This Week's Sky at a Glance, July 16 – 24

Nova Cas update. It's been four months since Nova Cassiopeiae 2021 erupted to magnitude 7.7. It has stayed roughly that bright ever since, making it officially a "slow nova," but with a brightness spike to 5.5 in early May and smaller bumps since. As of July 19th it was 7.0. Charts and comparison stars.

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FRIDAY, JULY 16

■ Venus continues to shine low in the western twilight this week. Tiny, distant Mars, a mere 1/200 as bright, is sliding away to Venus's lower right as shown below. Meanwhile, twinkly Regulus, a half magnitude brighter than Mars, is rapidly moving in on Venus from the upper left.

■ First-quarter Moon this evening and tomorrow evening (the Moon is exactly first quarter at 6:11 a.m. on July 17th EDT). Tonight, spot Spica about 6° to the Moon's lower left.

SATURDAY, JULY 17

■ Now the eastward-marching Moon, still essentially first-quarter, shines with Spica to its lower right.

■ Orange Antares and the rest of starry Scorpius are highest in the south just after dark, with Antares about three fists left of the Moon. Don't let the moonlight stop you! Scorpius is bright, and it's rich in double stars for binoculars and telescopes bright enough to shine through. Here's a rundown:

The head of Scorpius is the vertical row of three stars to the right of Antares. The top of the row is Beta Scorpii, a fine telescopic binary: separation 13 arcseconds, magnitudes 2.8 and 5.0.

Just 1° lower left of it is the very wide naked-eye pair Omega¹ and Omega² Scorpii, diagonal from upper right to lower left. They're 4th magnitude and ¼° apart. Binoculars show their slight color difference; they're spectral types B9 and G2.

Upper left of Beta by 1.6° is Nu Scorpii, separation 41 arcseconds, magnitudes 3.8 and 6.5. In fact this is a telescopic triple. High power in good seeing reveals Nu's brighter component itself to be a close double, separation 2 arcseconds, magnitudes 4.0 and 5.3, aligned almost north-south.

■ And the tail of Scorpius, the constellation's other end, is about two fists lower left of Antares. Look for the two stars especially close together in the tail. These are Lambda and fainter Upsilon Scorpii, the Cat's Eyes, 0.6° apart. They're canted at an angle; the cat is tilting his head to the right and winking (Lambda is brighter than Upsilon; they're magnitudes 1.6 and 2.6).

A line through the Cat's Eyes points right or lower right by nearly a fist toward Mu Scorpii, a much tighter pair known as the Little Cat's Eyes. These are oriented almost exactly the same way as Lambda and Upsilon, but they're only 0.1° apart, and fainter: magnitudes 3.0 and 3.5. A cat six times farther away?

SUNDAY, JULY 18

■ Cassiopeia is now well past its bottoming-out for the year. Look for its tilted W pattern slowly moving up in the north-northeast. The farther north you live, the higher it will be.

MONDAY, JULY 19

■ Now the waxing gibbous Moon is butting right into the head of Scorpius, passing between Beta and Delta Scorpii during evening for eastern and central North America. Antares is about 7° to the Moon's lower left.

TUESDAY, JULY 20

■ Now the Moon shines about a fist over the Cat's Eyes and the Little Cat's Eyes. See July 17 above.

WEDNESDAY, JULY 21

■ Low in the western twilight, Venus is now in conjunction with Regulus. The star glimmers only 1/140 as bright as Venus, about 1.1° lower left of it as shown below.

THURSDAY, JULY 22

■ Fourth star for the Summer Triangle. The next-brightest star near the Summer Triangle is Rasalhague, the head of Ophiuchus.

First identify the Triangle. Face east soon after dark and crane your neck high to spot bright Vega. Look two fists lower left of it for Deneb. Three or four fists lower right of Vega is Altair.

Next, Rasalhague. As you still face east, it's three fists to the right of Vega and three fists upper right of Altair.

Admittedly, at 2nd magnitude it's not as bright as the three Triangle stars. But include it and you've got a giant, flattened quadrilateral.

FRIDAY, JULY 23

■ Full Moon (exact at 10:37 p.m. EDT). The Moon is up in the east by late twilight. Look for Saturn about a fist at arm's length to its left.

By dawn on the 24th they shift to the southwest and twist around so that Saturn is above the Moon, as shown below.

SATURDAY, JULY 24

■ Now both Regulus and Mars are well down to Venus's lower right, by 4° and 7° respectively as shown below . Bye-bye!

■ And in the east after dark Saturn shines upper right of the Moon, and Jupiter shines farther to the Moon's left. Again, by dawn this scene of action shifts to the southwest and the pattern rotates clockwise. See the graphic for dawn on July 24–26 above.

■ As summer progresses, bright Arcturus moves down the western side of the evening sky. Its pale ginger-ale tint always helps identify it.

Off to Arcturus's right in the northwest, the Big Dipper scoops to the right.

 

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This Week's Planet Roundup

Mercury this week sinks even lower down toward the east-northeast horizon in bright dawn. But it does continues to brighten: from magnitude –0.8 to –1.5.

Venus (magnitude –3.9) continues to shine low in the west during twilight. Lower left of it is tiny Mars, 200 times fainter at magnitude +1.8. Mars gets lower every day. Both planets set before twilight ends.

Upper left of Venus you'll find Regulus early in the week, moving closer to Venus day by day. It's brighter than Mars by a half magnitude, not even counting their different amounts of atmospheric extinction. It passes 1° from Venus on the 21st.

Saturn (magnitude +0.2, in Capricornus) and brighter Jupiter (magnitude –2.8, in Aquarius) rise in the east-southeast in twilight, Jupiter an hour after Saturn. By late evening they make an impressive duo in the southeast. Jupiter will grab your eye first. Find Saturn 20° to its upper right.

They're highest in the south, at their telescopic best, around 2 a.m. daylight saving time. See "Action at Jupiter" in the July Sky & Telescope, page 50, and "Saturnian Challenges" starting on page 52. They'll reach opposition next month, so they're already about as close and big as they'll get.

Uranus (magnitude 5.8, in Aries) is well placed in the east before dawn begins.

Neptune (magnitude 7.8, in Aquarius 22° east of Jupiter) is higher in the south-southeast before dawn begins.

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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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