Some daily events in the changing sky for October 12 – 20.

Comet LONEOS. A nice little comet is currently visible in binoculars very low in the west at the end of twilight. Comet LONEOS, C/2007 F1, is below Arcturus at dusk, currently 6th magnitude and brightening toward a possible 4th magnitude at October's end. It's heading southeast (leftward above the dusk horizon as seen from mid-northern latitudes) day by day. Telescopes show it with a pale gray-green head and a long, thin gas tail. Finder map and ephemeris, courtesy

Looking southwest at dusk

Watch the waxing crescent Moon pass Antares and Jupiter low in the southwest after sunset this week. (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. The blue 10° scale is about the size of your fist held at arm's length. For clarity, the Moon is shown three times actual size.)

Sky & Telescope diagram

Friday, October 12

  • By late evening this week, the landmark W shape of the constellation Cassiopeia exactly balances on one end high in the northeast — and the Big Dipper lies exactly horizontally just above the north-northwest horizon (or below the horizon if you're as far south as Tampa or Houston).

    Saturday, October 13

  • A twilight challenge: If the sky is very clear about 15 or 20 minutes after sunset, use binoculars to scan low above the west-southwest horizon for the very thin crescent Moon — and tiny little Mercury about two Moon-diameters south (lower left) of it. This applies to North America; their relative positions are a bit different after sunset elsewhere. Good luck.

  • The dwarf planet 1 Ceres, now magnitude 7.1, is becoming a late-evening target in Taurus. Tonight, notes David Likuski, it forms a nearly isosceles triangle south of Xi and Omicron Tauri, magnitudes 3.7 and 3.6, the pair of stars forming the narrow rump of Taurus as the stick figure is drawn on Sky & Telescope maps. Late this evening Ceres is slightly more than 1° from each. Watch for many minutes and see if you can detect its movement.

    Sunday, October 14

  • The crescent Moon is thicker, higher, and easier to see this evening, but it's still quite low in twilight. It's more or less lined up diagonally with Antares and Jupiter, as shown above, depending on your location.

  • Venus and Saturn pose within 3° of each other before dawn this morning and Monday morning.

    Monday, October 15

  • The crescent Moon forms a triangle with Jupiter and Antares in the fading twilight, as shown above.

  • 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 1:13 a.m. Tuesday morning EDT (10:13 p.m. Monday evening PDT). Algol takes several additional hours to fade and to rebrighten. For all times of Algol's minima this month, good worldwide, see the October Sky & Telescope, page 70.

    Tuesday, October 16

  • As soon as the stars come out this evening, look very low below the Moon to catch your last glimpse this year of the Cat's Eyes, the paired stars Lambda and Upsilon Scorpii in the tail of Scorpius. You'll need an open view toward the south-southwest.

    Wednesday, October 17

  • This evening the Moon shines in the Teapot of Sagittarius (for the time zones of the Americas and Europe).

    Thursday, October 18

  • Algol should be at minimum light for a couple hours centered on 10:02 p.m. EDT.

    Friday, October 19

  • First-quarter Moon (exact at 4:33 a.m. Eastern Daylight Time).

    Saturday, October 20

  • Algol should be at minimum light for a couple hours centered on 6:51 p.m. EDT. Watch it slowly brighten during the course of the evening.

  • The annual Orionid meteor shower is scheduled to peak very late tonight, but activity should be nearly the same for several nights. An observer under a dark sky may see about 20 meteors per hour between 2 or 3 a.m. and dawn, when the shower's radiant is high in the sky. Any light pollution will reduce the numbers visible.

    Looking east-southeast in early dawn

    Saturn and Regulus remain near Venus in the dawn this week, but by the 20th they've climbed above it.

    Sky & Telescope diagram

    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 foldout map in each issue of Sky & Telescope, the essential magazine of astronomy. Or download our free Getting Started in Astronomy booklet (which only has bimonthly maps).

    Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of maps; the standard is Sky Atlas 2000.0) and good deep-sky guidebooks (such as Sky Atlas 2000.0 Companion by Strong and Sinnott, the even more detailed Night Sky Observer's Guide by Kepple and Sanner, or the enchanting though somewhat dated Burnham's Celestial Handbook). Read how to use them effectively.

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

    This Week's Planet Roundup

    Venus continues to climb higher in the morning sky. Sean Walker made this image from red, green, blue, and ultraviolet video stacks on the morning of October 5th.

    Sean Walker

    Mercury is lost deep in the glow of sunset.

    Venus (magnitude –4.7, in Leo) blazes high in the east before and during dawn. Near it are much fainter Saturn and Regulus. A telescope shows that Venus is a thick crescent, thickening from week to week even as the planet shrinks into the distance.

    Mars (magnitude –0.3, in the feet of Gemini) rises around 10:30 p.m. daylight saving time and shines very high toward the south before and during dawn — near the zenith, in fact, for mid-northern observers. Compare Mars's fiery color with that of Betelgeuse, which is off to its right after they rise and below it in early dawn.

    In a telescope, Mars appears 10.5 to 11 arcseconds in diameter this week. It will reach 16" diameter around its Christmas-season opposition. The Martian dust storms of July and August have abated, and while the planet's atmosphere is still bright and hazy with dust, surface features are showing through somewhat better. But they're still low-contrast.

    Sean Walker took this image of Mars on the morning of October 5th under near-perfect conditions. Sinus Meridiani, where the robust NASA rover Opportunity is exploring Victoria Crater, is about to transit the central meridian here. Note the big, newly regrown North Polar Hood of winter clouds at top.

    Sean Walker

    Jupiter (magnitude –2.0, in southern Ophiuchus) glares in the southwest during and after twilight. It sets by midevening. Look for Antares, much less bright, sparkling redly 8° or 9° below it at nightfall, as shown at the top of this page.

    Saturn (magnitude +0.7) is the brightest "star" near bright Venus in the dawn. Fainter Regulus (magnitude +1.4) is there too; it's upper right of Saturn.

    Uranus (magnitude 5.8, in Aquarius) and Neptune (magnitude 7.9, in Capricornus) are well placed in the south during evening. Finder charts for them are in the July Sky & Telescope, page 60, and online. With a big scope you can shoot for their faint moons! See the October Sky & Telescope, page 69.

    Pluto (magnitude 14.0, in Sagittarius) is sinking low in the southwest at dusk.

    All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's midnorthern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America. Eastern Daylight Time (EDT) equals Universal Time (UT, UTC, or GMT) minus 4 hours.

    To be sure you always get the current Sky at a Glance, bookmark this URL:

    "Rational and innocent entertainment of the highest kind."
    — John Mills, 19th century Scottish manufacturer and founder of Mills Observatory, on amateur astronomy.

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