Sky at a Glance | July 13th, 2012

Dawn view
The waning Moon now adds itself to the multifarious dawn scene. (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 actual size.)
Alan MacRobert

Friday, July 13

  • During dawn on Saturday morning, the waning crescent Moon in the east poses to the upper right of Jupiter and lower right of the Pleiades, as shown at right.

    Saturday, July 14

  • During dawn Sunday morning, the waning crescent poses dramatically with Jupiter and Venus, as shown here for North America. Think photo opportunity!

    The Moon actually occults Jupiter for most of Europe and parts of Asia; map and timetable. Jupiter disappears behind the Moon's sunlit limb.

    Sunday, July 15

  • Stars twinkle and planets (being extended objects) don't, right? That's what everyone is supposed to learn when they take up skywatching. But how true is it really? See for yourself by comparing Spica and Saturn as they sink together in the southwest these evenings. Saturn is the one on top. Sometimes the difference is dramatic, other times not so much, depending on the small heat waves rippling in the lower atmosphere.

    Monday, July 16

  • July is Scorpius month — at least in the evening hours. Scorpius is highest in the south right at nightfall this week. Its brightest star is orange-red Antares, "Anti-Ares," the "rival of Mars" in Greek. Compare its color with the real Mars moving lower in the west-southwest (to the lower right of the Saturn-and-Spica pair).

    Tuesday, July 17

  • The two brightest stars of summer are Vega, very high in the east as the stars come out, and Arcturus, very high in the southwest. Compare their colors. Vega is icy white with a hint of blue; Arcturus is pale yellow-orange. They're both relatively nearby as stars go. It's 25 light-years to Vega, 37 to Arcturus.

    Wednesday, July 18

  • Once you've found Vega and Arcturus (see yesterday), imagine a line between them. A third of the way along it from Vega is the dim Keystone of Hercules. Two thirds of the way is the little semicircle constellation Corona Borealis, the dim Northern Crown, with one modestly bright star, Alphecca or Gemma.

    Thursday, July 19

  • This is the time of year when the Big Dipper, in the northwest after dark, begins scooping down to the right as if preparing to scoop up water. And the dim Little Dipper, standing upright from the North Star at the end of its handle, begins to tip left starting its six-month downward fall.

    Dawn view
    Venus and Jupiter gradually move farther apart as they climb higher in the dawn. (The orientation of these scenes is exact for viewers at latitude 40° north.)

    Friday, July 20

  • The Teapot of Sagittarius is in the south-southeast at nightfall and highest in the south later in the evening. Hidden in the 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.

    Saturday, July 21

  • Draco the Dragon arches his back over the Little Dipper in the north at this time of year. With your scope, 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.


    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).

    Sky Atlas 2000.0 (the color Deluxe Edition is shown here) plots 81,312 stars to magnitude 8.5. That includes most of the stars that you can see in a good finderscope, and typically one or two stars that will fall within a 50× telescope's field of view wherever you point. About 2,700 deep-sky objects to hunt are plotted among the stars.
    Alan MacRobert

    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 on July 11, 2012
    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.”

    Mercury is lost in the sunset.

    Venus and Jupiter (magnitudes –4.7 and –2.1) shine dramatically in the east before and during dawn. They're stacked about 8° apart now, with Jupiter on top. Look for Aldebaran, much fainter, moving away from Venus to its upper right. Also in Venus's starry background are the Hyades, and above Jupiter are the Pleiades. See the scenes above.

    The asteroids Ceres and Vesta, magnitudes 9.1 and 8.4, are there 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 in the west-southwest at dusk. It's lower right of the Saturn-and-Spica pair, by about 16°. Mars is heading their way; it will pass right between them in mid-August. In a telescope Mars is gibbous and very tiny, only 6 arcseconds wide.

    Saturn (magnitude +0.8, in Virgo) shines in the southwest as the stars come out. Below it by nearly 5° is Spica, nearly the same brightness. 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 southeast and south before the first light of dawn. Finder charts.


    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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