Sky at a Glance | October 19th, 2007

Some daily events in the changing sky for October 19 – 27.

Comet Holmes finder chart
Periodic Comet Holmes (17P) is currently in east-central Perseus. Although this finder chart shows the northeastern horizon at dusk (click image for full version), the comet will be easier to spot in late evening when Perseus is higher in the northeastern sky.
Sky & Telescope
Huge Outburst of Comet Holmes. On October 24th news flashed around amateur networks that faint little Periodic Comet Holmes (17P) had undergone an enormous outburst — and was shining at about magnitude 2.5, easily visible to the naked eye. The presumed ejection of gas and dust was still so close to the comet's nucleus as to appear almost starlike, as if this were a nova rather than a comet. The cloud of ejected stuff is expected to enlarge into a more traditional cometary appearance in the next several days.

The comet had been magnitude 17! Such wild behavior has a precedent, though; Holmes was also in a great outburst when it was discovered in 1892, reaching 5th magnitude. Back then, in fact, it had two such eruptions; the second came 2 1/2 months after the first.

Comet Holmes is in Perseus, excellently placed high in the northeast in late evening. It's not far from Delta Persei and is moving slowly. Chart of its motion; ephemeris.

See our article.

Comet LONEOS. A much lesser little comet is currently visible in binoculars — sort of, with difficulty — just above the western horizon in the end of twilight. Comet LONEOS, C/2007 F1, is about 5th magnitude, nearly at its predicted peak. It's heading southeast (leftward above the dusk horizon as seen from mid-northern latitudes) day by day. Telescopes have shown it with a pale gray-green head and a long, thin gas tail. Ephemeris; charts; pictures.

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Looking east-southeast in early dawn
Saturn and Regulus remain near Venus in the dawn all week, climbing higher above it day by day.
Sky & Telescope diagram

Friday, October 19

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

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

    Saturday, October 20

  • The famous eclipsing binary star Algol (Beta Persei) should be at its minimum light, magnitude 3.4 instead of its usual 2.1, for a couple hours centered on 6:51 p.m. EDT. Watch it slowly brighten during the course of the evening.

    Sunday, October 21

  • When night falls, Cygnus, the Swan, is still high in the west. You can hunt out some of its lesser-known telescopic sights using Sue French's "Deep-Sky Wonders" column and map in the October Sky & Telescope, page 72.

    Monday, October 22

  • Pegasus and Aquarius are high after nightfall, and with them the globular star clusters M2 and M15. They'll show in a small scope even through a fair amount of light pollution. Check them out using Ken Hewett-White's "Suburban Star Hop" in the October Sky & Telescope, page 59.

    Tuesday, October 23

  • The red long-period variable star R Cassiopeiae should be at maximum light (7th magnitude) this week.

    Wednesday, October 24

  • The bright Moon this evening shines below the Great Square of Pegasus.

    Thursday, October 25

  • Full Moon tonight (exact at 12:52 a.m. Friday morning Eastern Daylight Time).

    Friday, October 26

  • 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? Every year around October 29th, Arcturus occupies a special place in your local sky. It closely marks the spot above your horizon where the Sun stood at the same time (by your watch) during June and July — in broad daylight. So at this season of the year, you can think of Arcturus as the Halloween ghost of the departed summer Sun.

    Saturday, October 27

  • The Moon occults the northern edge of the Pleiades for northeastern North America in early evening (see the October Sky & Telescope, page 70), and for Europe later in the night.

    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 in ultraviolet light
    On Monday morning, October 22nd, Sean Walker caught rich detail in Venus's clouds using an ultraviolet filter on his 12.5-inch Newtonian reflector. At visible wavelengths, Venus never shows more than the subtlest traces of detail at best. Try viewing through a deep blue or violet filter.
    S&T: Sean Walker
    Mercury is hidden in the glare of the Sun.

    Venus (magnitude –4.6, in Leo) blazes high in the east before and during dawn. Above it are much fainter Saturn and Regulus, as shown at the top of this page. A telescope shows that Venus is at its not-quite-half-lit phase.

    Mars (magnitude –0.4, in the feet of Gemini) now rises around 10 p.m. daylight saving time. It shines very high toward the south before and during dawn — near the zenith, in fact, for mid-northern observers. The "Red Planet" is actually much more like yellow-orange or even pure yellow, especially when its atmosphere is dusty. How does Mars appear to you?

    In a telescope, Mars is currently 11 arcseconds in diameter. It will reach 15.9" 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 –1.9, in southern Ophiuchus) glares low in the southwest during twilight. It sets about an hour after dark.

    Saturn (magnitude +0.8) is the brightest "star" above brilliant Venus in the dawn. Fainter Regulus (magnitude +1.4) is there too, 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 disappearing into the sunset.

    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.

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