Stars on a summer stream.
Vega lies near the direction we are going if "we" means the solar system judged with respect to the stars around us - those within, say, a few hundred light-years. This direction is called the "apex of the Sun's way" or the "solar apex." William Herschel, who coined the term in 1783, found it to be near Lambda ([Lambda]) Herculis. This is remarkably close to the modern position, which is at right ascension [18.sup.h] [06.sup.m], declination +30 [degrees], a point in Hercules 10 [degrees] southwest of Vega. The Sun is moving this way at 19.5 kilometers per second, or 4.1 astronomical units per year.
Deneb lies near the direction we are going if "we" means most of the stars in the solar neighborhood. Deneb roughly marks the forward point toward which our part of the galaxy is moving as it rotates around the galactic center. In this sense we are flying about 220 to 250 km/sec toward Cygnus.
Confusing? The clearest description I've seen of these two motions is Guy Ottewell's in his book The Astronomical Companion. Picture the solar system as a leaf floating on a river, suggests Ottewell. Many others leaves are floating too, drifting slightly this way and that; our own drift is toward the leaf called Vega. Meanwhile the entire river is flowing (much more rapidly) in the direction of Deneb.
THE SKY AT THE SUMMER TRIANGLE HOUR
Step outside on a late-summer evening and see for yourself where we're going. For most readers, Vega is now nearly overhead in the evening. It's at the center of the star map on the previous page, which shows Deneb not far east of overhead. Together with 1st-magnitude Altair they make up the giant, familiar Summer Triangle.
What other stars and constellations deck the sky at this Summer Triangle Hour?
Our gaze is pulled immediately to the south. The big piece of sky fire there is Jupiter, the brightest light of moonless evenings all summer. The giant planet burns just above the charming pattern of 2nd- and 3rd-magnitude stars called the Teapot, part of the constellation Sagittarius, the centaur Archer. By connecting the star dots of Sagittarius in the right way, it's possible to draw a cartoonish stick figure of an archer straining to pull a bow. This figure corresponds tantalizingly well to ancient descriptions of the stars' positions in Sagittarius's body. But on our constellation map on the previous page, we've drawn Sagittarius as the much easier-to-see Teapot.
If you're lucky enough to have a fairly dark observing site, something else will lead you inexorably south: the glorious band of the Milky Way running down the sky from near the zenith to its widest and most splendid in Sagittarius.
In the southeastern sky are the autumn constellations Capricornus and Aquarius and the western fish of Pisces, better known as the Circlet. All are famous but faint. Luckily, as the evening continues two bright objects come to the rescue: Saturn low in the east and Fomalhaut low in the southeast. They're visible as early as nightfall by the end of September.
Above Saturn, the Great Square of Pegasus balances on its southeast corner. Pegasus, the Flying Horse, extends his long neck and lunging head to the upper right. His 2nd-magnitude nose, Enif or Epsilon ([Epsilon]) Pegasi, points up toward tiny Delphinus, the Dolphin. From the lower-left corner of the ascending Great Square, Andromeda arcs toward her rising hero, Perseus, low in the northeast.
Above Perseus is the bright W of Cassiopeia, the Queen. Above her is her rather dim-starred husband, Cepheus, and his even dimmer modern companion, Lacerta, the Lizard.
The north is bare below Polaris. In the northwest on our map the Big Dipper is sweeping fairly low, and its constellation Ursa Major, the Great Bear, has already sunk almost knee-deep in horizon mist and murk.
Facing west, we find Arcturus at the same height as the Big Dipper. Above Arcturus is Corona Borealis like a letter C tilted backward, and above Corona is the similar-size Keystone of Hercules.
We complete our circuit around the rim of the sky by looking southwest. Here dramatic Scorpius is well past its prime height, but it's still not too late for good looks at twinkling Antares and other illustrious Scorpius treasures. This is an especially nice area for binoculars.
Now, do you have a lounge chair or sleeping bag to lie back on? Point your feet east, lie flat, and behold Deneb near the center of the sky. In this position, you can almost feel pressed against the surface of a world madly racing toward this brilliant star. Stretched across Deneb from north-northeast to south-southwest is the dreamy white Milky Way band. You are staring sideways into the disk of the Milky Way galaxy, toward which the galactic rotation is hurling us.
THE SUN, MOON, AND PLANETS IN SEPTEMBER
It's a rich month for solar-system displays. The most exciting is the total eclipse of the Harvest Moon visible from most of North America on the night of September 26th. The bright object kindling just 2 [degrees] from the Moon that night is Saturn, at its brightest and biggest of the year; see page 68. Saturn shines in the evening sky throughout September, as does Jupiter.
There is also spectacle before dawn: Venus is at its highest possible, with Mars, Castor, and Pollux near it early in September. Finally, Mercury pops into excellent view low in morning twilight near month's end.
Jupiter is at its highest in the south conveniently early in evening twilight during September. All month the brilliant cloud world stays enthroned just to the upper left of the top star of the Teapot, Lambda Sagittarii. Binoculars or a telescope shows the striped globe creeping slowly eastward against the background stars during September, passing just 1/2 [degrees] north of another magnificent ornament of heaven: the superb globular cluster M22. At least, M22 is superb when seen in a dark field. Jupiter, 1,000 times brighter, will tend to overwhelm it!
Jupiter in September is at the most southerly declination of its 12-year trip around the zodiac: -23 [degrees] 24 [minutes].
Pluto, in Ophiuchus just north of Scorpius, is heading down in the southwest right after dusk. If you're ready for a major challenge you can consult page 71 of the April issue for a finder chart.
Neptune and Uranus are a half constellation east of Jupiter near the Sagittarius-Capricornus border. They can be enjoyed in the south after nightfall with just a little optical aid. A map to identify them is on page 70 of the April issue.
Saturn is at opposition on the evening of September 26th, just hours before the Moon is at opposition (full phase). This is the night of the Moon's eclipse, and as a matter of fact the Moon is skirting closest to Saturn right around mid-eclipse, which should make for quite an unusual sight. As seen from North America the two will be only 2 [degrees] apart. Details are on page 69.
Saturn is an engaging object all September. It rises due east earlier in the dusk each week. Later in the evening it moves up into the southeastern sky, shining to us from a secluded section of Pisces at magnitude +0.5, its brightest in several years. The flattened globe of Saturn appears 19[inches] wide at the equator, its largest of the year, and the rings are narrow and dramatic, tilted 5 [degrees] from our line of sight. This is the last season we'll see the rings so narrow for a dozen years. In 1997 their tilt will be about 10 [degrees].
Venus and Mars rise together a little north of east around 3 a.m. daylight saving time as September begins. This is the peak of Venus's highest morning apparition (for midnorthern skywatchers) in its 8-year cycle of apparitions. Early risers will find the planet in interesting surroundings; it's paired with much fainter Mars to the lower right of Castor and Pollux. Venus starts the month just to the right of Mars; the planets stay within 3 [degrees] of each other on the mornings of September 2nd through 5th. For the rest of the month Venus moves increasingly far below Mars.
Shining at magnitude -4.2, Venus glides less than 3 [degrees] south of the center of M44, the Beehive star cluster in Cancer, on the mornings of September 12th to 14th. Mars, 200 times fainter at magnitude +1 1/2, follows behind, going right across the southern side of the cluster on the morning of September 21st. Both of these lovely events will be best seen in binoculars or a wide-field telescope.
By month's end Venus has descended to within 5 [degrees] of Regulus, which it will closely pass on October 3rd and 4th. Mars, meanwhile, hangs back in Cancer.
Telescopes show Venus's increasingly gibbous form shrinking from 21 [inches] to 16 [inches] in diameter during September. Mars's tiny disk appears only a quarter to a third this size. The red planet has a lot of growing to do before its opposition next March!
Mercury is in inferior conjunction with the Sun on September 17th. But each day thereafter it rises 7 or 8 minutes earlier (as seen from midnorthern latitudes), rocketing up into dawn visibility. On which morning can you first see Mercury low due east about 30 to 40 minutes before sunrise? By September's end, Mercury is in plain view near the peak of its best morning apparition of the year.
The last-quarter Moon occults Aldebaran on the morning of September 4th for Alaska, northern Canada, and the Arctic; elsewhere we see a near miss. On September 8th the slender lunar crescent stands only a few degrees to the right of Venus and Mars. The next morning it is underneath them.
New Moon occurs on September 12th, and first quarter comes on the 20th. That evening Jupiter shines to the Moon's lower left. The full Harvest Moon becomes totally eclipsed on the night of September 26-27 while just a couple degrees from Saturn; see page 69.
The Sun is at the September equinox point in Virgo at 2:00 p.m. Eastern Daylight Time on September 22nd. The Sun is crossing the equator heading south, ushering in autumn for the Northern Hemisphere and spring for the Southern Hemisphere.
RELATED ARTICLE: Light-Pollution Notes: Comet-Tail Photometers
Comet tails are sensitive indicators of sky quality. It's almost as if the tail were a sort of thermometer: but instead of measuring temperature, the length of the visible column measures the darkness of the sky.
The long tail of Comet Hyakutake last March provided some dramatic opportunities for documenting the effects of light pollution. Particularly striking to show the public would be photographs of the comet taken at the same time and revealing the vastly differing tail lengths that were visible at various distances from a city. The tail was a stubby 1 [degrees] long as seen from many urban centers and up to 80 [degrees] long in the country.
I'd like to receive comet photographs showing various degrees of light pollution. Please include all pertinent information about your pictures. Perhaps we can develop a more scientific plan to document the effects on Comet Hale-Bopp next spring! Ideally the only variable would be sky brightness due to light pollution, not haze, moonlight or altitude. And of course the camera settings, film, and development should be the same. Any volunteers?
Fred Schaaf welcomes mail at 681 Port Elizabeth-Cumberland Rd., Millville, NJ 08332, or e-mail at email@example.com.
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||includes related article on comet-tail photometers|
|Publication:||Sky & Telescope|
|Date:||Sep 1, 1996|
|Previous Article:||The evening sky: your guide to star-finding this month.|
|Next Article:||September's eclipse of the Moon.|