Sky at a Glance | October 21st, 2011

By: Alan MacRobert October 21, 2011 0

Friday, Oct. 21

The modest Orionid meteor shower should peak in the early morning hours of Saturday. You might count a dozen or more Orionids per hour under ideal observing conditions. Some are also visible for several mornings before and after.

The Moon, just past last quarter, shines near Regulus below Mars before and during dawn Saturday morning, as shown below.

On Saturday morning the 22nd, Mars shines above the Moon and Regulus.

Sky & Telescope diagram

Saturday, Oct. 22

Fomalhaut, the Autumn Star, shines low in the south-southeast after dusk. It's due south later in the evening. The western (right-hand) edge of the Great Square of Pegasus, very high in the sky, points down nearly to it.

Sunday, Oct. 23

The summery constellation Cygnus remains nearly overhead after dinnertime, even higher than bright Vega when you face west and crane your neck. Have you hunted out the subtle star clusters in Cygnus's Milky-Way-rich center? See Sue French's "Deep-Sky Wonders" article, finder chart, and photos in the October Sky & Telescope, page 56.

Monday, Oct. 24

The tiny black shadow of Jupiter's satellite Europa crosses Jupiter tonight from 12:26 to 2:54 a.m. Tuesday morning Eastern Daylight Time (9:26 to 11:54 p.m. Monday evening Pacific Daylight Time). Europa itself follows only about 15 minutes behind its shadow.

Tuesday, Oct. 25

By 9 or 10 p.m. the Autumn Star, Fomalhaut, shines at its highest in the south (not all that high). Fomalhaut is 25 light-years away — exactly the same distance as Vega, shining brighter high in the west. So, the difference in brightness that you see is the two stars' actual difference in true luminosity. Vega looks 1 magnitude brighter than Fomalhaut (in other words, 2.5 times brighter), and so it really is.

Wednesday, Oct. 26

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 centered on October 29th every year, Arcturus occupies a special place in the sky above your local landscape. It closely marks the spot there where the Sun stood at exactly the same time (by your clock) during warm June and July — in broad daylight, of course! So, in the last days of October each year, you can think of Arcturus as the chilly Halloween ghost of the departed summer Sun.

New Moon (exact at 3:56 p.m. EDT).

Thursday, Oct. 27

The shadow of Jupiter's moon Io starts crossing Jupiter around 11:19 p.m. EDT, followed by Io itself just 3 minutes behind almost on top of the shadow.

Then at 11:37 p.m. EDT, Ganymede disappears into eclipse by Jupiter's own shadow, just a hairsbreadth beyond Jupiter's western limb.

The waxing crescent Moon points the way to low Venus and even lower Mercury. (The visibility of the fainter objects in bright twilight is exaggerated. 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.)

Sky & Telescope diagram

Friday, Oct. 28

In bright twilight, look for the thin waxing crescent Moon very low in the southwest. Can you spot Venus to its lower right, as shown here? They're separated by roughly a fist-width at arm's length (depending on your longitude). Use binoculars to try for much fainter Antares and Mercury.

Jupiter is at opposition tonight, opposite the Sun as seen from Earth. Like last year's opposition, this is an unusually close one; see Jupiter under "This Week's Planet Roundup" below.

Saturday, Oct. 29

The crescent Moon is higher and easier to spot now after sunset than it was yesterday. The moon's round limb points to the lower right, toward very low Venus and Mercury as shown here.

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).

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 must have a detailed, large-scale sky atlas (set of charts). The standards are the 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 effectively.

You'll also want a good deep-sky guidebook, such as Sue French's new 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 classic if dated Burnham's Celestial Handbook.

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."

This Week's Planet Roundup

Mercury (about magnitude –0.3) is deep in the sunset below much-brighter Venus. If the air is very clear, use binoculars or a telescope about 15 or 20 minutes after sundown this week to see if you can pick up Mercury 3° lower right of Venus early in the week, and 2° below Venus later in the week.

Venus (magnitude –3.8) is a little above the west-southwest horizon 20 or 30 minutes after sunset. It's getting less difficult to spot every week.

Mars (magnitude +1.2, in Leo) rises around 1 or 2 a.m. daylight-saving time. By the beginning of dawn it's in good view high in the east-southeast. Mars is closing in on similarly bright Regulus below it. They're 10° apart on October 22nd and 7° apart on the 29th. In a telescope Mars is a tiny blob only 5.7 arcseconds wide.

Io and darker Callisto were just east of Jupiter when S&T's Sean Walker imaged the scene on the morning of September 19th. South is up. Note the reddish Oval BA, "Red Spot Junior," just past the central meridian in the South Temperate Belt.

S&T: Sean Walker

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Jupiter (magnitude –2.9, in southern Aries) shines low in the east-northeast in twilight, then blazes higher in the east to southeast all evening. Look above it for the stars of Aries and below it for the head of Cetus, rather dim. Jupiter is nearly at its highest in the south by midnight. It's a big 49 arcseconds wide, as big as it will appear at its October 28th opposition.

Moreover, this is an unusually close Jupiter opposition. Last year Jupiter came closer to Earth than it had since 1963. This year it's only an insignificant 0.4% farther. See our guide to observing Jupiter with a telescope.

Saturn is deep in the glow of sunrise.

Uranus (magnitude 5.7, in Pisces) and Neptune (magnitude 7.9, in Aquarius) are well placed in the south and southeast by mid-evening. Use our printable finder chart for both, or see the September Sky & Telescope, page 53.

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.

NEW BOOK: Sue French's DEEP-SKY WONDERS! This big, long-awaited observing guide by Sky & Telescope's Sue French is now available from Shop at Sky. Hefty and lavishly illustrated, it contains Sue’s 100 favorite sky tours (25 per season), with finder charts, from her 11 years of writing the Celestial Sampler and Deep-Sky Wonders columns for S&T. Don’t miss it!

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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.