Sky at a Glance | May 15th, 2009

Some daily events in the changing sky for May 15 – 23.

Friday, May 15

  • Face north at nightfall this week and look very high, almost overhead. There's the Big Dipper, floating upside down. The middle star of its bent handle is Mizar; can you see tiny Alcor hiding in its skirts? To check which side of Mizar to examine, notice that Vega is rising way off in the northeast. A line from Mizar through Alcor always points to Vega.

    Saturday, May 16

  • Jupiter is at western quadrature, 90° west of the Sun before and during dawn. In a telescope this month, can you see that its western limb is slightly less illuminated than its eastern limb?

    Looking southeast in early dawn
    Early risers can watch the waning Moon pass Jupiter in Capricornus. (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.)
  • On Sunday morning the last-quarter Moon shines close to Jupiter, as shown at right. Also Sunday morning: Jupiter's own moons Io and Callisto both cast their tiny shadows onto Jupiter's face from 3:56 to 5:16 a.m. Eastern Daylight Time.

    Sunday, May 17

  • Last-quarter Moon (exact at 3:26 a.m. EDT).

    Monday, May 18

  • A small telescope will almost always show Titan, Saturn's largest moon. Tonight Titan is four ring-lengths to Saturn's west. A 6-inch telescope will begin to show the orange color of its smoggy atmosphere. A guide to identifying other Saturnian satellites often visible in amateur scopes is in the May Sky & Telescope, page 47.

    Tuesday, May 19

  • Early Wednesday morning, Jupiter glides 4 arcminutes (6 Jupiter diameters) south of the 5.1-magnitude star Mu Capricorni, which will appear through a telescope like a fifth, out-of-place Jovian moon. And 0.6° to Jupiter's left is tiny, 8th-magnitude Neptune; it's almost stellar, with an apparent diameter of just 2.3 arcseconds. Use high power to see if you can identify Neptune by its slight fuzziness. Good luck.

    Wednesday, May 20

  • Late this evening the 10th-magnitude asteroid 8 Flora is just 4 arcminutes southwest of 3.4-magnitude Zeta Virginis. By tomorrow evening, it's 4′ southeast of the star. See the article and chart in the May Sky & Telescope, page 46.

    On Wednesday and Thursday mornings, the waning Moon guides the way to bright Venus and faint Mars in the dawn.
    Alan MacRobert
  • During dawn Friday morning for North America, the waning crescent moon forms a 6° equilateral triangle with brilliant Venus and, below them, much fainter Mars. See the illustration at right.

    Thursday, May 21

  • The Gemini twins stand upright in the west as twilight fades, with their head stars, Pollux and Castor, lined up nearly horizontally (4½° apart, about the width of three fingers at arm's length). To their lower left shines Procyon. Farther to their lower right shines brighter Capella.

    Friday, May 22

  • It's still May, but wave hello to the two brightest stars of summer already well up at the end of dusk. Arcturus is shining very high in the southeast. Vega is lower in the northeast. They're just 37 and 25 light-years away, respectively.

    Saturday, May 23

  • Look a third of the way from Arcturus to Vega for the dim semicircle of Corona Borealis, the Northern Crown. Look two-thirds of the way for the dim Keystone of Hercules.


    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 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
    Sky Atlas 2000.0 or the smaller Pocket Sky Atlas) and good deep-sky guidebooks (such as Sky Atlas 2000.0 Companion by Strong and Sinnott, the more detailed and descriptive Night Sky Observer's Guide by Kepple and Sanner, or the classic Burnham's Celestial Handbook). Read how to use them effectively.

    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 and a curious mind." Without these, they note, "the sky never becomes a friendly place."

    More beginners' tips: "How to Start Right in Astronomy".


    This Week's Planet Roundup

    Mercury is hidden in the glare of the Sun.

    Venus (magnitude –4.6, in Pisces) shines brightly low in the east during dawn. Don't confuse it with Jupiter, higher and far to the right in the southeast. In a telescope, Venus is now a thick crescent about 40% sunlit. The best telescopic views of it come in full early-morning daylight, when Venus is higher in steadier air.

    Mars (magnitude +1.2, in Pisces) still remains 6° lower left of Venus all this week. Bring binoculars; Mars is about 200 times fainter than Venus! There are four reasons for this: Mars is farther from the Sun so it gets illuminated less brightly; its surface is darker than Venus's white clouds; it's a smaller planet than Venus; and it's currently farther from us.

    Jupiter (magnitude –2.4, in Capricornus) shines brightly in the southeast before and during dawn.

    Saturn at 0:57 May 22, 2009 UT
    Saturn on the evening of May 21st, imaged by S&T's Sean Walker with a 14-inch scope. The bottom view is the same image processed to bring out five of Saturn's moons, including faint little Mimas. "All of them were visible in the eyepiece," he says.


    Have you noticed how dim Saturn's rings have been lately? It's because they've turned more nearly edge-on to the Sun than they are to Earth. Also, the black line across Saturn's globe is now a combination of the dimly sunlit surface of the rings themselves and their darker shadow on Saturn's globe. South is up, east is to the right.

    S&T: Sean Walker
    Saturn (magnitude +0.9, in Leo) is highest in the south at dusk and moves to the southwest later. Regulus, not quite as bright, sparkles 15° to its right at dusk, and to its lower right later. This week Saturn ends its retrograde (westward) motion toward Regulus and begins drawing away from it eastward again.

    In a telescope Saturn's rings appear 4° from edge on, their widest this year. But look at the picture at right; see how the rings have dimmed! The caption tells why.

    Uranus (6th magnitude, in Pisces) is between Venus and Jupiter in the dawn.

    Neptune (8th magnitude, in Capricornus) is in the background of Jupiter.

    Pluto (14th magnitude, in northwestern Sagittarius) is highest in the south before the first light of dawn.

    All descriptions that relate to your horizon or zenith — 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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