This Week’s Sky at a Glance, October 9 – 17

Venus, Mars, Jupiter, Moon on the morning of Oct. 10, 2015

In early dawn on Saturday morning the 10th, find the thin crescent Moon under Jupiter.

Venus, Mars, Jupiter, Moon, Mercury on the morning of Oct. 11, 2015

The next morning, a thinner, lower crescent guides the way to little Mercury. Binoculars will help.

Friday, October 9

• Now that we're well into October, Deneb is replacing brighter Vega as the zenith star after dark (for skywatchers at mid-northern latitudes). Accordingly, Capricornus has replaced Sagittarius as the most notable constellation down in the south.

• In early dawn on Saturday morning the 10th, look east for the waning crescent Moon below the Venus-Jupiter-Mars collection, as shown here.

Saturday, October 10

• A dawn challenge for Sunday morning the 11th: About 30 or 40 minutes before sunrise, scan low with binoculars almost due east for the tiny point of Mercury (magnitude +0.6) with the thin crescent Moon, as shown in the second panel. They're far below and perhaps a bit left of bright Venus and Jupiter.

Despite appearances, Mercury and the Moon are fairly similar places — except that Mercury is 40% larger and has twice as much surface gravity, and it's hotter and brighter in the daytime.

Sunday, October 11

• The little constellation Delphinus is about a fist at arm's length upper left of Altair early these evenings. It's a familiar group to scan with binoculars. But did you know about its twin orange variable stars for binoculars? See Gary Seronik's Binocular Highlight column and chart in the October Sky & Telescope, page 43. Also discover some deep telescopic targets in Delphinus in Ken Hewitt-White's Going Deep, page 57.

Monday, October 12

• Look low in the southeast in late twilight for Fomalhaut coming up. It stands highest in the south about 10 or 11 p.m.

• The eclipsing variable star Algol should be at its minimum light, magnitude 3.4 instead of its usual 2.1, for a couple hours centered on 11 p.m. Eastern Daylight Time, according to its recently revised timetable.

• New Moon (exact at 8:06 p.m. EDT).

Tuesday, October 13

• The Great Square of Pegasus balances on its corner high in the east at nightfall. Seen from your location, when is it exactly balanced? That is, when is the Square's bottom corner exactly below its top corner? It'll be sometime soon after the end of twilight, depending on both your latitude and longitude.

Wednesday, October 14

• Vega is the brightest star very high in the west at nightfall. Arcturus, equally bright, is getting low in the west-northwest. The brightest star in the vast expanse between them, about a third of the way from Arcturus back up toward Vega, is Alphecca, magnitude 2.2 — the crown jewel of Corona Borealis. Alphecca is a 17-day eclipsing binary, but its brightness dips are too slight for the eye to see reliably.

Moon, Saturn, Antares at dusk, Oct. 15-17, 2015

Back in the evening sky, the waxing crescent steps over Saturn (near Beta Scorpii) and Antares. Binoculars help.

Thursday, October 15

• Look for the crescent Moon, Saturn, and Antares lined up in the southwest in late twilight, as shown here.

Friday, October 16

• The Moon hangs over Saturn and Antares in the southwest at dusk, as shown here.

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

Saturday, October 17

• After dark, spot the W pattern of Cassiopeia standing on end high in northeast. The third segment of the W, counting from the top, points almost straight down. Extend it twice as far down and you're at the Double Cluster in Perseus. This pair of star-swarms is dimly apparent to the unaided eye in a dark sky, and visible in binoculars or a small, wide-field telescope from almost anywhere.


Want to become a better 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.

Pocket Sky Atlas

The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6 — which may sound like a lot, but it's still less than one per square degree on the sky. Also plotted are many hundreds of telescopic galaxies, star clusters, and nebulae.

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 (meaning heavy and expensive). 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 is having a fine dawn apparition. Look for it about 45 to 30 minutes before sunrise low in the east, well below and perhaps a bit left of brilliant Venus and Jupiter. Mercury brightens from magnitude +1.0 to +0.2 this week.

Venus, Mars, and Jupiter hang together in the east (in Leo) before and during dawn. Venus, on top, is the brightest at magnitude –4.6.  Jupiter is –1.8, and Mars, much closer to Jupiter, is much fainter at +1.8.

Jupiter and Mars pass through conjunction on October 17th and 18th, just 0.4° apart. Jupiter and Venus are also approaching each other. Look too for Regulus (magnitude +1.4) above Venus.

Saturn (magnitude +0.6, just off the head of Scorpius) sinks away in the southwest in twilight. Don't confuse it with orange Antares twinkling 10° to its left. Binoculars help.

Uranus (magnitude +5.7, in Pisces) and Neptune (magnitude +7.8, in Aquarius) are well up in the east and southeast, respectively, by 8 or 9 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) is Universal Time (UT, UTC, or GMT) minus 4 hours.


“This adventure is made possible by generations of searchers strictly adhering to a simple set of rules. Test ideas by experiments and observations. Build on those ideas that pass the test. Reject the ones that fail. Follow the evidence wherever it leads, and question everything. Accept these terms, and the cosmos is yours.”

— Neil deGrasse Tyson


4 thoughts on “This Week’s Sky at a Glance, October 9 – 17

    1. rocksnstarsrocksnstars

      Something to consider: with Uranus so far out, next month it will still be the same size (3.7 arcsec) and magnitude (5.7) as this month. In Dec, it only drops 0.1 arcsec and 0.1 magnitude, which is not really noticeable, and the advantage of waiting is that it will of course be higher in the sky earlier in the evening.


    Why was the minimal for Algol revised recently? I noticed the time for Oct. 12 is now 11:02 pm EDT instead of 1:01 am EDT Oct. 13 as predicted in the October Sky & Telescope magazine (same thing with the other predictions too). 2 hours is a big difference since the faintest time lasts about 2 hours. You could easily miss the minimum time. I looked all around, but couldn’t find any explanation anywhere. Thanks.

    1. Alan MacRobert Post author

      Daniel, The AAVSO recently found that minima of Algol had drifted quite far from the formula we (and they) had been using for too long. We’re using the new predictions online but the magazine lags behind; the change will start in the December issue (I think it is).

      For now, subtract 2 hours from the printed times and it’ll be close enough for all practical purposes.

      For reference, the new formula (heliocentric) for the minima in Julian Date is Time of Minimum = JD 2440953.5087 + 2.8673075E, where E is any integer.

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