This Week's Sky at a Glance, August 30 – September 8

FRIDAY, AUGUST 30

■ Venus, still very low in the west-southwest in bright twilight, has been creeping up very slightly day by day, while springtime Spica, barely a hundredth as bright, is finally sinking away for the year in the same vicinity (as shown below). Can you still detect Spica? Bring binoculars. They're two fists at arm's length apart.

Venus and Spica will pass each other on September 17th.

SATURDAY, AUGUST 31

■ Late these evenings as autumn approaches, Fomalhaut, the Autumn Star, makes its appearance above the southeast horizon. Its rising time will depend on where you live. Watch for it to come up two fists to the lower right of first-magnitude Saturn. By 10 or 11 p.m. you should have no trouble spotting Fomalhaut low in the southeast if you have an open view in that direction.

■ As dawn brightens on Sunday morning September 1st, look east-northeast for the hairline crescent Moon with Mercury 4° to its right, as plotted below. Binoculars will help in the brightening sky.

SUNDAY, SEPTEMBER 1

■ Look for bright Vega passing the zenith as twilight fades away, if you live in the world's mid-northern latitudes. Vega goes right through your zenith if you're at latitude 39° north (near Baltimore, Kansas City, Lake Tahoe, Sendai, Beijing, Ankara, Athens, Lisbon).

■ Another sign of the advancing season: Cassiopeia is now high in the northeast after dark, its W pattern tilting up. And below it, starry Perseus is reaching upward.

The highest part of Perseus includes the wintry Double Cluster. To find it, look back to Cassiopeia. Counting down from the top, note the third segment of the W. Continue that segment downward by twice its length, and there you are.

You're looking for what seems like a small spot of enhanced Milky Way glow. Binoculars or a finderscope will help you detect the Double Cluster even through a fair amount of light pollution. The pair are a glory in a telescope.

MONDAY, SEPTEMBER 2

■ As September begins to take hold and nights grow chilly, the Great Square of Pegasus displays itself in the east, balancing on one corner. Its stars are only 2nd and 3rd magnitude. Your fist at arm's length fits inside it.

From the Square's left corner, leftward or lower-leftward extends the backbone of the constellation Andromeda: three stars in a slightly curving line (including the corner) that are about as bright as those forming the Square.

This whole giant pattern was named "the Andromegasus Dipper" by the late Sky & Telescope columnist George Lovi. It's shaped sort of like a giant Little Dipper with an extra-big bowl. It's currently lifting its contents upward.

The actual Little Dipper, meanwhile, is tipping over leftward in the north. It's only 40% as long as the Andromegasus Dipper, and most of it is much fainter. As always, you'll find that it's oriented more than 90° counterclockwise compared to Andromegasus. It's dumping its contents out.

■ New Moon today (exact at 9:36 p.m. EDT).

TUESDAY, SEPTEMBER 3

■ The coming of September also means that Scorpius, the proudly starring highlight of the southern sky during July, is now tilting over and lying down in the southwest after dark, preparing to bed down and drift off for the season.

■ "This is my 100th column," writes Binocular Highlights columnist Matt Wedel in the September Sky & Telescope, "and to celebrate I'm going to revisit my all-time favorite target: the heart asterism at the center of Cygnus, the Swan." That was a new one on me. Could he mean the ragged ring of binoculars stars around Gamma Cygni, the center star of the Northern Cross? Well, that's part of it. Judge for yourself. He explains it on page 43 of the September issue, with a chart, in "Where the Secrets Are Kept." I'll never see that familiar field quite the same way again.

WEDNESDAY, SEPTEMBER 4

■ Find little Sagitta the Arrow barely a fist at arm's length above bright Altair high in the south these evenings. If your light pollution is too much, use binoculars.

Now imagine rotating the Arrow on its point a third of a turn counterclockwise. Its middle star would now rest just barely (0.4°) below M27, the big Dumbbell Nebula. At a total magnitude of 7½, the Dumbbell is a largish but subtle gray glow nearly 0.1° wide, easily seen in binoculars or a finderscope under a dark sky. In a 4- to 8-inch telescope it's a rectangle or hourglass. It's the brightest planetary nebula in the sky if you sum up all its spread-out light. Read all about observing it in Ken Hewitt-White's Suburban Stargazer column in the September Sky & Telescope, page 55.

By the way, the odd name Dumbbell Nebula was bestowed by John Herschel in 1828. He was referring to the exercise weights we still call dumbbells, but in his day they were sometimes made by connecting two heavy bells top-to-top by a short, thick rod. The bells were minus their clappers, so they were dumb bells. I saw an example once in a vintage image from a gym. It really did look like the nebula's hourglass shape.

A much more recent name for M27, more accurate to modern eyes, is the Applecore Nebula. The earliest use of this name that I find in print using Google Books is from 1997.

THURSDAY, SEPTEMBER 5

■ Starting about 15 or 20 minutes after sunset, look very low a little left of west for Venus and the two-day-old crescent Moon, as shown below. They're roughly 7° apart at dusk for North America. Binoculars help bring them out through bright twilight. They set before twilight ends.

FRIDAY, SEPTEMBER 6

■ Now the thickening crescent Moon shines about 18° (nearly two fists) left or upper left of Venus in bright twilight. And Spica is partway between them, as shown above. Bring binoculars.

■ Then several hours later as dawn in getting under way on Saturday the 7th, Mercury poses low over much fainter Regulus, as shown below. Again, bring binoculars.

SATURDAY, SEPTEMBER 7

■ A winter preview: Step out before the first light of dawn this week, and the sky displays the same starry panorama it will at dinnertime around New Year's. Orion is striding up in the southeast, with Aldebaran and then the Pleiades high above it. Sirius sparkles far down below Orion. The Gemini twins are lying on their sides well up in the east.

SUNDAY, SEPTEMBER 8

■ At nightfall, the crescent Moon shines low in the southwest. Look almost two fists upper left of it for orange Antares.

Two thirds of the way from the Moon to Antares is the near-vertical row of three stars marking the head of Scorpius. The brightest of these three is the middle one, Delta Sco. It's not all that much fainter than Antares.

■ On Monday morning the 9th, Mercury and Regulus will be in conjunction, 0.5° apart, low in the east-southeast by east. Bring binoculars. Regulus is to Mercury's right.

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This Week's Planet Roundup

Mercury is up in pretty good view during early dawn. Watch it brighten fast this week, from fairly faint to obvious. Look low in the east-northeast starting about 50 or 60 minutes before sunrise. On the morning of August 31st Mercury is still only magnitude +0.8, but by just a week later, September 7th, it's mag –0.7. That's four times as bright! ¹

Venus, magnitude –3.8, is very low above the west horizon in bright twilight. It's becoming very slightly more visible week by week. Look for it just left of due west starting 20 or 30 minutes after sunset. Much will depend on the clarity of your air. Humid means hazy.

Mars and Jupiter (magnitudes +0.7 and –2.3, respectively, in Taurus) continue pulling apart from each other in the early-morning sky. Watch for bright Jupiter to rise in the east-northeast around midnight daylight-saving time. Mars, much fainter, follows nearly an hour later, glowing to Jupiter's lower left. On Saturday morning August 31st, Jupiter and Mars are 8° apart. By a week later, the 31st, they've widened to 12° apart and Mars has edged into Gemini.

Mars forms a triangle with the horntips of Taurus, Beta and Zeta Tauri. Watch the triangle change shape daily as Mars moves eastward against the stars.

By the start of dawn each morning the two planets are very high toward the east-southeast, with Mars still to Jupiter's lower left. Mars-like Aldebaran shines to Jupiter's right or upper right. Week by week, Mars is starting to outshine Aldebaran.

Saturn (magnitude +0.6, south of the Circlet of Pisces) is at opposition this week: on the night of September 7th. Look for it glowing in the east-southeast as soon as the stars begin to come out. It's lower right of the Great Square of Pegasus, which is balancing on one corner. The Square's upper-right edge points diagonally down toward Saturn, two fists at arm's length away.

Saturn climbs higher through the evening. It shines highest in south after midnight, through less of our blurry atmosphere for better telescopic resolution.

Uranus (magnitude 5.7, in western Taurus) is some 20° west of Jupiter in the early morning hours. You'll need a good finder chart to identify it among surrounding faint stars.

Neptune (tougher at magnitude 7.8, near the Circlet of Pisces) is 12° east of Saturn. Again you'll need a proper finder chart.

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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 and graphics that also depend on longitude (mainly Moon positions) are for North America.

Eastern Daylight Time (EDT) is Universal Time minus 4 hours. UT is also known as UTC, GMT, or Z time.

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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 a more detailed 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 much more 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 all stars to magnitude 7.6.

Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5; nearly three times as many. (It's currently out of print.) The next up are the even larger Interstellarum atlas (stars to magnitude 9.5) or Uranometria 2000.0 (stars to mag 9.75). And read How to Use a Star Chart with a Telescope. It applies just as much to charts on your phone or tablet as to charts on paper.

You'll also want a good deep-sky guidebook. A beloved old classic is the three-volume Burnham's Celestial Handbook. An impressive more modern one is the big Night Sky Observer's Guide set (2+ volumes) by Kepple and Sanner. The pinnacle for total astro-geeks is the Annals of the Deep Sky series, currently at 10 volumes as it slowly works forward through the constellations alphabetically. So far it's only up to F.

Can computerized telescopes replace charts? Not for beginners I don't think, and not for scopes on mounts and tripods that are less than top-quality mechanically. Unless, that is, you prefer spending your time getting finicky technology to work rather than learning how to explore the sky. 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 and a curious mind." Without these, "the sky never becomes a friendly place."

If you do get a computerized scope, make sure that its drives can be disengaged so you can swing it around and point it readily by hand when you want to, rather than only slowly by the electric motors (which eat batteries).

However, finding faint telescopic objects the old-fashioned way with charts isn't simple either. Learn the essential tricks at How to Use a Star Chart with a Telescope.

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

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"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

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1 . Some of you have asked how to convert a magnitude difference into the brightness ratio. Here we go.

The stellar magnitude system is defined so that 5 magnitudes is a brightness ratio of exactly 100 to 1. So, one magnitude is a brightness difference of the fifth root of 100. Which is 2.512 for all practical purposes.

So, here's the formula to use: If Δm is the magnitude difference, then

brightness difference = 2.512^Δm

…which is just a few taps on your scientific calculator.