Sky at a Glance | September 13th, 2013

Watch Venus and Saturn change orientation all week.
If you live close to latitude 40° north, Saturn stands straight over Venus on Monday evening.
Mercury is gradually becoming less difficult after sunset.

Friday, September 13

  • Saturn has closed to within 6° of Venus low in the western twilight, as shown at right. They'll pass 3½° apart on Wednesday and Thursday.

    Saturday, September 14

  • Look for bright Vega close to the zenith as twilight fades away, if you live in the world's mid-northern latitudes. Vega goes right through your zenith if you're at latitude 39° north (near Baltimore, Kansas City, Lake Tahoe, Sendai, Beijing, Athens, Lisbon).

    Sunday, September 15

  • At dusk, look southeast high above the Moon for Altair. Lower in the east, the Great Square of Pegasus balances on one corner.

    Monday, September 16

  • Low in the western twilight, Saturn is now within 4° of brighter Venus. In a telescope at high power they'll be very blurry though the thick atmosphere, but you can certainly observe their wildly different surface brightnesses. Venus is 13 times closer to the Sun than Saturn is, so it's illuminated by sunlight that's about 169 times as bright!

    Tuesday, September 17

  • This is the time of year when, as the stars come out, bright Arcturus shines due west at the same height as the Big Dipper hangs in the northwest.

    Wednesday, September 18

  • Full Moon tonight and tomorrow night (exactly full at 7:13 a.m. Thursday morning EDT). The Moon is passing through dim Pisces, to the lower right of the Great Square of Pegasus both evenings.

    Thursday, September 19

  • A winter preview: If you're up before dawn this week, the southern sky displays the same starry panorama as it will at dusk next February. Orion stands high in the south, Sirius and Canis Major sparkle to its lower left, and Gemini, with Jupiter, occupies the high east. Come February, Jupiter will still be there.

    Friday, September 20

  • As summer nears its end, orange Antares blinks its bleary seasonal farewell low in the southwest at nightfall. Look for it twinkling well to the left of Venus and Saturn and perhaps a bit higher. The farther north you are, the sooner Antares sinks out of sight.

    Saturday, September 21

  • After the last of twilight has faded away, the Northern Cross in Cygnus floats near the zenith (for skywatchers at midnorthern latitudes). Without looking: Do you know which way its long end points? If you guessed southwest, you're right.

  • This is the last full day of summer in the Northern Hemisphere.


    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.

    This is an outdoor nature hobby. 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 guide to 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 with a telescope.

    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 beloved if dated Burnham's Celestial Handbook.

    Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically (able to point with better than 0.2° repeatability, which means heavy and expensive). 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 Sept. 9, 2013
    Oval BA. It shows up on all of my videos this morning."" credits="S&T: Sean Walker" width="" height="" align="right"]
    Mercury remains very deep in the glare of sunset.

    Venus and Saturn (magnitudes –4.1 and +0.7, respectively) are low in the west-southwest in evening twilight, far lower left of Arcturus. They set right after dusk.

    Venus is the brightest but lowest. Saturn slides from Venus's upper left at the beginning of the week to its right or upper right by week's end.

    Mars (magnitude 1.6) rises around 3 a.m. and glows due east in dim Cancer as dawn begins. Look for it far lower left of bright Jupiter. In a telescope Mars is still an extremely tiny 4.3 arcseconds wide.

    Jupiter (magnitude –2.1, in Gemini) rises around 1 a.m. and blazes high in the east-southeast by dawn. To its left are Castor and Pollux.

    Uranus (magnitude 5.7, in Pisces) and Neptune (magnitude 7.8, in Aquarius) are well up toward the southeast by 10 p.m. 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.


    Like This Week's Sky at a Glance? Watch our SkyWeek TV short, also playing on PBS.


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