Sky at a Glance | September 19th, 2008

Some daily events in the changing sky for September 19 – September 27.

Late this week, early risers can catch the waning crescent Moon passing Regulus and newly-arriving Saturn. (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 blue 10° scale is about the size of your fist held at arm's length. For clarity, the Moon is shown three times actual size.)

Friday, September 19

  • If you're in eastern North America and have a clear view to the east-northeast, plan to get out your telescope this evening and watch the waning gibbous Moon passing, and perhaps occulting, the Pleiades. See our article. If you're in northern Europe, the occultations will be seen later in the night with the Moon high in the sky.

    Saturday, September 20

  • As the evening grows late, look for Fomalhaut, the Autumn Star, rising in the southeast. At the same time Arcturus, the Spring Star, is getting low in the west. They'll be at exactly the same height some time between about 9 and 10 p.m., depending on where you live in your time zone. How accurately can you time this event?

    Sunday, September 21

  • Last-quarter Moon tonight (exact at 1:04 a.m. Monday morning EDT.)

    Monday, September 22

  • The September equinox occurs at 11:44 a.m. EDT. This is when the Sun crosses the celestial equator (i.e. crosses directly above Earth's equator) heading south for the year. Autumn begins in the Northern Hemisphere, and spring in the Southern Hemisphere.

  • Jupiter's moon Io reappears out of eclipse from Jupiter's shadow around 7:27 p.m. Pacific Daylight Time. Watch it swell into view just east of the planet (between Jupiter and Europa close by!). For a listing of all events among Jupiter's moons this month, visible worldwide, see the September Sky & Telescope, page 58.

    Tuesday, September 23

  • Now that fall is underway (in the Northern Hemisphere), skywatchers at mid-northern latitudes will see that Deneb is taking over from Vega as the "zenith star" after nightfall.

    Wednesday, September 24

  • Jupiter is passing 2 or 3 arcminutes north of a 5.9-magnitude star. Through a small telescope, the star will look like a fifth, out-of-place Jovian moon.

    Thursday, September 25

  • If you're up early in the dawn Friday morning, look east for the waning crescent Moon with Regulus just to its left, as shown at the top of this page. Look below them for Saturn.

    Friday, September 26

  • Early in dawn Saturday morning, look left of the Moon (low in the east) for Saturn, as shown at the top of this page. A telescope shows that Saturn's rings have turned nearly edge on.

  • The bright eclipsing variable star Algol should be in one of its periodic dimmings, magnitude 3.4 instead of its usual 2.1, for a couple hours centered on 11:56 p.m. Eastern Daylight Time. Algol takes several additional hours to fade and to rebrighten. (For all times of Algol's minima this month and next, good worldwide, see the October Sky & Telescope, page 68.)

    Saturday, September 27

  • Jupiter's moon Ganymede emerges from eclipse out of the planet's shadow around 8:02 p.m. Eastern Daylight Time. Watch it swell into view just east of the planet. (The other three moons are to Jupiter's west.)



    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 foldout 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).

    Pocket Sky Atlas
    The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6 — which may sound like a lot, but it's still less than one per square degree on the sky. Also plotted are many hundreds of good telescopic galaxies, star clusters, and nebulae.
    Sky & Telescope
    Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of maps; 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 even more detailed 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? 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 wisely say, "the sky never becomes a friendly place."

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


    This Week's Planet Roundup

    Mercury (about magnitude 1) is fading and getting lost in the glow of sunset, well below or lower right of Venus.

    Venus (magnitude –3.8) is gradually becoming more visible after sunset. Look for it above the west-southwest horizon in bright twilight, about 30 minutes after sundown.

    Mars (magnitude +1.7) is disappearing into the sunset to Venus's lower right. Binoculars required; good luck.

    The Great Red Spot was on Jupiter's central meridian when Christopher Go took this image at 13:19 UT August 27, 2008. The central-meridian longitude (System II) was 128°. Note the reddish Oval BA to the Red Spot's upper left, and the bright white point marking the start of a new rift in the dark North Equatorial Belt (below center). A more extensive white disturbance has developed in the North Equatorial Belt on the other side of the planet. South is up, to match the south-up view in many telescopes.
    Jupiter (magnitude –2.5, in Sagittarius) shines bright and steady in the south right at dusk, and lower in the southwest later. It's above the Sagittarius Teapot and below the end of the smaller, dimmer Teaspoon.

    Saturn is just emerging from the glow of sunrise. Look for it low above the eastern horizon an hour before sunrise. Don't confuse it with twinkly Regulus higher up. Despite the poor atmospheric seeing so low down, a telescope will show that Saturn's rings have turned nearly edge on.

    Uranus and Neptune (magnitudes 5.7 and 7.8, respectively, in Aquarius and Capricornus) are in the southeast to south during evening. Use our article and finder charts or the chart in the September Sky & Telescope, page 63.

    Pluto (magnitude 14.0, in the northwestern corner of Sagittarius) is still in the south-southwest right after dark. If you've got a big scope and a dark sky, use our article and finder chart.

    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 (known as UT, UTC, or GMT) minus 4 hours.


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