Sky at a Glance | July 20th, 2012

By: Alan MacRobert July 20, 2012 0

Venus and Jupiter gradually move farther apart as they climb higher in the dawn.

Friday, July 20

The Teapot star pattern of Sagittarius glitters dimly in the south-southeast at nightfall, and it reaches its highest in the south later in the evening. Hidden in the rich star fields above it is the magnitude-9.5 asteroid 18 Melpomene, which you can ferret out with a telescope and the finder chart in the July Sky & Telescope, page 52.

Jupiter and Venus blaze strikingly in the dawn this week, with an interesting (but much fainter ) starry scene behind them, as shown at right.

Saturday, July 21

Draco the Dragon arches his back over the Little Dipper in the north at this time of year. With your scope, here you can search out the Cat's Eye Nebula, some interesting double stars and galaxies, and (if your scope is big enough) a quasar with a look-back time of 8.6 billion years — using Sue French's "Deep Sky Wonders" column and charts in the July Sky & Telescope, page 56.

Sunday, July 22

Arcturus is the brightest star high in the west after dark at this time of year. It and Vega, almost overhead, are the two leading stars of summer. Look off to the right of Arcturus, in the northwest, to spot the Big Dipper.

Monday, July 23

As twilight begins to fade, use the Moon in the west-southwest to guide your way to Saturn, Spica, and Mars glimmering through the dusk (in that order of visibility) as shown below.

Watch the Moon wax thicker from night to night as it steps past Mars, Saturn, and Spica. (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. The Moon is shown three times actual size.)

Sky & Telescope diagram

Tues. July 24

The waxing Moon this evening forms a quadrangle with Saturn, Spica, and Mars.

Wed. July 25

The first-quarter Moon is left of Spica and Saturn this evening, as shown here.

Thurs. July 26

By 10 or 11 p.m. the Great Square of Pegasus is up in the east, balancing on one corner — an early warning of the inevitable approach of fall.

Friday, July 27

Look left of the Moon (by a fist-width at arm's length or more) for orange Antares. Much closer left of the Moon are the three fainter stars that mark the head of Scorpius, lined up about vertically.

Saturday, July 28

Fiery Antares shines lower right of the Moon tonight.

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'll need a detailed, large-scale sky atlas (set of charts). The standards are the little Pocket Sky Atlas, which shows stars to magnitude 7.6; the larger and deeper Sky Atlas 2000.0 (stars to magnitude 8.5); and the even larger 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 Sue French's Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, the bigger Night Sky Observer's Guide by Kepple and Sanner, or the classic if dated Burnham's Celestial Handbook.

Can a computerized telescope replace charts? I don't think so — not for beginners, anyway, and especially not on mounts and tripods that are less than top-quality mechanically (able to point with better than 0.2° repeatability). As Terence Dickinson and Alan Dyer say in their invaluable 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

Jupiter's North and South Equatorial Belts have swapped appearances since last year! South here is up. The South Equatorial Belt (above center)has turned narrow and dark, while the North Equatorial Belt has turned paler and very broad, the opposite of how they used to appear. Christopher Go in the Philippines took this superb image on July 11th with a 14-inch telescope in, he says, “perfect seeing.”

Christopher Go

Mercury is out of sight in conjunction with the Sun.

Venus and Jupiter (magnitudes –4.6 and –2.1) shine dramatically in the east before and during dawn. They've widened to about 10° or 12° apart now, with Jupiter higher. Look for Aldebaran, much fainter, below or lower right of Jupiter. Also in Jupiter's starry background are the Hyades, and above it are the Pleiades.

The asteroids Ceres and Vesta, magnitudes 9.1 and 8.4, are in the area too! See article Predawn Treats for Early Risers for the naked-eye aspect, and to find the asteroids, Ceres and Vesta: July 2012 – April 2013.

Mars (magnitude +1.0, in Virgo) glows orange low in the west-southwest at dusk, lower right of the Saturn-and-Spica pair by about 13°. It's heading their way; Mars will pass between Saturn and Spica in mid-August. In a telescope Mars is gibbous and very tiny, 6 arcseconds wide.

Saturn (magnitude +0.8, in Virgo) shines in the southwest as the stars come out. Below it by 4½° is Spica, nearly the same brightness but twinklier. After dark they move lower to the west-southwest.

Uranus (magnitude 5.8, at the Pisces-Cetus border) and Neptune (magnitude 7.8, in Aquarius) are high in the southern sky before the first light of dawn. Finder charts for Uranus and Neptune.

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.