This Week’s Sky at a Glance, July 15 – 23

The waxing gibbous Moon shines over Mars, Saturn, and Antares at dusk. (These scenes are always drawn for the middle of North America. European observers: move each Moon symbol a quarter of the way toward the one for the previous date. For clarity, the Moon is shown three times actual size.)

Friday, July 15

• Look south during and after twilight. The Moon, Saturn, and Antares form a roughly vertical stack there, as shown above. Mars blazes off to their right. Watch them all tilt westward until they set around 2 to 3 a.m.

Saturday, July 16

• Venus-Mercury conjunction challenge: About 15 minutes after sunset, use binoculars to look for Venus just above the west-northwest horizon — with fainter Mercury only ½° above it. Venus is currently magnitude –3.9, and Mercury is –1.0, so you may be able to detect at least Venus naked-eye once you locate them in binoculars. Good luck.

Sunday, July 17

• Right after dark, the bright waxing gibbous Moon stands over the top of the Sagittarius Teapot, which rests nearly level. Can you see it through the moonlight? Shield your eyes from the Moon itself. The Teapot is about the size of your fist at arm's length, with its handle to the left and its spout to the right.

Monday, July 18

• Arcturus shines as the brightest star high in the west these evenings, pale yellow-orange. The kite pattern of its constellation, Bootes, extends upper right from it. Off to Arcturus's right in the northwest glitters the Big Dipper.

Tuesday, July 19

• Full Moon (exact at 6:57 p.m. EDT). The Moon rises around sunset. As the Moon climbs higher and the stars come out, look for Altair high to its upper left.

Wednesday, July 20

• We're only a third of the way through summer, but already W-shaped Cassiopeia, a constellation of fall and winter evenings, is climbing up in the north-northeast as evening grows late. And the Great Square of Pegasus, emblem of fall, comes up to balance on one corner just over the eastern horizon.

Thursday, July 21

• The tail of Scorpius lies low due south right after dark. How low depends on how far north or south you live: the farther south, the higher. Look for the two stars especially close together in the tail. These are Lambda and fainter Upsilon Scorpii, known as the Cat's Eyes. They're canted at an angle; the cat is tilting his head and winking.

The Cat's Eyes point west (right) by nearly a fist-width toward Mu Scorpii, a much tighter pair known as the Little Cat's Eyes. It takes very sharp vision to resolve Mu without using binoculars.

Friday, July 22

• Starry Scorpius is sometimes called "the Orion of Summer" for its brightness, its blue giants, and its prominent red supergiant (Antares in the case of Scorpius, Betelgeuse for Orion). But Scorpius is a lot lower in the south for those of us at mid-northern latitudes. That means it has only one really good evening month: July. Catch Scorpius due south just after dark now, before it starts to tilt lower toward the southwest. It's full of deep-sky objects for binoculars and telescopes. Not to mention Mars and Saturn now nearby!

Saturday, July 23

• After nightfall, Altair shines in the east-southeast. It's the second-brightest star on the southeastern side of the sky, after Vega high to its upper left. Above Altair by a finger-width at arm's length is its sidekick, little orange Tarazed. And a bit more than a fist-width lower left of Altair, little Delphinus, the Dolphin, leaps away from it.

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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 guide to astronomy.

The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6 — which may sound like a lot, but it's less than one per square degree on the sky. Also plotted are many hundreds of telescopic galaxies, star clusters, and nebulae. Shown above is the new Jumbo Edition for easier reading in the night. Click image for larger view.

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 new 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, is the even larger Uranometria 2000.0(stars to magnitude 9.75). And read how to use sky charts with a telescope.

You'll also want a good deep-sky guidebook, such as Sue French's Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, or the biggerNight 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 theirBackyard 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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This Week's Planet Roundup

Jupiter, Io, and its shadow, imaged by Damian Peach on June 16th at 22:27 UT. The image is so sharp that Io's dark polar regions are distinctly resolved. When Io is resolved less well on a dark background, they can make Io seem egg-shaped.

Mercury and Venus are very low in bright twilight. About 15 minutes after sunset, start looking from them just about the west-northwest horizon; binoculars help. Venus is magnitude –3.9, Mercury, at magnitude –1, is only a tenth or a fifteenth as bright. Mercury is begins the week (July 15) less than 1° to the right of Venus, passes closely over Venus in the 16th, and ends the week (July 22) 4° to Venus's upper left. Good luck.

Mars (magnitude –1.0, in Libra to the right of upper Scorpius) is still bright, though fading. It's the yellow-orange light in the south-southwest at dusk, lower in the southwest later in the evening. In a telescope, Mars is still about 14 arcseconds in diameter and very plainly gibbous.

Jupiter (magnitude –1.8, at the hind foot of Leo) is getting rather low in the west at dusk. It's on the far side of its orbit from us and 33 arcseconds wide, almost the smallest it ever appears.

Saturn (magnitude +0.2, in southern Ophiuchus) and Antares below it (magnitude +1.0) are about 15° east (left) of Mars. Still near the middle of the Mars-Saturn-Antares triangle is the strange variable Delta Scorpii (Dschubba). See our telescopic guide to Saturn in the June Sky & Telescope, page 48.

Uranus (magnitude 5.8, in Pisces) and Neptune (magnitude 7.8, in Aquarius) are very high now in the southeast to south before the first light of dawn. Background and finder charts.

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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 (UT, UTC, or GMT) minus 4 hours.

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“This adventure is made possible by generations of searchers strictly adhering to a simple set of rules. Test ideas by experiments and observations. Build on those ideas that pass the test. Reject the ones that fail. Follow the evidence wherever it leads, and question everything. Accept these terms, and the cosmos is yours.”

— Neil deGrasse Tyson