Some daily events in the changing sky for October 23 – 31.

Watch the Moon pass by Jupiter Sunday through Tuesday evenings. (Moon positions are for evening in North America.) If you have sharp eyes, Alpha (α) Capricorni is a naked-eye double star. The smallest binoculars resolve it easily — and will also show Beta (β) to be double.

Sky & Telescope diagram


Friday, October 23

  • This is the second night of the Galilean Nights worldwide star party, which continues through Saturday. Check for an event near you.
  • It's also the last night for the Great World Wide Star Count. Go out and report what Cygnus (or Sagittarius) looks like from your location, if you haven't done so already!
  • During early evening for eastern North America, Jupiter's moon Ganymede occults (crosses over) Europa partially, from 8:31 to 8:40 p.m. EDT. In a telescope they'll appear to merge for a number of minutes, then separate. Ganymede is slightly the brighter of the two. How well can you resolve this event? It kicks off a busy week of Jovian mutual-satellite occultations.

    Saturday, October 24

  • This is the third and last night of the Galilean Nights international star party.
  • The bright 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 8:26 p.m. EDT. Algol takes several additional hours to fade and to rebrighten. Use our comparison-star chart.
  • Jupiter's moon Io occults Europa partially from 9:19 to 9:24 p.m. EDT.

    Sunday, October 25

  • First-quarter Moon (exact at 8:42 p.m. EDT).
  • Spring begins in Mars's northern hemisphere.

    Monday, October 26

  • Jupiter this evening is 3° or 4° lower left of the waxing Moon, as shown above (during dusk for North America).

    Tuesday, October 27

  • The Ghost of Summer Suns. Halloween is approaching, and this means that Arcturus, the star sparkling low in the west-northwest in twilight, is taking on its role as "the Ghost of Summer Suns." What does this mean? For several days every year around October 29th, Arcturus occupies a special place in the sky above your local landscape. It closely marks the spot there where the Sun stood at the same time (by the clock) during warm June and July — in broad daylight, of course. So at this season every year, you can think of Arcturus as the chilly Halloween ghost of the departed summer Sun.

    Jupiter's Great Red Spot remains clearly separated from the South Equatorial Belt (SEB) by a wide, white Red Spot Hollow. Note the small, very dark red barges following behind. The SEB seems to be calming and fading; the NEB is darker and much busier. South is up.

    Christopher Go took these images on October 26th at 11:12 and 12:07 UT. Stacked-video images like these show much more detail than you're ever likely to see visually on Jupiter.

    Christopher Go

  • Jupiter's Great Red Spot should cross Jupiter's central meridian (the imaginary line down the center of the planet's disk from pole to pole) around 10:18 p.m. Eastern Daylight Time. The "red" spot appears pale orange-tan. It should be visible for about an hour before and after in a good 4-inch telescope if the atmospheric seeing is sharp and steady. A light blue or green filter helps. The Red Spot transits about every 9 hours 56 minutes; for all of the Red Spot's central-meridian crossing times, good worldwide, use our Red Spot calculator or print out our list for the rest of 2009.

    Wednesday, October 28

  • Orion preview. Stay up till about 10 p.m. (depending on where you live in your time zone) and you'll find the bright winter constellation Orion already making his appearance low in the east. Above Orion glares orange Aldebaran. Above Aldebaran are the fingertip-size Pleiades.

    Thursday, October 29

  • Mars is at quadrature, 90° west of the Sun. So this week the planet appears as gibbous in a telescope as it's going to get (with its disk 88% illuminated).
  • Jupiter's moon Ganymede occults Io partially from 10:04 to 10:10 p.m. EDT.
  • Jupiter's Great Red Spot transits around 11:11:57 p.m. EDT.

    Friday, October 30

  • Jupiter's moon Ganymede occults Europa partially tonight from 11:55 p.m. to 12:03 a.m. EDT; 8:55 to 9:03 p.m. PDT.

    Saturday, October 31

  • Jupiter's moon Io occults Europa partially from 11:38 to 11:43 p.m. EDT; 8:38 to 8:43 p.m. PDT.
  • Mars shines in the east during the early-morning hours of Sunday. Binoculars will show that it's smack in the center of the Beehive star cluster, M44.

    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 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, "the sky never becomes a friendly place."

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

    This Week's Planet Roundup

    Two planets accompany stars of Virgo in the east-southeast at dawn. This is their configuration on the morning after Halloween night. (The visibility of faint objects in bright twilight is exaggerated here; you may need binoculars for all but Venus.)

    Sky & Telescope diagram

    Mercury has become lost in the glow of sunrise.

    Venus (magnitude –3.9, in central Virgo) is also getting lower in the east at dawn, but much more slowly; it's still moderately well up. Venus is so bright that it's easy to spot if you look low in the east 60 to 30 minutes before your local sunrise time. Saturn, only a hundredth as bright, is above Venus and somewhat to the right; their separation widens from 12° to 20° this week. Late in the week, use binoculars to look below Venus for twinkly little Spica.

    Mars (magnitude +0.5, in central Cancer) rises around midnight and is very high in the southeast before dawn. It's below Gemini's head stars, Pollux and Castor. Use binoculars to watch Mars closing in on the Beehive Star Cluster; it will cross the cluster from the mornings of October 31st to November 2nd.

    In a telescope Mars is still only about 7.6 arcseconds wide: a tiny, fuzzy gibbous blob. Mars is on its way to an unremarkable opposition late next January, when it will be 14.1 arcseconds wide.

    On this side of Jupiter away from the Great Red Spot, the South Equatorial Belt (dark band above center) is straight and double. The North Equatorial Belt is darker and busier. The black dot is the shadow of Io. Note the small red oval in the north edge of the NEB just past (left of) the central meridian. It used to be white but picked up dark material from the belt. South is up.

    Christopher Go took this image on October 25th at 11:12 UT, when the System II longitude on the central meridian was 10°.

    Christopher Go

    Jupiter (magnitude –2.5, in Capricornus) shines brightly in the south after dark and lower in the southwest later in the night. It sets around 1 a.m.

    Saturn (magnitude 1.1, in the head of Virgo) is getting higher the east-southeast during early dawn. Look for it to the upper right of low Venus — by 12° at the beginning of the week, 20° at week's end.

    Uranus (magnitude 5.8, below the Circlet of Pisces) is well up in the southeast to south during evening.

    Neptune (magnitude 7.9, in Capricornus) is 6° east of Jupiter.

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

    Pluto (14th magnitude, in Sagittarius) is sinking into the sunset.

    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

    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.

    • 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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