightest of the head stars, the long-term eruptive variable Delta Scorpii. It's been holding fairly steady at magnitude 1.8 since 2010.

This Week's Planet Roundup

Mercury (magnitude 0.0) is very deep down in the sunset, 15° lower right of Venus this week. You might have a chance at it with binoculars or a wide-field scope. About 15 or 20 minutes after sunset, scan for it just above your horizon due west. Good luck.

Venus, brilliant at magnitude –4.0, shines in the west-southwest during twilight. It still sets around twilight's end.

Jupiter and Saturn shine in the southeast to south these evenings. They're magnitudes –2.8 and +0.3, respectively, in dim Capricornus.

Jupiter starts the evening as slightly the lower of the two. lowest. Saturn glows 17° (almost two fists) to Jupiter's upper right. They level out around 10 p.m. daylight-saving time. By then they're about at their highest in the south, at their telescopic best. See "Saturnian Challenges" starting on page 52 of the July Sky & Telescope, also "Dog Days with the Gas Giants" on page 40 of August.

Three satellites on Jupiter, two shadow transits, and more! "Today we had a grand slam event," wrote Christopher Go from the Philippines on August 15th. "A very rare triple transit on Jupiter with the moons Callisto, Ganymede and Europa in transit. As a bonus, a partially eclipsed Io was seen just before it went behind Jupiter. After the triple transit there was a very rare event when Ganymede occulted Europa followed by Ganymede eclipsing Europa! Seeing the shadow of Ganymede over Europa was surreal!" In the images here, north is up.

But the best way to follow the sequence of events is by video! The Washington Post put it all together along with a step-by-step guide: Watch this ‘surreal’ Jupiter eclipse that you probably missed.
Saturn on July 25, 2021
Saturn on July 25th. North is up. This exquisite stacked-video image shows detail in the broad B Ring, as well as the thin Encke Gap just inside the outer edge of the outer A Ring. Saturn's Equatorial Zone is bright as usual, and its North Equatorial Belt is distinct. The narrow dark band seemingly on the other side of the Equatorial Zone is actually the dim, inner C ring. Agapios Elia of Nicosia, Cyprus, took video across 45 minutes on July 25th using a 9.25-inch Schmidt-Cassegrain scope. He de-rotated the globe during processing and then stacked large numbers of the of best frames.

Uranus (magnitude 5.7, in Aries) gets high in the east after midnight.

Neptune (magnitude 7.8, at the Aquarius-Pisces border) is fairly high in the southeast as early as 10 p.m.

All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America.

Eastern Daylight Time, EDT, is Universal Time minus 4 hours. Universal Time is also known as UT, UTC, GMT, or Z time. To become more expert about time systems than 99% of the people you'll ever meet, see our compact article Time and the Amateur Astronomer.

Want to become a better astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.

This is an outdoor nature hobby. For an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential magazine of astronomy.

Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas (in either the original or Jumbo Edition), which shows stars to magnitude 7.6.

Pocket Sky Atlas cover, Jumbo edition
The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6, and hundreds of telescopic galaxies, star clusters, and nebulae among them. Shown here is the Jumbo Edition, which is in hard covers and enlarged for easier reading outdoors by red flashlight. Sample charts. More about the current editions.

Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5; nearly three times as many. The next up, once you know your way around, are the even larger Interstellarum atlas (stars to magnitude 9.5) or Uranometria 2000.0 (stars to magnitude 9.75). And be sure to read how to use sky charts with a telescope.

You'll also want a good deep-sky guidebook, such as the big Night Sky Observer's Guide by Kepple and Sanner.

Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically, meaning heavy and expensive. And as Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."

Audio sky tour. Out under the evening sky with your
earbuds in place, listen to Kelly Beatty's monthly
podcast tour of the heavens above. It's free.

"The dangers of not thinking clearly are much greater now than ever before. It's not that there's something new in our way of thinking, it's that credulous and confused thinking can be much more lethal in ways it was never before."
            — Carl Sagan, 1996

"Facts are stubborn things."
            — John Adams, 1770



Image of Rod


September 3, 2021 at 3:04 pm

Last night I observed 1930-2130 EDT. I star tested the 10-inch Newtonian after cleaning the primary mirror and collimating with a collimation tool I recently purchased. Excellent views! At Saturn using 14-mm eyepiece at 86x, I could see rings, Cassini division, and 5 Saturnian moons. Faint stars in FOV too some 10th to 12th magnitude. While observing Saturn, between 2030-2100 EDT, 3 satellites passed by the FOV close to Saturn's position, perhaps 3rd to 5th magnitude. I star tested using Altair at 34x using 2-inch eyepiece and 1.25 inch, 14-mm eyepiece at 86x views. Altair defocused, nice circular disk shape and tack sharp image when focused. Same for Albireo but lovely double star at 86x, gold and blue colors. At Altair, numerous faint stars visible in FOV, 9th to 12th magnitude or fainter. Same at Albireo, many faint stars in FOV all around in Cygnus. Quite a lovely sight. Using 14-mm eyepiece at 86x, true FOV ~ 50 arcminute. I checked star fields visible at Altair using Stellarium and Starry Night that showed stellar magnitudes. Skies were clear, temperature 17C with lighter NW winds. Wonderful viewing weather.

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mary beth

September 3, 2021 at 3:32 pm

Very good report Rod, so glad you had such nice conditions. Sounds colorful too! Is the Newtonian new or one you’ve had and put back into service?

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Image of Anthony Barreiro

Anthony Barreiro

September 3, 2021 at 3:48 pm

A clean well collimated mirror is a thing of beauty.

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Image of Rod


September 4, 2021 at 9:45 am

mary beth, Anthony Barreiro. I purchased my 10-inch Newtonian brand new back in June 2018 and first light was 28-June-2018 observing Jupiter. I noticed after some 3 years of regular use out in my horse pastures and fields during spring, summer, and winter nights, the big primary mirror was looking dirty and not as bright when I shined my light into the tube at night. I used cotton balls and water to lightly clean the mirror and a tab or two of rubbing alcohol on some spots if needed. It cleaned up nicely. Collimation was more difficult, first time for me. I purchased a set of metric hex keys (like Allen wrenches), some of those are used on the secondary mirror to adjust if needed. Three adjustments screws on both mirrors like my Telrad. Now I can take of the Newtonian and keep it in very good shape for regular use. Anthony is correct. *A clean well collimated mirror is a thing of beauty* 🙂 My 90-mm refractor is easier to clean too and no collimation needed. Keeping your telescopes clean, aligned correctly, and clean eyepieces goes a long way to enjoyable stargazing for years to come.

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