Sky at a Glance | May 11th, 2012

Crescent Venus waning in May
Seen in a telescope, Venus enlarges in diameter while waning in phase this month. It's coming nearer to Earth is it swings (in its faster orbit) toward the line between Earth and Sun. The 10° scale refers to Venus's height above the horizon, as seen from latitude 40° north, 45 minutes after sunset.

Friday, May 11

  • Brilliant Venus is dropping lower in the west-northwest every evening this month. A telescope shows it enlarging and waning in phase, as shown here; it's swinging ever closer to the line between us and the Sun. Look a little to its right for the much fainter star Beta Tauri (not shown here). As of tonight the planet and star are 1½° apart.

    Saturday, May 12

  • Last-quarter Moon (exact at 5:47 p.m. Eastern Daylight Time). The Moon rises in the middle of the night, looking lopsided and awkward in dim Aquarius.

    Sunday, May 13

  • Use Venus, the bright landmark point in the west-northwest at dusk, to identify stars in the May twilight. Capella is to Venus's upper right. Pollux and Castor are farther to Venus's upper left, and Procyon is lower left of Pollux. These four stars form the enormous Twilight Arch of Spring.

    Monday, May 14

  • The pair of points you'll find shining fairly high in the southeast at nightfall are Saturn and (to its lower right) Spica. Look to their lower right for the four-star pattern of Corvus, the Crow. Look farther to their upper left for brighter Arcturus, the "Spring Star."

    Tuesday, May 15

  • Arcturus shines high in the southeast after dark. Vega, equally bright, shines lower in the northeast. A third of the way from Arcturus to Vega look for dim Corona Borealis, the Northern Crown, with its one modestly bright star, Alphecca.

    Two thirds of the way from Arcturus to Vega is the dim Keystone of Hercules.

    Wednesday, May 16

  • This is the time of year when the Big Dipper floats upside down at its highest due north when the stars come out. Far below it is Polaris. Far below Polaris, near or even below the horizon depending on your latitude, is W-shaped Cassiopeia.

    Thursday, May 17

  • The Summer Triangle is fully up in the east by about 11 p.m. Its top star is Vega, the brightest in the eastern sky. About two fist-widths to Vega's lower left is Deneb. Farther to Vega's lower right is Altair. If you have a dark sky, you'll see that the Milky Way runs through it.

    Friday, May 18

  • Arcturus is the brightest star high in the southeast these evenings. It's the leading light of the constellation Bootes, the Herdsman. The main stars of Bootes form a narrow, bent kite shape extending left of Arcturus, nearly three fists at arm's length long. Or maybe the kite is a pointy-toed shoe, with Arcturus the tip of the toe.

    Saturday, May 19

  • Venus, moving lower in the west every evening as twilight fades, is now 2° left of much fainter Beta Tauri, which has been descending almost in parallel with it.

  • Partial/annular solar eclipse tomorrow! On Sunday afternoon, all but easternmost North America will experience at least a partial eclipse of the Sun. So will the Pacific and (on the morning of May 21st local date) the eastern half of Asia. The eclipse will become annular — with the rim of the Sun a brilliant ring surrounding the dark silhouette of the Moon — along a path from south China and parts of Japan across the Pacific to the California-Oregon coast and from there southeastward to end at sunset in Texas. See our article May 20th's Annular Eclipse of the Sun, which has links to detailed maps, local timetables, and many other resources, including How to Watch Safely. Reminder: Looking at any part of the bright Sun without a proper, safe solar filter can permanently damage your vision.


    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).

    Pocket Sky Atlas
    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

    Mercury is hidden deep in the glow of sunrise.

    Venus (magnitude –4.7 in Taurus) blazes as the "Evening Star" in the west-northwest in twilight. It's dropping lower every day now (see article, Venus Takes the Plunge).

    Look high to Venus's upper right for Capella. Look just to Venus's right for Beta Tauri, also known as El Nath, glimmering only 1/300 as bright (at magnitude +1.6). They're slowly moving apart; Beta Tauri is 1.5° from Venus on May 11th and 2.0° from it by the 18th.

    In a telescope, Venus is a crescent becoming more interesting all the time. It has enlarged to about 46 arcseconds tall while waning to about 15% sunlit; watch it changing daily. You may even see Venus's crescent shape with good, firmly braced binoculars.

    Venus is plunging toward inferior conjunction, when it will transit the face of the Sun on June 5–6 (on the afternoon of the 5th for North America); see our article in the June Sky & Telescope, page 50, or online. This will be the last transit of Venus until 2117.

    Mars on April 21, 2012
    Although Mars was a mere 10.8 arcseconds wide on April 21st, Damian Peach recorded this extraordinarily fine image showing the brown heights of Olympus Mons (right of center) and the three other large Tharsis volcanos (above center) poking through a broad equatorial band of thin white clouds. South is up. In the North Polar Cap at bottom, note the dark Chasma Borealis extending inward from the cap's dark collar.
    Damian Peach

    Mars (magnitude +0.2) shines fire-orange under the belly of Leo. It's high in the south-southwest at dusk and lower in the southwest to west later in the evening. Spot Regulus 8° or 9° to Mars's right in twilight, and to its lower right later. They're moving farther apart daily. Fainter Gamma Leonis is 8° above Regulus. Mars in a telescope is gibbous and small, 9 arcseconds wide, fading and shrinking.

    Jupiter is in conjunction behind the Sun.

    Saturn (magnitude +0.4, in Virgo) shines in the southeast in twilight and highest in the south around 11 p.m. The star 5° to Saturn's lower right in the evening is Spica, slightly fainter and bluer.

    Uranus (magnitude 5.9, at the Pisces-Cetus border) is very low in the dawn.

    Neptune (magnitude 7.9, in Aquarius) is low in the east-southeast before dawn.


    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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