Space & Astronomy

home *** CD-ROM | disk | FTP | other *** search

/ Space & Astronomy / Space and Astronomy (October 1993).iso / pc / text / spacedig / v10_3 / v10_307.txt < prev next >

Wrap

Internet Message Format | 1991-07-08 | 34KB

Return-path: <ota+space.mail-errors@andrew.cmu.edu> X-Andrew-Authenticated-as: 7997;andrew.cmu.edu;Ted Anderson Received: from beak.andrew.cmu.edu via trymail for +dist+/afs/andrew.cmu.edu/usr11/tm2b/space/space.dl@andrew.cmu.edu (->+dist+/afs/andrew.cmu.edu/usr11/tm2b/space/space.dl) (->ota+space.digests) ID </afs/andrew.cmu.edu/usr1/ota/Mailbox/kZSq2KG00VcJE8rU4a>; Tue, 5 Dec 89 01:30:46 -0500 (EST) Message-ID: <EZSq21600VcJA8pk5k@andrew.cmu.edu> Reply-To: space+@Andrew.CMU.EDU From: space-request+@Andrew.CMU.EDU To: space+@Andrew.CMU.EDU Date: Tue, 5 Dec 89 01:30:25 -0500 (EST) Subject: SPACE Digest V10 #307 SPACE Digest Volume 10 : Issue 307 Today's Topics: Electronic Journal of the ASA, Vol. I, No. V ---------------------------------------------------------------------- Date: 4 Dec 89 15:18:02 GMT From: cica!ctrsol!emory!eedsp!chara!don@tut.cis.ohio-state.edu (Donald J. Barry) Subject: Electronic Journal of the ASA, Vol. I, No. V THE ELECTRONIC JOURNAL OF THE ASTRONOMICAL SOCIETY OF THE ATLANTIC Volume 1, Number 5 - December 1989 ########################### TABLE OF CONTENTS ########################### * ASA Membership Information * How True is Our Astronomical Knowledge? The Case of the Planets - William G. Bagnuolo, Jr. * Orion: Winter's Mighty Hunter - Michael S. Wiggs * Observing the Wreaths of Winter - Don Barry ########################### ASA MEMBERSHIP INFORMATION The Electronic Journal of the Astronomical Society of the Atlantic is published monthly by the Astronomical Society of the Atlantic, Inc. The ASA is a non-profit organization dedicated to the advancement of amateur and professional astronomy and space exploration, and to the social and educational needs of its members. Membership application is open to all with an interest in astronomy and space exploration. Members receive the ASA Journal (hardcopy sent through U.S. Mail), the Astronomical League's REFLECTOR magazine, and may additionally purchase discount subscriptions to SKY & TELESCOPE, ASTRONOMY, DEEP SKY, and TELESCOPE MAKING magazines. For information on membership, contact the Society at: Astronomical Society of the Atlantic c/o Center for High Angular Resolution Astronomy Georgia State University Atlanta, GA 30303 U.S.A. asa%chara@gatech.edu or call the Society recording at (404) 264-0451 ASA Officers and Council - President - Don Barry Vice President - Bill Bagnuolo Secretary - Scott Mize Treasurer - Alan Fleming Board of Advisors - Bill Hartkopf, David Dundee, Anita Kern EJASA Editor - Larry Klaes Observatory Co-Chair - Michael Wiggs, Max Mirot Observing Coordinator - Eric Greene Georgia Star Party Chairman - Patti Provost Advertising Committee - Paul Pirillo Community Coordinator - Becky Long Regional Planetary Society Coordinator - Jim Bitsko Society Librarians - Julian Crusselle, Toni Douglas Telephone the Society Info Line at (404) 264-0451 for the latest ASA News and Events. ARTICLE SUBMISSIONS - Article submissions on astronomy and space exploration to the EJASA are most welcome. Please send your on-line articles to Larry Klaes, EJASA Editor, at the following net addresses: klaes@wrksys.dec.com, or ...!decwrl!wrksys.dec.com!klaes, or klaes%wrksys.dec@decwrl.dec.com, or klaes@wrksys.enet.dec.com, or klaes%wrksys.enet.dec.com@uunet.uu.net If you cannot send your articles to Larry, please submit them to Don Barry, ASA President, at the following net addresses: don%chara@gatech.edu, or chara!don@gatech.edu, or don@chara.UUCP You may also use the above net addresses for EJASA backissue requests and ASA membership information. Please be certain to include either a net or U.S. Mail address where you can be reached, a phone number, and a brief biographical sketch. DISCLAIMER - Submissions are welcome for consideration. Articles submitted, unless otherwise stated, become the property of the Astronomical Society of the Atlantic, and although they will not be used for profit, are subject to editing, abridgment, and other changes. Copying or reprinting of the EJASA, in part or in whole, is encouraged, provided clear attribution is made to the Astronomical Society of the Atlantic, the Electronic Journal, and the author(s). This Journal is Copyright (c) 1989 by the Astronomical Society of the Atlantic. HOW TRUE IS OUR ASTRONOMICAL KNOWLEDGE? THE CASE OF THE PLANETS by William G. Bagnuolo, Jr. As Will Rogers would say, it isn't what we don't know that's a problem, it's what we know for sure that ain't so. How "sure" is our current astronomical knowledge? Could firmly held beliefs by most astronomers be completely mistaken? Sometimes it is instructive to look at the past record. Patrick Moore's book, GUIDE TO THE PLANETS shows the state of "conventional wisdom" about our solar system in the year 1960. Now, nearly thirty years later, we can see how many of those "facts" turned out to be true. It should be kept in mind that whereas by the last year of the 1980s we had sent probes to all of the known planets - except Pluto - and two comets, in 1960 only a few Soviet probes had studied Earth's Moon and three probes had been placed in orbit around the Sun, two of which had lost power only days from Earth. Successful preliminary space missions to the other planets were two years in the future. Table 1 below lists some of the properties of the planets as known in 1960 and 1989. Some properties, notably the distances, have not "changed" significantly. Distances are known from Kepler's Third Law and the planets' periods. Other properties, such as detailed surface features, were completely unknown in 1960, so that no comparison can be made. The following is therefore mainly a summary of what we "knew" to be true in 1960 that "ain't so." Mercury - Books like Moore's had vivid descriptions of Mercury as both the hottest and coldest planet, because it turned one face perpetually toward the Sun, like the Moon towards Earth. None of this is true! In 1965 in was discovered that Mercury turned out to have a rotation period two-thirds as long as its revolution period, and thus each hemisphere is exposed to the Sun. It is not quite as hot as Venus or as cold as the three outermost planets - Uranus, Neptune, and Pluto. The surface features confidently mapped by 1960 were found to be mostly spurious, although a few feature names are adopted for modern maps made from 1974 and 1975 images of the planet by the United States MARINER 10 spacecraft. Because Mercury has no moon, its mass could only be measured by its perturbations on other planets in pre-MARINER days. Somewhat surprisingly, it was found that Mercury is fifty percent more massive than previously thought and has a large iron core. Venus - This cloud-shrouded planet looks as bland through the telescope as a white tennis ball; therefore, even though it is the closest planet to Earth - coming within 42 million kilometers (26 million miles) in its orbit around the Sun - little was known about it in 1960. The 1960 rotation period was completely wrong. The surface temperatures were grossly under-estimated. The high temperatures found on Venus are due to a "runaway Greenhouse Effect" under its very dense atmosphere (one hundred times Earth's surface pressure). On the other hand, the basic mass, size, and density of this planet were well known by 1960. The surface features that have been radar-mapped from Earth and the U.S. PIONEER Venus Orbiter (PIONEER 12) and the Soviet VENERA 15 and 16 spacecraft were of course unknown in 1960. Mars - Mars is the one planet whose surface features were fairly well known by 1960. The rotation period was consequently very well known. Unfortunately, it turned out that there is not much correspondence between these surface (albedo) features and the physical surface features. The largest surface relief features, such as the huge Valle Marineris canyon and the Olympus Mons volcano were unknown in 1960, although the latter was observed as a white spot and named Nix Olympia. It was also generally accepted in 1960 that the "wave of darkening" seen on Mars' surface with the melting of the polar icecaps was due to some primitive form of life, such as lichens. Unfortunately, the darkening is from winds blowing away lighter colored dust from a darker underlying and not due to life. That there are "canals" on Mars was not generally accepted by 1960, but it did turn out paradoxically to have ancient dry "riverbeds". Jupiter - The diameter, rotation period, mass, density, and diameter of the largest planet were well known by 1960. It was also accepted that the Great Red Spot was a hurricane-like atmospheric feature. The temperatures of the cloud tops was also about right. What is known currently about Jupiter thus comes mainly from addition, rather than subtraction (of false knowledge), through the PIONEER and VOYAGER probe missions of the 1970s. Some of the new facts about Jupiter are its thin ring, powerful magnetosphere, and an increase in the number of known moons from twelve to sixteen. The facts about the moons - each a world it itself - were new and almost completely unexpected, from the volcanoes of Io to the icy slush of Europa. Saturn - The diameter and mass of Saturn were known thirty years ago. The rotation period has "increased" from about 10.25 to 10.7 Earth hours; it was hard to discern spots on Saturn with a telescope. Titan is about ten percent smaller in diameter than previously thought, and has a dense atmosphere composed more of nitrogen than methane. The detailed structure of the rings and moons and the increase in the known moons from nine to seventeen are post-1960 contributions to our knowledge through PIONEER and VOYAGER. There were hints from telescopic observation of various gaps in the rings and other moons that are now known to exist. Uranus - The outer planets can barely be resolved through most telescopes, so that it is not surprising that not much was known about them in 1960. The mass and diameter of Uranus were both found to be slightly smaller than estimated then when VOYAGER 2 flew by the planet in January of 1986. The surface features glimpsed with telescopes turned out to be spurious, thus it is not surprising that the rotation period was inaccurately known in 1960. The experience with Mercury and Uranus teaches us to be conservative about what we can actually see. For example, the spiral arms of the galaxy Messier 51 (M51) can be "seen" with a 30-centimeter (12-inch) telescope, but they were first recognized only with the Earl of Rosse's 180-centimeter (72-inch) reflector. The largest diameter of Uranus' moons is about thirty-five percent smaller than thought in 1960. On the other hand, we now know three times as many moons! The dark rings of Uranus, discovered in 1977 from Earth, were unexpected as well. Neptune - The biggest increment of knowledge about this planet will come when all of the recent VOYAGER 2 data is digested. The rotation period of the planet was surprisingly well known in 1960 (a lucky guess or excellent spectroscopic radial velocities?). The diameter of Neptune's largest moon, Triton, is not much more than half of what we thought we knew. Unfortunately, the moon also turns out to be too cold for the methane seas that some thought could have evolved a different form of life. The diameter of the planet itself is about ten percent less than that estimated in 1960 - not bad considering that you can barely resolve the planet's two-arcsecond disk in most telescopes. As might be expected from the discussion of the other gas giants, Neptune has a ring system and six more moons. Pluto - This planet has been gradually "disappearing" since 1960: Its currently estimated diameter is only about forty-one percent of the 1960 value. A popular theory then was that Pluto was really a much larger planet - we were only seeing a specular reflection from its surface, like from a ball bearing. This would explain how Percival Lowell was able to predict the planet's location from its perturbations on Uranus and Neptune's solar orbits. Pluto is now estimated to have only about 0.2 percent the mass of Earth, and its discovery near Lowell's position in 1930 was a lucky guess. The discovery of Pluto's moon, Charon, was done in 1978 with photography at the telescope and could have been done in 1960. Conclusions - 1 - Observations near the limits, e.g., looking at surface features of Mercury, are often completely wrong. The diameters of several moons in the outer solar system, such as Uranus' Titania and Neptune's Triton, were greatly over-estimated. 2 - Data based on fundamental physical laws, such as the distances to the planets, is very accurate and will not be revised much. 3 - Wishful thinking can influence results: The life-on-Mars theory of the "wave of darkening" and the picture of Venus as a warmer Earth are examples. 4 - Some discoveries could have been made with 1960 techniques much earlier than they were, such as the discovery of Pluto's moon Charon. Also, at least one of Neptune's newly discovered moons, tentatively designated 1989 N1, is larger than the previously known moon Nereid, discovered in 1949. 5 - Expect the unexpected, particularly on a planet's moons! Some worlds like Jupiter's Io or Uranus' Miranda were unimaginable in 1960. TABLE 1 - Planetary Data, Then and Now PARAMETERS 1960 1989 Mercury Diameter 4,985 km 4,878 km Rot. Period 88 d 58.6 d Mass/Me 0.04 0.06 Density 3.8 5.4 Temp (max) 410 C 427 C Moons 0 0 Venus Diameter 12,383 km 12,104 km Rot. Period 30? d 243.0 d Mass/Me 0.83 0.82 Density 5.2 5.2 Temp (max) 60 C 450 C Moons 0 0 Atmosphere mostly CO2 97% CO2 (very dense) Mars Diameter 6,755 km 6,796 km Rot. Period 24.62 hr 24.62 hr Mass/Me 0.11 0.11 Density 4.0 3.9 Temp (max) 29 C 20 C Moons 2 2 Largest Moon Diam (Phobos) 16 km 24 km Atmosphere mostly CO2 95% CO2 Jupiter Diameter 142,650 km 142,796 km Rot. Period 9.85 hr 9.9 hr Mass/Me 318 317.89 Density 1.3 1.3 Temp (max) 144 K 200 K Moons 12 16 Largest Moon Diam (Ganymede) 5,178 km 5,262 km Saturn Diameter 120,778 km 120,660 km Rot. Period 10.25 hr 10.7 hr Mass/Me 95 95.15 Density 0.7 0.7 Temp (max) 122 K 143 K Moons 9 17 Largest Moon Diam (Titan) 5,630 km 5,150 km Uranus Diameter 51,460 km 50,800 km Rot. Period 10.8 hr 17.3 hr Mass/Me 15 14.54 Temp (max) 83 K 63 K Moons 5 15 Largest Moon Diam (Titania) 2,410 km 1,590 km Neptune Diameter 44,390 km 48,600 km Rot. Period 15.67 hr 16.05 hr Mass/Me 17 17.23 Density 2.2 1.7 Temp (max) 55 K 55 K Moons 2 8 Largest Moon Diam (Triton) 5,300 km 2,900 km Pluto Diameter 5,790 km 2,400 km Rot. Period 6.4 d 6.4 d Mass/Me ? 0.2 Density ? 0.9 Temp (max) 33 K 35 K Moons 0 1 Largest Moon Diam (Charon) none 1,200 km About the Author - Bill Bagnuolo, Ph.D. Astronomy 1976, Caltech, is the Society's Vice President and Program Chairman. An active amateur, Bill learned the craft of telescope mirror-making at Chicago's Adler Planetarium, with his chief accomplishment the 1/20 wave 31.25-centimeter (12.5-inch) mirror whose telescope is featured at most ASA observing events. Bill's professional interests include interferometry, galactic evolution, spectroscopy, and theoretical-optical techniques. ORION: WINTER'S MIGHTY HUNTER by Michael S. Wiggs The mighty Orion has, in virtually all ancient cultures, been associated with national heroes, demi-gods, or renowned warriors. The name Orion probably comes from the Greek word for warrior. According to one account, Orion was known to the Greeks as an immensely skilled hunter and was also a son of Neptune, god of the seas. He came to love Diana, sister of Apollo. Our hero, Orion, was mistakenly killed by Diana who was provoked by her overprotective brother. The gods honored the great hunter by placing him in the sky. And they did a magnificent job, for Orion boasts many very bright stars and is thus one of the most easily recognizable of the Northern Hemisphere constellations. This celestial marvel is easily seen immediately to the southeast at about 9 o'clock in early December, 8 o'clock later that month, and about 7 o'clock in early January (This is because the stars rise four minutes earlier each day). Going outside in the winter and looking directly southeast, the first thing you may notice about fifty degrees above the horizon are three stars close to each other arranged in a line. This is the belt of Orion. It is interesting to note that the belt happens to lie roughly on the celestial equator (The celestial is simply Earth's equator projected onto the sky). From the belt, sometimes known as the Three Kings or the Three Marys, one can easily find the other bright stars defining Orion's shape. Slightly north (up) and east (left) of the belt one finds the brilliant Betelgeuse, a red supergiant star which is about six hundred times larger than our Sun. If Betelgeuse were placed where our Sun is in the solar system, Earth's orbit would actually be inside the star! Betelgeuse's red color is quite evident to the naked eye. A bit to the west (right) of Betelgeuse is Bellatrix, which marks the left shoulder of Orion, whereas Betelgeuse marks the right. Now looking slightly south and west of the belt, one can see the left leg of the Hunter, the star Rigel. Rigel is a blue supergiant star which is a little brighter than Betelgeuse. Rigel is a brilliant white star about nine hundred light years from Earth and shines about fifty thousand times brighter than our Sun. Next we move a bit to the east of Rigel to find Saiph, the right knee of Orion. Saiph is about the same brightness as Bellatrix, and is another of Orion's array of blue giant stars. Our naked eye tour of Orion is not complete without mentioning the sword. Midway between the belt and a line connecting Rigel and Saiph is a small line of three stars known as Orion's sword, which looks as if it is hanging from Orion's belt. The central star of the three in the sword is not a star at all, but Messier 42 (M42), the famous Orion Nebula. Called a "smoking star" by American Indians, this nebula is a vast complex of gas and dust where stars are being born (We will return to the Orion Nebula shortly). As for Orion's head, look just a little above a line joining Betelgeuse and Bellatrix and you will see a little triangle of stars similar in brightness to the ones in the sword. One can see that Orion's head is his least impressive feature. Now that you are familiar with the naked eye splendors of Orion, the time has come to grab those binoculars or a small telescope to observe one of the most famous "deep-sky" objects in the heavens: M42, the Orion Nebula. First observed telescopically in 1611 by Nicholas Peiresc, the Orion Nebula has captivated all who have seen it ever since. Binoculars will reveal the central "star" in Orion's sword to be an extended, greenish glow with many faint stars peppered around it. The view in binoculars will be quite similar to what Peiresc first saw in the early Seventeenth Century! In any event, the Orion Nebula is a fascinating sight with any type of astronomical instrument. Observers with medium to large telescopes would do well to try and find NGC 2024 just east of and in the same field as the star Zeta, in Orion's belt. Also try for M78 above and east of the belt. Both of these are gaseous nebulae, similar to M42, but fainter. If double stars are your game, then take a peek at Rigel, which is a challenge due to the brightness difference between the two stars. Lambda Orionis is a fine double for medium telescopes, with both stars appearing white. Our friend Orion also holds other deep-sky objects of interest. NGC 2194, near the star Xi, in the northeast corner of the constellation, is readily visible in 20-centimeter (8-inch) telescopes or larger. The same holds true for another open cluster near Xi, NGC 2169. The brighter stars in this region, incidentally, represent Orion's right arm wielding a club at the "charging" constellation of Taurus the Bull. An easy open star cluster for the small telescope is NGC 1981, located just north of M42, the Orion Nebula. This is a small, scattered group of about ten stars. Do you like to observe variable stars? If so, you might be interested in watching U Orionis' activity early in December. U Orionis is a pulsating red giant star of the well-known Mira class of variables. Varying by about seven magnitudes over a mean period of 372 days, U Orionis is now approaching maximum light, (i.e., when it appears brightest in the sky). Predicted to reach its maximum of about magnitude 6 on December 7, one can check this star daily by comparing its brightness with nearby non-variables, and watch it rapidly rise to maximum and then slowly fade after the seventh of the month. U Orionis is easy to find: It lies near the star Chi-1 at the tip of Orion's club. It will be easily recognized due to its deep red color. See the November, 1989 issue of SKY & TELESCOPE magazine for more details on U Orionis. Another object worth mentioning is the well known Horsehead Nebula, located very near the star Zeta, in the belt. This nebula is seen in photographs in nearly all introductory astronomy textbooks, and from time to time in basic science and astronomy magazines. Unfortunately for amateurs, this object is virtually impossible to detect visually, requiring only the very darkest skies, large telescopes, and low power eyepieces. This concludes our tour of Orion, the Great Hunter in the sky. Now that you have a basic idea as to what he looks like in your mind, go out and see the real thing! Orion makes for a perfect way to begin learning about the other constellations and celestial objects of the winter skies. References and Suggested Readings - Allen, Richard H., STAR NAMES: THEIR LORE AND MEANING, Dover Books Burnham, Robert Jr., BURNHAM'S CELESTIAL HANDBOOK (3 volumes), Dover Books, 1978 Menzel, Donald H., and Jay M. Pasachoff, FIELD GUIDE TO THE STARS AND PLANETS, Houghton Mifflin Company, 1983 About the Author - Michael Wiggs, co-chair of the ASA Observatory Committee, is pursuing advanced study in astronomy at Georgia State University. An active amateur, Mike is interested in telescope construction and observing. His professional interests include interacting hot binary stars, stellar formation, and cosmology. OBSERVING THE WREATHS OF WINTER by Don Barry During the crystal nights of autumn, observing becomes almost a habit for the serious amateur. Evening after evening, the skies open up with pristine transparency, and the white murk of summer is long gone. Then December arrives, and soon gossamer cirrus clouds make the sky their loom, heralding the onrush of slate-gray winter skies. At times like these, even the hardiest amateur astronomer can wish himself a meteorologist. But in denying views of the ultimate heavens, the petulant atmosphere occasionally puts on a show of its own - and what a wonderful consolation prize it can be. Most people have seen the Sun ringed by a circle of light 22 degrees away. Many have seen the same around the Moon. "Sun dogs", bright spots to the left and right of the Sun along the 22-degree ring, are common enough to earn a nickname. Rarer, but not as rare as one might think, however, are great ice crystal displays, where the sky may be filled with bows, rings, and haloes, shifting as the Sun and clouds change. These unique displays are among the most awe- inspiring sights of nature, and also among the easiest to observe. Perhaps most important, though, is that the amateur can make a real contribution to meteorology by recording or photographing these rare displays, as much remains to be discovered about them. As mentioned above, the 22-degree ring is the most common feature observable around the Sun. It is produced when atmospheric conditions are just right for the formation of crystals with a long, hexagonal, pencil-like shape. As these crystals fall in quiet, non-turbulent air, they orient themselves horizontally, and light reflecting and refracting through them gets preferentially thrown out at just the right angle to make the ring. The "Sun-dogs", or parhelia that often accompany the ring, are manifestations of the same crystal type, but require the light to take a different path enroute. Due to the geometry involved, they are only visible when the Sun is lower than about 45 degrees above the horizon. But what a sight they make when they become very bright, and form a faint rainbow tint from the prismatic effect of refraction at the surface of the crystals! Under just the right conditions, the parhelia become only a starting point for a line of light that extends out horizontally from the Sun ring, and which can encircle the entire horizon. This wreath, called the parhelic circle, also forms reflections inside ice crystals, but requires more perfect ice to allow the light to take a more complicated path when bouncing inside the crystal. Like the parhelia, it can only form when the Sun is below 45 degrees from the horizon. Sometimes, a brighter spot on it will form opposite the Sun, at the same altitude, forming the so-called anthelic spot. This may even be complete with its own anthelic parhelia, 22 degrees away on the parhelic circle. The Sun may appear suspended on a pillar of fire, or it may be capped by a spire. These phenomena, caused by flat crystals descending like Frisbees, result from light reflecting squarely off the tops or bottoms of their surface like little mirrors between the Sun and the observer. At the top of the 22-degree ring, other arcs may appear, including the common upper contact arc, or the rare Parry arc. Depending on the altitude of the Sun, either may appear as an upward-facing parabola or as a gull's-wings touching the tip of the ring. Surrounding this may be the rare 46-degree ring, again circling the Sun. Higher still, and exceptionally rare, are the circumzenithal ring, and descending from it toward the anthelic spot, Wegener's anthelic arc. Only a few great displays are recorded in history which show most of these spectacular phenomena. Most are named for cities or the scientists who recorded them; but I suspect they are more common than one might believe. On December 20, 1986, I watched a magnificent display in Atlanta which lasted for hours, constantly changing and bringing in new arcs as others faded away. Most of the phenomena above were represented at some point, as well as some which may not have been described before. Yet no mention of it was made by the local news media. The 22-degree ring or parhelia are probably visible to some extent at some time on half of the days that show high cirrus clouds. Partial horizon arcs and upper contact arcs may appear once or twice a month during winter. Whenever the latter start forming, though, there is always the chance that the sky may be working up to something big. It only takes twenty minutes or so for a simple ring to be transformed into a glorious vault of arches, rings, and bows. What should one do if a big display suddenly unfolds overhead? Probably a kind thing, although somewhat altruistic, is to miss a few minutes while calling others out to share it. Secondly, though, one should grab a sketch pad or a camera, in order to make some record of what is going on. Because sky photographs usually turn out to be poor, and because most sky haloes require special wide-angle lenses to properly photograph, the amateur's best tools are a sketch pad and pencil, with which to make the so-called all-sky diagram. This is simply a circle, representing the horizon, in which a sketch is prepared showing the presence of any arcs, spots, or unusual glows in the sky above. The center of the circle would represent the zenith, of course, and the Sun may either be drawn in, or more traditionally, left out because its presence would be located anyway by the ubiquitous 22-degree ring, which is instantly recognizable in the all-sky diagram. These sketches are valuable to scientists who study displays, because they may confirm the existence of suspected types of arcs, or even lead to the discovery of new ones. The Parry and Wegener arcs, mentioned previously, are named for the Twentieth Century scientists who first described them. Other arcs, such as the claimed "90-degree ring", or "horizontal tangent ring", have never been confirmed. Perhaps the most comprehensive work on ice-crystal phenomena, as well as being the best introduction to it, is Robert Greenler's RAINBOWS, HALOES, AND GLORIES, available through most bookstores. It also includes many color photographs of these apparitions, most of them recorded from the undisputed capitol of these displays - the South Pole, Antarctica. But one doesn't need to venture there to catch a view. Next time the sky turns pearly, take a peek out. And if you're lucky, you'll be in for the view of a lifetime. About the Author - Don Barry, ASA President, is a researcher and Ph.D. candidate at the Center for High Angular Resolution Astronomy, Georgia State University. An active amateur and professional astronomer, his amateur interests include telescope making, antique instruments, and amateur-professional collaborative projects, while his professional interests include optical interferometry, binary astrometry, and innovative instrumentation. THE ELECTRONIC JOURNAL OF THE ASTRONOMICAL SOCIETY OF THE ATLANTIC December 1989 - Vol. 1, No. 5 Copyright (c) 1989 - ASA -- Donald J. Barry (404) 651-2932 | don%chara@gatech.edu Center for High Angular Resolution Astronomy | President, Astronomical Georgia State University, Atlanta, GA 30303 | Society of the Atlantic ------------------------------ End of SPACE Digest V10 #307 *******************