Sky at a Glance | September 20th, 2013

Mercury is gradually becoming less difficult after sunset.

Friday, September 20

  • Saturn is still 4° to the upper right of bright Venus in twilight, as shown here. Watch it move farther from Venus each evening.

  • As summer nears its end, orange Antares blinks its bleary seasonal farewell low in the southwest after dusk. 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 mid-northern latitudes). Without looking: Do you know which way its long end points? If you guessed southwest, you're right.

    Sunday, September 22

  • The September equinox occurs at 4:44 p.m. EDT today, when the Sun crosses the equator heading south for the season. Fall begins in the Northern Hemisphere, spring in the Southern Hemisphere.

  • As summer ends, the Sagittarius Teapot moves to the west of south during evening and tips increasingly far over, as if pouring out summer's last drops.

    Monday, September 23

  • Altair shines due south very high after dark. It's the leading light of the constellation Aquila, the Eagle. Off the tail of Aquila lies the Scutum Star Cloud, with the grand open cluster M11 just below the dark nebula Barnard 111. See Gary Seronik's Binocular Highlight column and finder chart in the September Sky & Telescope, page 45.

    View in bright twilight
    While twilight is still bright, use binoculars to try the challenge of catching Mercury and Spica in conjunction very low.

    Tuesday, September 24

  • Mercury and Spica are in conjunction just 3/4° apart, very low in the west-southwest in bright twilight. About 20 minutes after sunset, use binoculars to look for them 22° to the lower right of Venus. Mercury is magnitude –0.1. Spica is only a third as bright at magnitude +1.0.

    Wednesday, September 25

  • The Moon rises late this evening (around 11 p.m. depending on your location), with Aldebaran well to its upper right and brighter Capella farther to its upper left. Earlier, spot these two stars when they're low to judge where the Moon will rise.

    Thursday, September 26

  • Last-quarter Moon tonight (exact at 11:55 p.m. EDT). The Moon rises around 11 or midnight local time, shining in the feet of Gemini. Jupiter is to its lower left, as shown below, and Orion is farther to its right.

    Before the first light of dawn
    Before dawn, watch the Moon pass Jupiter in Gemini.

    Friday, September 27

  • The Moon, a day past last quarter, rises by 1 a.m. tonight (Saturday morning the 28th), with Jupiter shining to its upper left as shown here. Can you spot Delta Geminorum, magnitude 3.6, just ½° below Jupiter?

    Saturday, September 28

  • This is the time of year when, in mid-evening, W-shaped Cassiopeia stands on end halfway up the northeastern sky — and when, off to its left, the dim Little Dipper extends straight leftward from Polaris in the north.


    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 once you know your way around, 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 (magnitude –0.1) is deep in the glow of sunset. Twenty minutes after sunset, use binoculars to look for it about 22° lower right of Venus. Don't confuse it with fainter, twinklier Spica near it. Mercury and Spica pass just 3/4° apart on Tuesday evening September 24th.

    Venus and fainter Saturn (magnitudes –4.2 and +0.6, respectively) shine low in the west-southwest in evening twilight. Find Venus far lower left of Arcturus. Saturn moves increasingly far to Venus's right this week. They set just after dark.

    Mars (magnitude 1.6, crossing from Cancer to Leo) rises around 3 a.m. and glows due east as dawn begins. Look for it far lower left of bright Jupiter. Below Mars is Regulus, similarly bright and moving closer to it daily. Compare their colors.

    Jupiter (magnitude –2.1, in central Gemini) rises around midnight or 1 a.m. and blazes high in the east-southeast by dawn. Left of it 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.


    To be sure to get the current Sky at a Glance, bookmark this URL:
    http://SkyandTelescope.com/observing/ataglance?1=1

    If pictures fail to load, refresh the page. If they still fail to load, change the 1 at the end of the URL to any other character and try again.