September 2008
Skywatcher’s Guide for September 2008
PLANETS
HIGHLIGHTS: Two planets at one time might be glimpsed with the unaided eye in bright evening twilight this month, with four possible in binoculars as Mercury and faint Mars will be very low and challenging. Venus, Mars and Mercury group very low in bright evening twilight from September 3-19 in Virgo, the Virgin when they fit within a five degree circle. Venus and Mars are closest on September 11 when only 18 arc seconds apart in the sky, a very interesting sight in a telescope.
Bright Jupiter dominates the southern sky in September for much of the first half of the night, especially during the first half of the month. Jupiter appears as a very bright, almost brilliant white starlike object, well above the southern horizon on September evenings. Through September 8 Jupiter is near maximum brightness for 2008 (brighter than magnitude -2.5) and large in a telescope (43 arc seconds), though by September 30 has faded slightly (to magnitude -2.3) from its brightest and largest back in July. September is still a very good time to view the giant planet this year in the convenient evening hours just after sunset. Look for bright, whitish Jupiter just east (or roughly ‘left’) of the Teapot of Sagittarius: It’s the brightest starlike object well up in the evening sky in September. Jupiter is easily found 1 hour after sunset in the southern sky. Jupiter is highest (at around 30 degrees in altitude for much of the Desert Southwest, 34 degrees for southern Arizona) and due south at 8:30 p.m. in early September, and in evening twilight mid-month into the third week of September. By the last week of September Jupiter starts to lose altitude in the southern sky. Jupiter sets in the south-southwest around 1:30 a.m. on September 1, around 12:30 a.m. on September 15 and around 11:30 p.m. in late September. Also, the waxing gibbous Moon will slide below (east) of Jupiter in the southern sky on Tuesday evening, September 9.
When looking at Jupiter in a telescope to view its cloud belts and festoons, the sharpest views should be in evening twilight, when it is near its highest in the sky this month. As with any planet, viewing is dependent on atmospheric stability and conditions, and of course, the quality and size of the telescope used. However to view its moons, a good pair of binoculars is all that’s needed.
The brilliant planet Venus very gradually moves out of the glare of the Sun above the western horizon and into the evening twilight this month. Being so low until the last 10 days of the month, it won’t be easy to find. Look for Venus to become easier to see later this fall. Venus is loosely grouped to Mars and Mercury from September 3-19, and the three fit within a 4 degree circle on 6-13. However the three are so low that only Venus will be easily visible to the naked eye, with Mars and Mercury requiring binoculars and a clear horizon. Venus and Mars are closest on September 11 when only 18 arc seconds apart in the sky, a very interesting sight in a telescope. Venus starts September just a few degrees above the western horizon about 30 minutes after sunset, but creeps higher each evening and reaches 8 degrees above the west-southwestern horizon at that time by September 30.
As mentioned previously, the elusive planet Mercury groups loosely with Venus and Mars from September 3-19. However, the glare of the Sun brightens the sky and makes naked eye viewing for Mercury and dimmer Mars very difficult; Use binoculars to find them starting 30 minutes after sunset. Mercury will be brighter than Mars, at magnitude 0.0 on September 1, and fades each evening slightly, reaching 0.5 magnitude on September 19. Mercury is closest to brilliant Venus above the western horizon on September 1, and then closest to Mars on September 4/5.
Although not very bright (at around magnitude 1.8), spotting Mars is made much easier when it is closest to Venus around September 10-13 (use binoculars). As stated previously, Venus and Mars are closest on September 11 when only 18 arc seconds apart in the sky, a wonderful opportunity to see two planets at once in a telescope. Orange-red Mars will appear less than half as bright as Mercury. Don’t expect to see any surface contrast features or polar caps as Mars is far too small in apparent size for viewing any features, even in large, high quality telescopes.
Saturn emerges out of the glare of the Sun starting September 16-20 in the morning twilight above the eastern horizon. At that time Saturn appears as a near 1st magnitude starlike object, 45 minutes before sunrise, just a few degrees above the eastern horizon (Saturn will be dimmed some by its low position in the sky). Saturn rises earlier and creeps higher above the eastern horizon each morning and reaches 7 degrees in altitude, one hour before sunrise, by September 30. Saturn is found in the constellation Leo the Lion, nearly 15 degrees below its bright star Regulus. Also, the thin waning crescent Moon is found closest to Saturn and above it on Saturday morning, September 27 (binoculars might be needed to spot Saturn).
Visitors to Flandrau’s 16-inch telescope should note that only the planet Jupiter is visible this month in the telescope after it opens at 7 p.m. (Wednesday-Saturday nights only, weather permitting). The telescope normally closes at 10 p.m.
Finally, remember that in spotting planets there is a general rule: ’stars twinkle, planets don’t’. This is because stars are point sources of light; therefore starlight is easily disturbed and shifted by air currents in the Earth’s atmosphere. However when looking at Mercury right now, note that it may appear to twinkle somewhat; however, this twinkling is of a slower nature than the bright stars.
SEPTEMBER STARS
September evenings are an excellent time to see the bright constellations of the summer months (especially Scorpius and Sagittarius), along with the Milky Way. To see the brightest areas of the Milky Way in the south, look before 10 p.m., as the star rich regions in Scorpius and Sagittarius start to lose altitude in Arizona skies early. Start by finding the brightest star in Scorpius: Antares. These evenings, its orange-red twinkling enlivens the southern sky, giving reason to its name “the rival of Mars”. From the southern United States Antares does not get very high, only about 30 degrees (or three fists held at arm’s length) above the southern horizon, because of its position in the southern sky. Antares is the brightest part of a most impressive scroll of stars, sort of like a long letter S fallen halfway forward. And in this case the S stands for the constellation it is part of: Scorpius, the Scorpion.
To the ‘left’ or east of Scorpius is the “Teapot” of Sagittarius. Not technically a constellation, the Teapot is actually part of the larger constellation Sagittarius, the Archer. Like steam emerging from the spout of a real teakettle, look for bright portions of the Milky Way off the spout of the Teapot; this region marks the central area of our galaxy and is fascinating to scan in binoculars. High power binoculars and telescopes reveal the Milky Way to be clouds of stars, millions of them, with attendant star clusters and gas clouds making up some of the “fuzzier” spots you’ll see.
Overhead in the September evening sky look for the bright stars of the Summer Triangle: Vega, Deneb, and Altair. Each night every September the widely spaced triangle of Vega, Deneb, and Altair brighten the evening sky. Vega, Altair, and Deneb are each the brightest stars of three constellations that make up the Summer Triangle. Vega is the brightest star in the small constellation of Lyra, the Lyre (a small ancient harp). Vega, one of the closer stars to Earth (only 25 light years away), was the Pole Star (our North Star) some 12,000 years ago, and will again be the North Star in 12,000 A.D. when Earth’s cyclical wobbling motion brings in near the pole once more. Deneb is the tail of Cygnus the Swan, but also marks the bottom o0 of the cross-shaped pattern whose other name is the Northern Cross. Deneb is a huge super-giant star over 1,600 light years from Earth. Aquila, the Eagle is the home of the bright star Altair, the first star of the three in the triangle to set late at night and the closest of the three to Earth at only 16 light years distant. Aquila looks more like a diamond or kite than an Eagle. Aquila and Cygnus (or the Northern Cross) mark some of the brightest areas of the northern Milky Way.
The Big Dipper sets early in September, low in the northern half of the evening sky. Look for the Big Dipper low in the north starting 1 hour after sunset. Take the two end stars off of the bowl (the ‘pointer stars’) and point down to Polaris, the North Star. Polaris is a star of ordinary brightness, and marks the end of the handle of the Little Dipper. Polaris can also be found about 30 degrees (or 3 fists) above the northern horizon. Polaris stays put in the sky; to the eye it doesn’t appear to move. This is because Earth’s axis is pointed toward Polaris in space.
The Milky Way Prominent into the Late Night Hours!
Look for the Milky Way after the end of evening twilight on moonless September evenings, as its brightest portions ride high in the southern sky. You’ll need to look right after the end of evening twilight as these bright regions of our home galaxy (around the constellation Scorpius, the Scorpion) set early. The Milky Way, the galaxy of stars, gas and dust our solar system resides in, is prominent all summer long, but its brightest portions in Scorpius and Sagittarius are best seen around 8 p.m., through 9 p.m. in September when they are highest in the sky. Additional bright portions can be seen overhead into the late night hours around the ‘Summer Triangle’ of bright stars: Vega, Deneb, and Altair. To see the Milky Way, you’ll need to get away from city lights to a dark location, and look on a night when there’s not much interference from the moon. Our home galaxy will appear as a “cloudy” band stretching clear across the sky. High power binoculars and telescopes reveal the Milky Way to be clouds of stars, millions of them, with attendant star clusters and gas clouds making up some of the “fuzzier” spots you’ll see.
Sept. 22nd marks the Autumnal Equinox, The Start of Autumn for Arizona
Fall begins for Tucson and much of Arizona on Monday Sept. 22 at 8:44 a.m. MST (Mountain Standard or Tucson time), as the Sun, in Virgo, stands directly overhead from the equator at noon. Fall now begins for the northern hemisphere (and spring for the southern hemisphere) as the Sun heads south of the equator, losing altitude in our northern hemisphere sky.
MOON PHASES
The September first quarter Moon will occur on Saturday evening, September 6. The September Full Moon, which can be called the Fruit Moon but is better known as the Harvest Moon, rises on the evening of Sunday, September 14 and sets on the morning of Monday, September 15. The September last quarter Moon occurs on Monday morning September 22. The September 2008 new moon will occur on Sunday, September 28.
Note: Additional information can be accessed by phone at Flandrau Science Center’s Astronomy Newsline at (520) 621-4310 or as a menu option at 621-S-T-A-R.
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