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The sky at summer's end.

A crystal clear sky is not the only weather condition that astronomers long for. Critical for telescopic observers is a steady atmosphere that produces sharp images good "seeing" - at high power. This is especially true when a special planetary or lunar sight is on the night's agenda - such as the remarkable views of Saturn's rings near edgewise that we get all this summer and fall.

If you don't own a telescope, consult the listings in the center of this issue for astronomy clubs and planetariums near you. If you do have a telescope, you may want to review the article on seeing in the April issue, page 40. Do you suspect that your telescope is giving subpar images even in good seeing? Then consult the article on star-testing a telescope's optical quality starting on page 42 of the March issue.

Remember too that those wonderful, crisp nights we often get in late summer and early autumn (after an air-clearing cold front passes through) may not be the best nights for sharp images of Saturn. A hazy, milky summer sky is often the steadiest. And don't expect good seeing when the weather map shows the jet stream overhead, when the weather has changed dramatically in the last few hours, when there is local convective activity, or when you are downwind from nearby mountains. A meteorologist friend of mine in Colorado complains about frequent bad seeing caused by the Rockies but says September is about the steadiest time of year.

THE SKY AT THE SUMMER TRIANGLE HOUR

Saturn has rings around its waist, angels have rings over their heads, but at the time of our all-sky map one page back, most of our readers have a triangle over their heads. Up near the zenith shines the Summer Triangle formed by the bright stars Vega, Deneb, and Altair.

The Triangle is highest at summer's end, late on August evenings and early on September ones. It will stay visible in the west all fall right into the beginning of winter. For months now we've been following its three bright stars and their constellations of Lyra, Cygnus, and Aquila, but there is more to see and enjoy in this region.

Lyra and Cygnus are home to many double stars rivaling the acclaimed orange and blue Albireo [Beta] Cygni) and the Double Double, Epsilon ([Epsilon]) Lyrae. These two constellations hold nebulae as renowned as the Ring, North America, and Veil, and variable stars as famous as Beta Lyrae and Chi ([Chi]) Cygni.

But there are many less well-known sights here just as fascinating for the naked eye, binoculars, and telescopes. You can blink a planetary nebula (NGC 6826) in Cygnus or locate the pinpoint "companions" of Vega in Lyra. Or you can search for a nova - one of the rare, elusive "ghost ships in the night" in the words of novafinder Kenneth Beckmann - near the Milky Way of the Triangle and points south. Twenty years ago this August a nova in Cygnus brightened to magnitude 1.8, rivaling Deneb.

Don't forget the two other, tiny constellations wedged into the Triangle: Sagitta, the Arrow, and Vulpecula, the Little Fox. The latter is home to the Dumbbell Nebula, the most prominent planetary nebula in the heavens.

What sights decorate the lower heavens at our Summer Triangle Hour?

Scorpius and bright Jupiter still shine low in the southwest. Arcturus is somewhat higher due west. Hercules and Draco's front half are very high in the north and west, bordering on zenithal Lyra. The Big Dipper, however, has sunk much lower in the northwest, and the feet of Ursa Major are already lost in the horizon glow.

The northeastern sky offers a rising Perseus, Cassiopeia above it, and much dimmer Cepheus still higher and a little left. From northeast to almost southeast a big arc of 2nd-magnitude stars is formed by the brightest lights of Andromeda, the Great Square of Pegasus, and the nose of Pegasus, Epsilon Pegasi, also known as Enif. Lower from east to south stretches a broad, dimmer strip of stars, mostly difficult to see in light-polluted skies, formed by Pisces, Aquarius, and Capricornus. Inside that strip burns Saturn.

DRACO CLOSE-UP, PART 3

We left off our tail-to-head survey of winding Draco, the Dragon, last month [TABULAR DATA OMITTED] with Zeta ([Zeta]) Draconis, also known as Nodus Primus. Just off the line from Zeta to Delta ([Delta]) Draconis is the north ecliptic pole, marked quite accurately by the object on the cover of our April issue: the 9th-magnitude planetary nebula NGC 6543, known as the Cat's Eye Nebula.

The line from Zeta to Delta cuts across a huge meander of Draco. If we return to Zeta in the Dragon's body, the next star working headward is actually a pair, [Psi.sup.1] and [Psi.sup.2] ([[Psi].sup.1] and [[Psi].sup.2]), known together as Dsiban. [Psi.sup.2] is a type-GO dwarf only a little more luminous than the Sun and about 60 light-years away. It shines at magnitude 5.4 in your binoculars about a degree from 4.6-magnitude [Psi.sup.1], which itself turns out to be a pretty binocular double. The primary is an F5 sun; north of it by 30[feet] is a 6th-magnitude companion.

Another few degrees along the body of Draco brings us to 4th-magnitude Chi and Phi ([Phi]). Just 2 [degrees] north of them is the galaxy NGC 6643. This many-armed spiral, 11th magnitude and 4[feet] by 2[feet], is beautiful in photographs. How large a telescope do you need to see good structure in it? This object was once known as "Tuttle's Variable Nebula"!

As we round the foremost coil of Draco we encounter the stars Epsilon ([Epsilon]) or Tyl, Sigma ([Sigma]) or Alsafi, and Delta, named Altais or Nodus Secundus. Sigma is an orange dwarf, magnitude 4.7, just 18 light-years away and only a third as luminous as the Sun. This is one of the very few naked-eye stars with less than solar luminosity.

And at last we come to the famous head of Draco. The 2nd-, 3rd-, 4th-, and 5th-magnitude stars that comprise this "Lozenge" are, respectively, Gamma ([Gamma]) or Eltanin, Beta ([Beta]) or Rastaban, Xi ([Xi]) or Grumium, and Nu (v) or Kuma. Eltanin is a K5 orange beauty that passes directly overhead as seen from England's Greenwich Observatory, a fact that earned it the name of the Zenith Star. Nu is a lovely binocular double composed of identical white stars 62[inches] apart. Marginally dimmer Mu ([Mu]) Draconis, Alrakis, which forms a tall triangle with Beta and Nu, is itself a pair of nearly identical F stars currently separated by only 1.9[inches].

THE SUN, MOON, AND PLANETS IN SEPTEMBER

Every planet is above the horizon at dusk by mid-September, but only two are easy to see. Jupiter is the brilliant light glowing steadily in the southwest. Saturn is low in the east at nightfall, getting a little higher each day.

Jupiter in the southwest is the first "star" to appear in the darkening sky. Watch it shining steadily 5 [degrees] or 6 [degrees] to the north (upper right) of orange Antares, which is only 1/16 as bright. This is an ideal time to check the old adage that stars twinkle while planets (usually) don't. Jupiter passes 5.1 [degrees] due north of Antares on September 19th, the last of the three conjunctions Jupiter has with it this year.

In binoculars you can watch Jupiter marching eastward against the stars to pass just 1.1 [degrees] south of 4.5-magnitude Psi ([Psi]) Ophiuchi on September 9th.

Jupiter's close conjunction with Omega Ophiuchi on the evening of September 24th is something very special. Telescope users in Ontario, Quebec, and the Midwest and Northeast will get a precarious sight, just after dusk, of the giant planet's edge grazing the 4.4-magnitude star. Those in the South can watch the star barely miss Jupiter's south pole. See the map in the February issue, page 73. This is one to mark on the calendar!

Saturn, in Aquarius near Pisces, rises around sunset. It reaches opposition on September 14th, meaning we see Saturn at its closest and biggest in this month when the sunlit face of its rings is tilting into view from edge-on - temporarily. In a telescope the rings should appear as a thickening line of radiance on either side of the golden globe.
THE MOON
(Times and dates are Universal Time)


First Quarter            Aug. 4, 3:16
Full Moon                Aug. 10, 18:15
Last Quarter             Aug. 18, 3:04
New Moon                 Aug. 26, 4:31
First Quarter            Sept. 2, 9:03
Full Moon                Sept. 9, 3:37
Last Quarter             Sept. 16, 21:09
New Moon                 Sept. 24, 16:55
Greatest and Least Distances


                Apsis              Distance         Diameter


Perigee    Aug. 8, [14.sup.h]     362,863 km    32[feet]56[inches]
Apogee     Aug. 20, [12.sup.h]    404,763 km    29[feet]31[inches]
Perigee    Sept. 5, [1.sup.h]     367,912 km    32[feet]29[inches]
Apogee     Sept. 17,[6.sup.h]     404,262 km    29[feet]34[inches]
Perigee    Sept. 30, [4.sup.h]    369,505 km    32[feet]20[inches]


The ball of Saturn is 19[inches] wide at the equator. We will not see it both so big and so nearly unblocked by the rings for four more decades. An observer's guide to Saturn's globe is in last month's issue, page 76.

The rings are still so thin in September that the brightest Saturn can get overall is magnitude +0.7. But on nights of good seeing, even small telescopes (properly collimated) will start revealing glimpses of the rings and perhaps condensations along them. Don't be fooled into thinking you've discovered a bunching of ring particles when the beautiful speck is really one of Saturn's satellites! A diagram showing where the satellites are all September is on page 68.

Mars, passing from Virgo into Libra, is now far less conspicuous than Jupiter or Saturn. At midtwilight during early September look for it low in the west-southwest just to the upper left of Spica. As September advances, Spica slides down into the Sun's afterglow but Mars races upstream against the westward current of the stars fast enough to remain in view well into the fall.

Of Mars's several conjunctions with stars in September, the best is its passage on September 29th less than 1 [degree] south of the wide, 3rd-magnitude binocular double Alpha Librae (Zubenelgenubi).

Mercury is visible with binoculars very low in the west at mid-dusk early in September. Mercury is at its greatest eastern elongation on the evening of September 8th, 27 [degrees] from the Sun, but is poorly placed for northern latitudes throughout this apparition.

Venus gradually begins to show itself just above the western sunset horizon by the end of September. Use binoculars to look for it barely off the horizon 15 or 20 minutes after the Sun goes down. If you succeed, you are seeing the very start of Venus's slow ascent to become the blazing "evening star" this winter.

Pluto, in Libra, is getting low in the southwest at nightfall, but a large telescope can still show it. Use the chart on page 71 of last April's issue.

Uranus and Neptune, in easternmost Sagittarius, are well placed in the south after nightfall. The chart on page 70 of the April issue will help you locate them with binoculars.

The Moon at first quarter is above Jupiter on the evening of September 1st. On the night of September 8th we get nearly the earliest full Harvest Moon possible. It shines well above Saturn. The Moon is at last quarter on September 16th. The waxing crescent struggles up slowly for several days after new Moon on the 24th, becoming noticeable very close to Mars on the 27th. On the evening of September 29th the Moon is above Jupiter and Antares.

The Sun crosses the celestial equator at 8:13 a.m. Eastern Daylight Time September 23rd, heading south. This equinox ushers in autumn in the Northern Hemisphere.

RELATED ARTICLE: Light-Pollution Notes: Lighting for Visibility

Last month in this space I discussed how to reply to those who think "more lights" is the answer to every outdoor lighting problem. Tell them what they actually want is not more lights but better visibility.

Explaining this distinction is a matter of common sense. Point out that we all know there can be too much of a good thing; if a good thing is overdone in the wrong place it becomes a bad thing. Light sent in unnecessary directions, or excessive amounts of it, can be distracting and even disabling. Maybe an increase in the total light will help us see better. But more often what's needed is better shielding or pointing so that you can better see people and terrain, not the dazzling surfaces of light bulbs.

Give examples. Ask someone how often they have been driving along a dark road and been blinded by dazzling floodlights shining directly into their eyes from a parking lot or car dealership. Such lights on a rain-streaked windshield can make the road totally invisible. That's a case where more light obviously means less visibility.

Lights should be shielded so that you see no bright bulbs, only lit ground. That's good visibility. It's also just what astronomers need to keep most light pollution out of the sky.

F. S.

Fred Schaaf welcomes mail at 681 Port Elizabeth-Cumberland Rd., Millville, NJ 08332.
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Title Annotation:includes related article; Summer Triangle
Author:Schaaf, Fred
Publication:Sky & Telescope
Date:Sep 1, 1995
Words:2246
Previous Article:Stairway to the stars.
Next Article:CH Cygni, a symbiotic variable.
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