Sky at a Glance | July 22nd, 2011

By: Alan MacRobert July 22, 2011 0

Night owls can watch the waning Moon pass bright Jupiter.

Friday, July 22

Last-quarter Moon tonight (exact at 1:02 a.m. Eastern Daylight Time Saturday morning). The Moon rises around the middle of the night with Jupiter below it. By dawn they're very high in the southeast.

Saturday, July 23

Look low in the south-southeast (depending on your latitude) for the Teapot pattern of Sagittarius. It's starting to tilt to the right — as if to pour out the rest of the summer from its spout.

This week, keep an eye out for occasional Southern Delta Aquarid meteors shooting away from the southeast late at night.

Sunday, July 24

Arcturus is the bright star shining high in the west-southwest at dusk, high above Saturn and Spica. Arcturus is a yellow-orange giant 37 light-years away and about 150 times as luminous as the Sun. Its pale ginger-ale color is plain to the unaided eye. Compare this to the icy blue-white of Vega overhead.

Look far to the right of Arcturus, at roughly the same height, for the Big Dipper.

Monday, July 25

The Moon is near the Pleiades and Aldebaran before dawn this morning and Tuesday morning, as shown below.

Tuesday, July 26

Use binoculars in bright twilight to look for Mercury and twinklier Regulus just 3° apart very low above the western horizon.

Around the first light of dawn on these mornings.

The Moon before dawn now lights the way to Mars and Aldebaran. (These scenes are 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 its actual size.)

Sky & Telescope diagram

The waning crescent Moon hangs near Mars before dawn Wednesday morning. They're between the horns of Taurus, as shown here. The Moon occults (covers) Mars as seen from parts of South America and the Pacific Ocean, mostly during daylight.

Wednesday, July 27

Bright Vega crosses near the zenith late these evenings, depending on your location. When does it appear closest to straight overhead from your site? Like all stars, it reaches the same position in your sky about a half hour earlier each week.

Thursday, July 28

The brightest asteroid, 4 Vesta, is now magnitude 5.8 as it nears its August 6th opposition. It's in Capricornus, easily visible in binoculars in late evening; use our finder chart. The Dawn spacecraft has taken up orbit around Vesta and should be starting its science observations around now!

Friday, July 29

Look southeast after dark, a little more than halfway from the horizon to straight overhead, for the bright star Altair. It's the eye of Aquila, the Eagle. Altair is one of our closest stellar neighbors at a distance of 17 light-years. It's a fast-spinner, so much so that its profile is strongly elliptical rather than round — not that you can see any sign of this with any ordinary instrument!

Saturday, July 30

Now that summer is far advanced, the glowing band of the Milky Way forms a vast arch high across the sky after darkness is complete — if you're one of the few lucky people not living under serious light pollution. The Milky Way runs from Perseus and Cassiopeia low in the north-northeast, up and across the big Summer Triangle very high in the east, and down to Sagittarius and Scorpius low in the south. How much of it can you see?

New Moon (exact at 2:40 p.m. Eastern Daylight Time).

Want to become a better amateur astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.

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. Or download our free Getting Started in Astronomy booklet (which only has bimonthly maps).

The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6 — which may sound like a lot, but that's less than one star in an entire telescopic field of view, on average. By comparison, Sky Atlas 2000.0 plots 81,312 stars to magnitude 8.5, typically one or two stars per telescopic field. Both atlases include many hundreds of deep-sky targets — galaxies, star clusters, and nebulae — to hunt among the stars.

Sky & Telescope

Once you get a telescope, to put it to good use you must have a detailed, large-scale sky atlas (set of charts). The standards are the Pocket Sky Atlas, which shows stars to magnitude 7.6; the larger Sky Atlas 2000.0 (stars to magnitude 8.5); and the even larger and deeper Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts effectively.

You'll also want a good deep-sky guidebook, such as Sky Atlas 2000.0 Companion by Strong and Sinnott, or the more detailed and descriptive Night Sky Observer's Guide by Kepple and Sanner, or the classic if dated Burnham's Celestial Handbook.

Can a computerized telescope take their place? I don't think so — not for beginners, anyway, and especially not on mounts that are less than top-quality mechanically. 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."

This Week's Planet Roundup

This week, use binoculars to watch Regulus pass fading Mercury after sunset. Their visibility in the bright twilight a half hour after sundown is exaggerated here.

Alan MacRobert

Mercury fades from magnitude +0.4 to +1.0 this week as it sinks even lower toward the sunset horizon. Use binoculars to look for it very low in the west-northwest in bright twilight — and to spot twinkly Regulus, slightly fainter, nearby. Mercury and Regulus pass 3° from each other on July 26th.

Venus is hidden in the sunrise.

Mars (magnitude +1.4, near the horns of Taurus) can be found in the east-northeast as dawn begins to brighten. Look for it very far to the lower left of brilliant Jupiter. Don't confuse Mars with Aldebaran to its upper right. Upper left of Mars is brighter Capella.

In a telescope Mars is just a tiny blob only 4.4 arcseconds in diameter. It's on its way to a poor opposition (13.9 arcseconds wide) next March.

Jupiter (magnitude –2.4, in southern Aries) rises in the east around midnight daylight saving time. Once it's well up, look below it for the head of Cetus, rather dim. By dawn Jupiter shines high in the southeast.

Saturn (magnitude +0.9, in Virgo) is sinking ever lower in the west-southwest at dusk. Shining 13° left of it is similar, but bluer, Spica. About 1½° to 2° to Saturn's right is fainter Porrima (Gamma Virginis).

Uranus (magnitude 5.8, in western Pisces) and Neptune (magnitude 7.8, in western Aquarius) are well up in the early-morning hours. Here's our printable finder chart for both.

Pluto (magnitude 14.0, in northern Sagittarius) is highest in the south around 11 p.m. A big finder chart for it is in the July Sky & Telescope, page 64.

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) equals Universal Time (also known as UT, UTC, or GMT) minus 4 hours.

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About Alan MacRobert

Alan M. MacRobert became an avid Sky & Telescope subscriber in 1966 at age 14, joined the editorial staff in 1982, and is now a senior contributing editor, semi-retired. He played a role in practically every part of the magazine and the company's other products for more than a generation, both on the amateur-observing side and the science-reporting side. In 1994 a book collection of his observing how-tos and telescopic sky tours was published as Star Hopping for Backyard Astronomers. He has produced This Week's Sky at a Glance online every week since 1989.