Some daily events in the changing sky for September 14 – 22.

Looking south-southwest in twilight

Watch the Moon triangulating on Jupiter and Antares around midweek. (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. For clarity, the Moon is shown three times actual size.)

Sky & Telescope diagram

Friday, September 14

  • Shortly after sunset today, a thin crescent Moon is very low in the west-southwest; use binoculars. To its right is Spica. Farther right of Spica (and probably a bit lower) is Mercury. Catch them early before they set!

    Saturday, September 15

  • As soon as the stars come out, look for the Big Dipper in the northwest and W-shaped Cassiopeia at the same height in the northeast. At this moment they're in balance with each other. Later in the evening, the Dipper dips lower and Cassiopeia climbs higher.

    Sunday, September 16

  • Early Sunday and Monday mornings, Mars is passing a bit less than 1° north of M1, the dim Crab Nebula. On Monday through Wednesday mornings, Mars is barely 2° north of 3rd-magnitude Zeta Tauri.

    Monday, September 17

  • The waxing Moon shines lower right of Jupiter and Antares low in the dusk, as shown above.

  • Mars is at western quadrature, 90°, west of the Sun. So this month Mars shows us its most pronounced gibbous phase.

    Tuesday, September 18

  • This evening the Moon forms a nearly isosceles triangle with Jupiter and Antares. The Moon is to their left, as shown above.

    Wednesday, September 19

  • First-quarter Moon (exact at 12:48 p.m. EDT).

    Thursday, September 20

  • Chi (χ) Cygni, one of the brightest long-period variable stars in the sky, should be at maximum light. It's predicted to shine at magnitude 5.2 this week, but last year it hit magnitude 3.7. Chi Cygni tends to alternate between bright and faint maxima — so will this year's peak be dimmer than expected? As of September 13th the star was magnitude 5.9. Keep track of its doings (in the shaft of the Northern Cross) using binoculars and the comparison-star chart in the September Sky & Telescope, page 58, or online. Then compare your estimates to those being reported to the AAVSO.

    Friday, September 21

  • Soon after sunset, use binoculars to try for Mercury and Spica in conjunction low above the west-southwest horizon, as shown below. Mercury passes a mere 5 arcminutes north of Spica around 10 hours Universal Time on the 22nd — good timing for evening viewers in Hawaii (on the 21st local date) and especially Australia and the Far East (on the 22nd local date).

    Saturday, September 22

  • The equinox occurs at 5:51 a.m. EDT Sunday morning. This is when the Sun crosses the equator heading south for the year. Fall begins in Earth's Northern Hemisphere, spring in the Southern Hemisphere.

    Looking very low west-southwest in bright twilight

    Use binoculars to catch Mercury and Spica in conjunction on Friday evening the 21st. (Their exact positions with respect to each other will depend on where you are.) The old rule is that stars twinkle and planets don't, but when you're viewing through this much of Earth's unsteady atmosphere, even a planet is likely to twinkle a bit.

    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

    Mercury (magnitude 0) is deep in the glow of sunset, in the midst of a poor apparition for the Northern Hemisphere. Try looking for it with binoculars about 20 minutes after sundown just above the horizon — due west early in the week, west-southwest at week's end. Day by day, watch Mercury closing in on fainter, sparklier Spica to its upper left. Spica and Mercury come to a close conjunction on Friday the 21st, as shown above.

    Venus on Aug. 30, 2007

    Venus was still a very a thin crescent just 14° from the Sun when Sean Walker took this stacked video image, using a 12.5-inch Newtonian reflector, after sunrise on August 30, 2007. He boosted contrast to darken the blue sky.

    S&T: Sean Walker

    Venus (magnitude –4.7) shines in the east at dawn, getting higher every day. A telescope shows that it's a crescent, waxing in phase as Venus always does when you see it as the "Morning Star."

    Mars (magnitude +0.1, between the horns of Taurus) rises around 11 p.m. daylight saving time and shines very high in the southeast before dawn. Compare its fiery color with that of fainter Aldebaran to its upper right late at night, and with th color of Betelgeuse a similar distance to Mars's lower right.

    In a telescope Mars is gibbous and 9 arcseconds in diameter. It will reach 16" diameter around its Christmas-season opposition. The last couple months of Martian dust storms have abated, and while the planet's atmosphere is still bright and hazy with dust, surface features are showing through a little better. But they're still low-contrast; don't expect to see much, especially with Mars still so small.

    Mars on Sept. 13, 2007

    The Martian air is clearing. On the morning of September 13th, Sean Walker shot this view showing the dark band of Mare Tyrrhenum (left) and Mare Cimmerium (center to lower right). Jutting up from Mare Cimmerium, just lower right of center, are the two little prongs of Gomer Sinus (actually, Gomer Sinus is just the one on the right) — well-known among Mars observers as an indicator of excellent resolution. North is up, celestial east is left.

    S&T: Sean Walker

    Jupiter (magnitude –2.1, in southern Ophiuchus) glares in the south-southwest during twilight and sets around midevening. Get your telescope on it as early in twilight as you can. Antares, less bright, sparkles redly 6° or 7° below it.

    Saturn (magnitude +0.7) is low in the glow of sunrise, about 16° lower left of brilliant Venus. If you pick up Saturn in binoculars or a wide-field scope, you can see that it's within 2° or 3° of twinkly Regulus (magnitude +1.4). Regulus is to Saturn's upper right.

    Uranus (magnitude 5.7, in Aquarius) and Neptune (magnitude 7.8, in Capricornus) are well placed in the southeast to south during evening. Finder charts for them are in the July Sky & Telescope, page 60, and online.

    Jupiter Sept. 2, 2007

    Jupiter is bidding telescopic observers a long farewell, but even so, Christopher Go caught this fine shot on September 2nd (at 10:35 UT) from his low latitude in the Philippines. North is up. The South Equatorial Belt has become a pair of narrow dark lines separated by a wide white zone (at least on this side of Jupiter). Lower right of center, South Tropical Disturbance #1 is very prominent.

    Christopher Go

    Pluto (magnitude 14.0, in the northwestern corner of Sagittarius) is already starting to creep down in the south-southwest when darkness falls. It's about 15° east-northeast of Jupiter. If you have at least an 8-inch scope and a good sky, try for Pluto while you still can. A finder chart is in the July Sky & Telescope, page 60.

    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 get the current Sky at a Glance, bookmark this URL:
    http://SkyandTelescope.com/observing/ataglance?1=1

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