This Week's Sky at a Glance, August 13 – 21

RS Ophiuchi erupts. For the first time since 2006, on August 8th the famous recurrent nova RS Ophiuchi lept from its normal magnitude 11.2 to 4.8, dim naked-eye magnitude. In the first week after its past outbursts RS Oph has faded smoothly by about two magnitudes, then it declines more slowly down to normal. See Bob King's Recurrent Nova RS Ophiuchi Just Blew its Top! with finder charts and comparison stars. Ophiuchus is ideally placed high in the evening sky.

Nova Cas fades. Nova Cassiopeiae 2021 has bounced around in brightness ever since it erupted from 15th magnitude to 7.7 in March. Among its bumps were brightenings to magnitude 5.5 in early May and 6.0 around July 27th. As of August 12th it was down to about 8.5. Charts and comparison stars. Cas too is excellently placed in the evening sky.

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FRIDAY, AUGUST 13

■ A twilight challenge: as twilight fades, examine Venus with binoculars or a small telescope. You may notice a tiny spark barely 0.1° from it (below it as seen from mid- North America). That's Beta Virginis. At visual magnitude 3.6, Beta Vir is 1,000 times fainter than Venus.

■ Much easier: Look for 1st-magnitude Spica lower right of the Moon, by a little less than a fist at arm's length. Much higher to their upper right shines brighter Arcturus.

SATURDAY, AUGUST 14

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

SUNDAY, AUGUST 15

■ First-quarter Moon (exact at 11:20 a.m. EDT). This evening the Moon shines near the head of Scorpius. Look left of the Moon for the roughly vertical row of Beta, Delta, and fainter Pi Scorpii, between the Moon and Antares. Delta's proper name, Dschubba, is from the Arabic for "Forehead" of the scorpion.

Delta is the brightest of the three. It's an irregular variable star: a fast-rotating blue subgiant throwing off luminous gas from its equator. It has a smaller orbiting companion that seems to trigger more such activity at 10.5-year intervals. Long presumed to be stable, Delta unexpectedly doubled in brightness in the summer of 2000 and has remained nearly that bright, with fluctuations, for many of the years since.

Astronomers are waiting to see whether it will display another flareup soon, when the companion makes its third pass by the primary star since the instability started in 2000.

MONDAY, AUGUST 16

■ Now the Moon shines upper left of Antares, as shown below (for North America).

■ Regarding the Moon in your scope, have you ever made a study of lunar craters' central peaks? Explore them and the physics behind them with Chuck Wood's "Hills in the Middle" in the August Sky & Telescope, page 52.

TUESDAY, AUGUST 17

■ After nightfall, look far below the Moon for the Cat's Eyes in the tail of Scorpius (as shown above for mid-twilight). These are Lambda and fainter Upsilon Scorpii, 0.6° apart and canted at an angle; the cat is tilting its head to the right. And the cat has a bleary eye; Upsilon is fainter than Lambda (they're magnitudes 2.6 and 1.6). Both are blue-white giants, 500 and 700 light years away, respectively. Yes, the fainter one is the nearer one.

WEDNESDAY, AUGUST 18

■ Way, way, way down low in the bright afterglow of sunset, Mercury and Mars are in conjunction only 0.1° apart (for North America). About 15 or 20 minutes after sunset, use binoculars or a small telescope to try to pick up Mercury a few degrees above the horizon. It's a trace to the right of due west, far lower right of Venus. You've got a fairly good chance; Mercury is currently magnitude –0.5. Mars is another story; at magnitude +1.8 it's only an eighth as bright. Good luck with that. . . .

■ After dark, look above the waxing gibbous Moon for 2nd-magnitude Sigma Sagittarii (Nunki). It's the brightest star in the handle of the Sagittarius Teapot.

The Moon's dark limb occults Nunki for Florida and other parts of the southernmost US, Mexico, Central America, and the Caribbean; map and timetables.

THURSDAY, AUGUST 19

■ The Moon forms a great, curving arc with Saturn and brighter Jupiter, which are to its left.

■ Jupiter is at opposition, opposite the Sun as seen from Earth. Among other things, this means the tiny black shadows of Jupiter's moons cross the face of the planet very close to the moons themselves. Both the shadows and the moons enter and exit Jupiter's edges just a few minutes from each other. See timetables for all these events (as well as for when Jupiter's Great Red Spot transits the planet's central meridian) in the August Sky & Telescope, pages 51-52.

FRIDAY, AUGUST 20

■ The bright Moon shines under Saturn this evening, and Jupiter shines to their left, as shown below.

■ The two brightest stars of summer are Vega, now overhead shortly after nightfall, and Arcturus, shining in the west. Draw a line down from Vega to Arcturus. A third of the way down, the line crosses the dim Keystone of Hercules. Two thirds of the way down the line crosses the dim semicircle of Corona Borealis with its one modestly bright star, Alphecca or Gemma.

Vega and the Keystone's star closest to it form an equilateral triangle with Eltanin to their north: the nose of Draco the Dragon. Eltanin is the brightest star of Draco's quadrilateral head. He's eyeing Vega.

SATURDAY, AUGUST 21

■ Now the Moon shines under Jupiter, with Saturn to their right.

■ As summer progresses and Arcturus moves down the western sky, the kite figure of Bootes that sprouts up from Arcturus tilts to the right (depending on your latitude). The kite is narrow, slightly bent with its top leaning right, 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.

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This Week's Planet Roundup

Mercury and Mars are very deep in the sunset. You might have a chance for Mercury. About 15 or 20 minutes after sunset, look for it with binoculars or a wide-field scope just above your west horizon about 20° (two fists at arm's length) lower right of Venus. Mercury dims from magnitude –0.8 to –0.4 this week, so earlier in the week is better.

Much-fainter Mars is in Mercury's vicinity, passing it by a mere 0.1° at their conjunction on the 18th (as seen in twilight for the Americas). But since Mars is only magnitude 1.8, you probably have zero chance. Prove me wrong?

Venus, brilliant at magnitude –3.9, shines pure white in the west during twilight. It sets around twilight's end.

Jupiter and Saturn shine in the southeast in late twilight and after dark. They're magnitudes –2.9 and +0.2, respectively, in or very near Capricornus.

Jupiter starts the night lowest. Saturn glows 19° (about two fists) to Jupiter's upper right. The pair levels out around 11 or midnight daylight-saving time. By then they're about at their highest in the south, at their telescopic best.

Saturn reached opposition on August 1st. Jupiter does so on August 19th. So this month they're at their closest, biggest, and brightest of the year. See "Saturnian Challenges" starting on page 52 of the July Sky & Telescope, also "Action at Jupiter" in the August issue, page 50, and "Dog Days with the Gas Giants" on page 40 of August.

Uranus (magnitude 5.7, in Aries) and Neptune (magnitude 7.8, at the Aquarius-Pisces border) are high in the southeast to south in the pre-dawn hours.

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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 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.

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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 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.

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 Sky Atlas 2000.0 Companion by Strong and Sinnott, or the bigger (and illustrated) 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."

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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 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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