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

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

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

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.

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.