Some daily events in the changing sky for September 25 – October 3.

Friday, Sept. 25

  • First-quarter Moon tonight (exact at 12:50 a.m. on the morning of the 26th Eastern Daylight Time).
  • Mars, in the early-morning sky, is passing less than 1° north of Delta Geminorum, magnitude 3.5.

    Saturday, Sept. 26

  • Jupiter's moon Io crosses Jupiter's face from 8:45 to 11:02 p.m. Eastern Daylight Time tonight, followed by its tiny black shadow from 9:44 p.m. to 12:01 a.m. EDT.
  • Jupiter's Red Spot, meanwhile, should transit the planet's central meridian around 9:39 p.m. EDT. (For all of the Red Spot's transit times, good worldwide, use our Red Spot calculator, or print out our list for the rest of 2009.)

    Sunday, Sept. 27

  • Now that summer has changed to fall, the zenith star at nightfall has changed from Vega to Deneb (for skywatchers at mid-northern latitudes).
  • Jupiter's moon Io emerges out of eclipse from Jupiter's shadow around 9:11 p.m. EDT. A small telescope will show it swelling into view just east of the planet.

    Monday, Sept. 28

  • The bright "star" left of the Moon this evening is Jupiter, as shown below.
  • The eclipsing variable star Algol (Beta Persei) should be in one of its periodic dimmings, magnitude 3.4 instead of its usual 2.1, for a couple hours centered on 12:20 a.m. Tuesday morning EDT (9:20 p.m. Monday evening PDT). Algol takes several additional hours to fade and to rebrighten.
  • Jupiter's Red Spot transits the planet's central meridian around 11:17 p.m. EDT.

    Watch the Moon passing Jupiter on its way to full.

    Sky & Telescope diagram

    Tuesday, Sept. 29

  • Jupiter and the Moon shine together tonight, as shown above. Although they look close together, Jupiter is currently 1,610 times farther away — and it's 40 times larger in diameter.
  • Two mutual events occur tonight between Jupiter's moons Io and Europa. From 10:18 to 10:25 p.m. Pacific Daylight Time, Io occults Europa partially, passing in front of it. Then from 12:07 to 12:13 a.m. Pacific Daylight Time, Io's shadow eclipses Europa partially, dimming it by 81%.

    For a complete list of such mutual events among Jupiter's satellites that are visible from North America through the end of the year, see the October Sky & Telescope, page 56.

    Wednesday, Sept. 30

  • Jupiter's Red Spot transits the planet's central meridian around 12:56 a.m. Thursday morning EDT; 9:56 p.m. Wednesday evening PDT.
  • Happy 129th anniversary to deep-sky astrophotography. On this date in 1880, Henry Draper took the first successful photograph of the Great Orion Nebula.

    Thursday, Oct. 1

  • Ganymede, Jupiter's biggest satellite, casts its shadow onto Jupiter's face from 6:49 to 10:26 p.m. Eastern Daylight Time. Meanwhile, the Great Red Spot transits Jupiter's central meridian around 8:47 p.m. EDT.
  • Algol should be at minimum light for a couple hours centered on 9:09 p.m. EDT.

    Friday, Oct. 2

  • Look upper left of the bright Moon this evening for the Great Square of Pegasus, balancing on one corner. Your fist at arm's length will fit inside it. Binoculars show that the Moon itself is skimming the lower edge of the smaller Circlet of Pisces (during evening for North America). The Circlet is 6° in diameter, about as wide as a typical binocular's field of view.

    Saturday, Oct. 3

  • Full Harvest Moon tonight (exact at 2:10 a.m. Sunday morning Eastern Daylight Time). The Harvest Moon is the full Moon closest to the autumnal equinox.
  • Jupiter's Red Spot transits Jupiter's central meridian around 10:25 p.m. EDT.

    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 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 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, "the sky never becomes a friendly place."

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

    This Week's Planet Roundup

    Looking east in bright dawn

    Before sunrise, look carefully below Venus for Mercury and Saturn becoming a little more visible each morning. Binoculars help.

    Sky & Telescope diagram

    Mercury barely emerges low in the sunrise by about September 29th. Look for it below Venus, as shown at right.

    Venus (magnitude –3.9, in Leo) shines low in the east before and during dawn. Look for Regulus, much fainter, above it — by a rapidly increasing amount each day.

    Mars (magnitude +0.8, in Gemini near Pollux and Castor) rises around midnight or 1 a.m. and is very high in the east before dawn. In a telescope it's still only 6.5 arcseconds wide: a tiny, fuzzy blob, though noticeably gibbous. Mars is on its way to an unremarkable opposition late next January, when it will be 14 arcseconds wide.

    Jupiter (magnitude –2.7, in Capricornus) shines in the southeast as twilight fades — the first "star" to appear after sunset. It's highest in the south by about 9 p.m.

    Saturn follows Mercury up into dawn view by the end of the week. Use binoculars to look for it below Mercury after about September 29th, as shown here.

    Uranus (magnitude 5.7, below the Circlet of Pisces) is well up in the southeast during evening. Also catch the 8th-magnitude asteroid 3 Juno in Uranus's vicinity.

    Tiny Mars on Sept. 21, 2009

    Mars is gradually growing bigger and brighter in the morning sky. The planet was still only 6.3 arcseconds in diameter when S&T's Sean Walker took this stacked-video image on the morning of September 21st, but recognizable details are coming into high-resolution imagers' views. South is up. On the right (the disk's celestial east or following side), the diagonal dark band of Sinus Sabaeus ends at Sinus Meridiani near the limb. At left, dark Syrtis Major is approaching the sunset terminator. Bright Hellas is near top.

    Note the north polar clouds. In coming months, as Martian northern winter gives way to spring, the clouds should clear to reveal the bright white north polar cap. Walker used a 12.5-inch reflector and RGB color filters.

    S&T: Sean Walker

    Neptune (magnitude 7.8, in Capricornus) appears 6° or 7° east of Jupiter — and 16,000 times fainter.

    See our finder charts for Uranus and Neptune. And for a guide to spotting the challenging satellites of Uranus and Neptune (you'll need a fairly big scope), see the October Sky & Telescope, page 59.

    Pluto (14th magnitude, in northwestern Sagittarius) is getting lower in the southwest after dark. Use the finder chart in the June Sky & Telescope, page 53, before it's too late. Good luck.

    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.

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

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    • About Alan MacRobert

    Alan M. MacRobert became an avid Sky & Telescope subscriber in 1966 at age 14, joined the editorial staff in 1982, and is now a senior contributing editor, semi-retired. He played a role in practically every part of the magazine and the company's other products for more than a generation, both on the amateur-observing side and the science-reporting side. In 1994 a book collection of his observing how-tos and telescopic sky tours was published as Star Hopping for Backyard Astronomers. He has produced This Week's Sky at a Glance online every week since 1989.

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