This Week's Sky at a Glance, August 23 – September 1

FRIDAY, AUGUST 23

■ The brightest star high in the southeast these moonless evenings is Altair, with little orange Tarazed above it by a finger-width at arm's length.

A little more than a fist-width to Altair's left is delicate Delphinus, the Dolphin, leaping left.

Above Altair, slightly less far, is smaller, subtler Sagitta, the Arrow. It too points leftward. You'll need a nice dark sky.

SATURDAY, AUGUST 24

■ August is prime Milky Way time whenever the Moon is out of the evening sky... like it is this week. After evening darkness is complete, the Milky Way runs up from between Sagittarius and Scorpius in the south-southwest, then tilts leftward across Aquila, continues left through the big Summer Triangle very high, then trails on down through Cassiopeia to Perseus low in the north-northeast.

■ The Moon, nearly last quarter, rises around 10 or 11 p.m. Once it's well up an hour or two later, look for the Pleiades to its lower left by about a fist at arm's length.

SUNDAY, AUGUST 25

■ The last-quarter Moon rises late, around 11 p.m. (It's exactly last quarter at 5:26 a.m. Monday morning.) Spot the Pleiades a few degrees to the Moon's upper right. Once it rises high it shines above much else in Taurus, as shown below.

■ And while you're at it, Algol in Perseus shines two fists upper left of the Moon (off the chart above). Here you can catch Algol doing its famous act: It should be in one of its self-eclipses, magnitude 3.4 instead of its usual 2.1, for a couple hours centered on 3:13 a.m. EDT Monday morning August 26th; 12:13 a.m. PDT. Algol takes several additional hours to fade and to rebrighten. Comparison-star chart, with north up. (Celestial north is always the direction in the sky toward Polaris. Outside at night, turn the chart around to match.)

MONDAY, AUGUST 26

■ As August proceeds and nights begin to turn chilly, the Great Square of Pegasus looms up in the east, balancing on one corner. Its stars are only 2nd and 3rd magnitude. Your fist at arm's length fits inside it.

Leftward and down from the Square's left corner extends the backbone of the constellation Andromeda: three stars in a slightly curving line (including the corner) 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.

TUESDAY, AUGUST 27

■ "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, AUGUST 28

■ Arcturus is the brightest star high in the west on late-summer evenings. In astronomy lore nowadays, Arcturus may be best known for its cosmic history: It's an orange giant some 7 billion years old, older than the solar system, racing by our part of space on a trajectory that indicates it was born in another galaxy: an ancient dwarf galaxy that fell into the Milky Way and merged with it.

But in the astronomy books of our grandparents, Arcturus had a different claim to fame: It turned on the lights of the 1933 World's Fair in Chicago celebrating "a century of progress." Astronomers rigged the newly invented photocell to the eye end of big telescopes around the US and aimed them where Arcturus would pass at the correct moment on opening night. When the star's light crept onto the photocells, the weak signals were amplified and sent over telegraph wires to Chicago, and on blazed the massive lights to the cheers of tens of thousands.

Why Arcturus? Astronomers of the time thought it was 40 light-years away (modern value: 36.7 ±0.2 light-years). So the light would have been in flight since the previous such great event in Chicago, the World's Columbian Exhibition in 1893.

And earlier? Arcturus was famous as one of the first stars discovered to show proper motion, its own independent motion on the celestial sphere. In 1718 Edmond Halley realized that Arcturus, Sirius, and Aldebaran had moved more than half a degree from where the Greek astronomer Hipparchus had carefully measured them to be some 1,850 years earlier.

And before that? Arcturus was the first nighttime star to be seen in the daytime with a telescope: by Jean-Baptiste Morin in 1635.

THURSDAY, AUGUST 29

■ As summer progresses and Arcturus moves down the western sky, the kite figure of Boötes that sprouts up from it tilts to the right. The kite is narrow, slightly bent, and 23° long: about two fists at arm's length. Arcturus is its bottom point where the stubby tail is tied on.

The Big Dipper now slants at about the same height in the northwest, to the Kite's right.

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. Can you still detect Spica? Bring binoculars.

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 a 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

■ A 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 up.

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.

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This Week's Planet Roundup

Mercury barely peeps through the glare of sunrise late this week; it's still quite faint. By August 29th or so, try looking for it just above the east-northeast horizon starting about 30 or 40 minutes before sunrise. Use binoculars. It's still only magnitude +1.3 that morning, but next week it will brighten fast.

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 due west starting 20 or 30 minutes after sunset. Much will depend on the clarity of your air; humid means hazy. Binoculars will help.

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 roughly a half hour later, glowing to Jupiter's lower left.

On Saturday morning August 24th the two planets are 5° apart. By a week later, on the 31st, they've widened to 8° apart.

By the first sign of dawn each morning they're high toward the east-southeast, with Mars still to Jupiter's lower left. Mars-like Aldebaran shines to Jupiter's right. The horntip stars of Taurus, Beta and Zeta Tauri, keep the planets company as shown at the top of this page. Mars crosses the line between the horntips on the morning of August 27th. Orion sparkles below.

Saturn (magnitude +0.6, south of the Circlet of Pisces) glows low in the east as the stars 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 shines high toward the south by midnight, through less of our blurry atmosphere then for better telescopic resolution. Saturn is nearing its opposition on the night of September 7th.

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 top of the hill 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 the sky. 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 and a curious mind." Without these, "the sky never becomes a friendly place."

If you do get a computerized scope, make sure 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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