Unless otherwise noted, information contained in each edition of the Kansas School Naturalist reflects the knowledge of the subject as of the original date of publication.
Volume 32, Number 1 -
The Return of Comet Halley
ABOUT THIS ISSUE
Published by EMPORIA STATE UNIVERSITY
Prepared and Issued by THE DEPARTMENT OF BIOLOGICAL SCIENCES
Editor: Robert F. Clarke
Editorial Committee: Tom Eddy, Gilbert A. Leisman, Gaylen Neufeld, John Parrish
Online edition by: Terri Weast
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The Kansas School Naturalist is published in October, December, February, and April of each year by Emporia State University, 1200 Commercial, Emporia, Kansas 66801-5087. Second-class postage paid at Emporia, Kansas.
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ABOUT THE AUTHOR
DeWayne Backhus is Associate Professor of Earth Science at Emporia State University. In addition to teaching earth science and astronomy courses, he supervises and holds public lectures in the Peterson Planetarium, located in Cram Hall on the ESU campus.
The Return of Comet Halley
by DeWayne Backhus
The Comet cometh. The period of time from late in the year 1985 to mid-1986 will be remembered by many as their turn to observe one of the most famous of comets - Comet Halley. Because it orbits the sun once every 75 to 76 years, its passage near the sun provides us with a once-in-a-lifetime opportunity. That only opportunity for most of us is here.
In this issue of the Naturalist, a number of topics will be considered: (1) the nature of comets in general; (2) Edmund Halley's contributions to astronomy and historical circumstances associated with some of the return visits of the comet named after Edmund Halley, who, did not discover it; (3) and observational aspects unique to this particular visit of the Comet.
The attention of the reader is alerted in advance to terms or ideas which may be unfamiliar. These items will be "footnoted" with numbers and discussed in a section of "End Notes" near the end of this issue.
COMETS . . . IN GENERAL
To casual observers, which includes most persons, events involving the sky seem regular, constant, and predictable. The sun seems to rise and set. And so do the moon, planets, and most stars. More careful observations reveal that the moon shifts its completely with respect to the star background in 27.3 days, and that the moon cycles through a complete set of phases in 29.5 days. The sun in a year's time can also be observed to move completely through the star background. But with the pace of contemporary times, most have become unconscious of these subtle celestial rhythms. Our response to these daily, monthly, and annual phenomena is often "so what!"
Just as was the circumstance, I think, with ancient man, we are occasionally aroused by the more spectacular of celestial happenings, particularly the rare ones that occur. And our once-in-a-lifetime opportunity for one of those rare events is imminent.
A comet when first observed by the ancients must have evoked fear. It was an object which did not follow the basic (monotonous?) patterns in the sky. It could be observed to become brighter on successive nights (as it rose and set with other celestial objects due to the rotation of the Earth). And then it may have been observed to develop a "wispy," or "fuzzy" extension - a tail. Appropriately, the object became known to those using the Latin language as a "comet," which means "hairy star." In a few days or weeks, the tail would grow in length - and it always would point away from the sun. Because the comet is orbiting the sun, it would have disappeared from view for a few days. But its orbit would then let it be seen again after it passed by the sun. The story would conclude with the object dimming in brightness and its tail fading from sight. This object presented a serious challenge to a fundamental belief: that the sky is perfect and unchanging ("immutable"). Ah! Maybe this phenomenon of a comet is not occurring in the sky - in the heavens. Perhaps this phenomenon is occurring in Earth's atmosphere; after all, Earth is properly considered a place of change! Such was the prevalent thinking up to a couple of hundred years ago.
So it was that the observation of comets became associated with fear and superstition. See Table 1 for events associated with past returns of Comet Halley. To some the comet with its associated tail had the appearance of a "sword." Comets thus became associated with undesirable, or unhappy, events - famines, floods, earthquakes, and wars - never with happy events.
Much of what we currently know about comets is credited to the thinking of Harvard astronomer Fred Whipple. Whipple's model of a comet is best thought of as a mass of frozen material with bits of dust and chunks of rocky or rocky-metallic material frozen into the mass. An analysis of the frozen material indicates the presence of frozen water (ice), carbon dioxide, methane, ammonia, and more complex molecules such as cyanogen.(1) We can expect the dust to be small particles of "heavy," nongaseous elements or substances like those commonly found on Earth. Organic (carbon-based) molecules have been identified as part of the dust; hence, we can think of part of the material as being "sooty" dirt. Furthermore, the chunks of larger debris would match the composition of stony, metallic, or stony-metallic meteorites(2) found on Earth. So we can visualize a comet as a "dirty snowball" or "iceberg" in space.
While a comet is approaching the sun and still at a distance greater than 2-3 AUs(3) it will be a mass of ice and dust about 4 miles (6.4 kilometers) in diameter. This may sound large and massive, but this is fairly flimsy by celestial standards. (A comet is "the closest anything can be to nothing and still be something," some have stated.) This approximately 4-mile diameter chunk is commonly referred to as the "nucleus," or the head, of the comet. It is solid - composed of the frozen gases (the ice) with the dust and chunky material locked inside. As a comet travels in its orbit toward the sun, and when it is approximately 2-3 AU from the sun, incident solar energy will cause the frozen gases to begin to vaporize (or sublime, as solids are being coverted to gases). At that time a shroud of gases called the "coma" will surround the nucleus. The comet will appear larger in size because sunlight will be reflected from the coma, and the comet will appear to get brighter.
PAST RETURNS OF COMET HALLEY
|DATE of PERIHELION||ASSOCIATED HISTORIC EVENTS and/or COMMENTS|
|239||May 11||First reliable sighting recorded by the Chinese. Often referred to as 240 B.C. sighting.|
|163||Nov 13||Not recorded.|
|86||Aug 6||Observed by and inspiration for Julius Caesar.|
|66||Jan 26||Described by historian Josephus as "a star that appeared like a sword."|
|451||June 28||Thought to have signaled the defeat of Attila the Hun by the Romans.|
|530||Sep 27||A plague sweeps Europe.|
|1066||Mar 21||Visible in spring sky before Normans invaded and conquered England. Inspiration for Bayeux Tapestry.|
|1222||Sep 29||Regarded as omen to Genghis Khan to conquer the world.|
|1301||Oct 26||Impressed the artist Giotto di Bondone sufficiently to produce a fresco at Padua titled "Adoration of the Magi." Thus, the space probe Giotto!|
|1531||Aug 26||Studied by Western Medieval astronomer Petros Apianus, purported to be the first to observe that a comet's tail always points away from the sun.|
|1607||Oct 28||Observed by Johannes Kepler, who later deduced three laws of planetary motions.|
|1682||Sep 15||Observed by Edmund Halley. He speculated about its permanence in the solar system. If it is a permanent member of the solar system, had it visited us before?|
|1759||Mar 13||Commonly referred to as the comet of 1758. The comet returned just as Halley had predicted. Halley was dead, but the Comet was named in his honor.|
|1835||Nov 16||The year of Mark Twain's birth; Twain "predicted" he would leave when Halley's Comet reappeared.|
|1910||Apr 20||Mark Twain died April 21, 1910. For the superstitious: Paris flooded; Mt. Etna erupted; King Edward VII died.|
|1986||Feb 9||Actually spotted October, 1982. Largest coordinated effort to study any celestial object.|
|2061||c||???? Next anticipated return.|
a. Perihelion data are from materials compiled by / for Sky Publishing Corporation, Cambridge, MA. Historic notes and comments were gleaned from a variety of sources.
b. Dates for 1531 and earlier are based on the Julian Calendar; subsequent dates are Gregorian.
c. Since the Comet's orbit may be affected gravitationally by other celestial objects, the perihelion date will be unknown until calculations are made based on first sightings of that return.
The "solar wind"(4) and an associated radiation pressure from the sun will cause the gases and dust being released at the comet's solid surface to be forced in a direction away from the sun. We observe that fact as the formation of a comet's tail. Often a comet's tail has two components: a gas (or ion) tail and a dust tail. The gas tail, visually, will be radially away from the sun's direction; the dust tail will often be curved slightly away from the gaseous component and in a direction from which the comet is coming. (See Figure 1). We must remember that a comet is a very "flimsy" object when compared to other members of the solar system. Isaac Asimov, a popular science writer, has suggested that all of the dust in a developed comet's tail could be fitted into an ordinary suitcase! Even though a tail is noticeable on a comet, the density is so small that the tail region would represent a good vacuum by terrestrial standards.
FIGURE 1. The main features of a comet: the head (nucleus and coma), and the tail(s) (dust and gases or ions).
How do these flimsy objects fit into the "big picture" of the solar system? Some observations provide clues. The orbits of "periodic"(5) comets are very elongated. Their closest points of approach to the sun (called perihelion points) are very small, often less than 1AU of distance. But the greatest distance of a comet from the sun (called the aphelion distance) is very great. For Comet Halley the perihelion distance on this visit will be about 0.6 AU (0.59 AU), but at its aphelion it is nearly 40 AU from the sun. See Figure 2. Even though Comet Halley lies within the orbit of Pluto, most comets appear to have orbits that place them well beyond that of Pluto.
FIGURE 2. A schematic drawing of Halley's elliptical orbit. The semimajor axis (OB or OA), which is the distance from the center of the orbit (O) to the end of the major axis (AB), is 18 astronomical units (AUs). (The semimajor axis is also the comet's average distance from the sun.) The perihelion distance for this return (apparition) of Comet Halley will be 0.59 AU. The sun is said to be located at one "focus" of the elliptical orbit.
Also, comets exhibit orbits that are inclined greatly to the orbital plane of planet Earth around the sun. In contrast, planets orbit the sun in nearly the same plane - in a disk-shaped region of space. This means that the plane of the planets in the solar system is surrounded by an orbiting "swarm" of cometary objects. This swarm, or spherical cloud, is called the Oort Cloud (after work done by the Dutch astronomer, Jan Oort). The Oort Cloud can be visualized as a "deep freeze" of space in which comets have their primary residence. See Figure 3.
These comets which reside in the Oort Cloud can be considered to have formed at the time of other solar system objects, including the sun, about 5.0 billion years ago. Gravitational forces would cause gases and dust of a nebula(6) to come together (accrete or aggregate). Most of the material in the nebula from which the solar system formed accreted to form the sun. Less massive chunks accreted to form the planets which were gravitationally controlled by the more massive sun. Some less massive objects were undoubtedly perturbed gravitattionally) into random orbits by the more massive planets, but were not lost into space beyond the gravitational influence of the sun. The result is the sun with the planets in a rather restricted plane of space, and a larger, spherical "swarm" of comets in the so-called Oort Cloud.
FIGURE 3. The realm of the planets, the "PLANE OF PLANET ORBITS," is shown schematically relative the "OORT CLOUD OF COMETS." Sizes are not to scale. The planets vary in average distance from the sun from 0.4 AU (Mercury) to about 40 AU (Pluto). The Oort Cloud is considered to have a radius up to 50,000 AU. Some periodic comets, such as Halley, are in orbits around the sun near the realm of the planet.
These "deep freeze" objects, the comets, are considered potentially important clues to the history of the solar system. They are in an environment of space that has preserved their qualities since the time of their formation. That is why space probes are being readied to rendezvous with this return visit of Comet Halley.
Occasionally, comets that reside in the distant Oort Cloud (up to 50,000 AU from the sun) are gravitationally coaxed out of the "deep freeze." The coaxing may be done by a star, for example. One such comet was coaxed out and has been orbiting the sun since at least 240 B.C. It is the comet that we call Comet Halley.
EDMUND HALLEY AND COMET HALLEY
Edmund Halley - it's pronounced to rhyme with "valley" - did not discover the comet which we name in his honor. His contribution to astronomy is much greater than the discovery of a "new" comet.
Edmund Halley was born in 1656 near London, England. His life of 86 years was during an exciting period of intellectual history. He was a contemporary of the great humanitarian and architect Christopher Wren; he met with Isaac Newton and others about problems related to science. He died in 1742, the year of the first public presentation of Handel's "The Messiah." This was the rich context in which he lived.
The famous observatory, the Royal Greenwich Observatory, was established just outside of London on a hill near the Thames River in 1676. Halley served as Director of the Royal Observatory and Second Astronomer from 1720 to 1742.
But how did Halley become involved with and associated with a particular comet? Several events seem to provide the link.
Halley had been meeting regularly with others - Christopher Wren and physicist Robert Hooke, in particular. They would apparently discuss the works of other scientists or "natural philosophers." The works of Brahe (1546-1601, a Danish astronomer) and Johannes Kepler (1571-1630, an European mathematician) could not have escaped their attention. Highlights of their work are instructive.
The eccentric Tycho Brahe had established an exceptional "naked-eye" observatory (the telescope was yet to be invented by Galileo in about 1610). The Observatory, Uraniborg, was on an island near Denmark. The Observatory, actually a self-contained community, was used Brahe and his associates from 1576 to 1597. His observations allowed new courses to be charted in astronomy.
In 1577 Tycho had the opportunity to observe an fine comet (not Comet Halley). With his instruments for measuring positions of objects in the sky, Tycho successfully demonstrated that the comet was at a distance far beyond Earth's atmosophere. In fact, the comet was in the realm of the planets (the "heavens"). This challenged the prevailing assumptions of the Greeks that the "heavens" were and unchanging (immutable). Aside from his observations afforded by the appearance of the comet, Tycho and his associates accurately (meticulously) recorded positions of the planet Mars. These data on Mars were obtained by Johannes Kepler just prior to Tycho's death in 1601. Kepler now had the best data ever made available for the study of planetary motion. But more important, he had the desire and capability to derive important conclusions from the data.
The Mars data were to become for Kepler an important key for unlocking "secrets" of planetary motion (or the motion of any object orbiting another object.) After about four years of study, Kepler was able to show that planets (or comets) orbit the sun in an elliptical orbit, with the sun located at one focus of the ellipse. See Figure 2. (This realization overturned early Greek assumptions about circular - the "most perfect" of shapes of celestial motions.)
Following on that discovery was a law concerning rates of planetary motion. The Law of Areas showed that planets move fastest at perihelion and slowest at aphelion.
Kepler then concluded his study of the Mars data after working out a third law two decades later. Called the "Harmonic Law," it is a relationship between planetary (or cometary) distances from the sun and the time (the "period") for one revolution around the sun. This was all accomplished by about 1620. Furthermore, during this time Galileo Galilei had arranged lenses to form a refracting telescope, and was also performing experiments with moving objects. All of these studies were profound, and Halley was reading about them and discussing them with others.
During conversations of Halley with Wren, Hook, and others, items of confusion would arise. In the year 1684, Halley, Wren, and Hooke went to visit with Isaac Newton, a professor, at nearby Cambridge University in England. In particular, they discussed the laws of Kepler. On one occasion Newton discussed a force controlling the motions of planets and other orbiting celestial objects. He spoke of an "inverse-square force," and suggested that Kepler's third law was a logical consequence of such a force. The three visitors were perplexed; they encouraged him to write his thoughts. Newton agreed. In 1687 Newton published Philosophia Naturalis Principia Mathematica, (or Principia in which he established, among other things, the concept of gravitation and the basic laws describing gravitational interactions. Even though Halley and his cohorts may have been confused, Halley persisted with some additional thinking.
Edmund Halley had had an opportunity to observe a fine display of a comet in 1682 and contemplated whether the object might be a permanent member of the solar system. After all, the work of Tycho Brahe showed that comets were not Earthly, atmospheric phenomena. If the comet was a member of the solar system, gravitationally controlled by the sun, then maybe the comet of 1682 had been observed on previous occasions. After checking records of comet observations in 1456, 1531, and 1607, he suggested that the comet of 1682 was the same as the one which had been observed on those previous occasions. Implicit was the notion that the comet was being observed at intervals of 75 to 76 years. (Recall Kepler's third law!) Finally, Edmund Halley predicted that the comet would return in 1758. It did. But Edmund Halley had died in 1742. In recognition of the insight which he provided concerning theory and observation, that comet which returns for a visit to the inner solar system every 75 to 76 years is called Halley's, or Comet Halley.(7)
COMET HALLEY 1985/86: WHEN AND WHERE DO I LOOK?
After five years of unsuccessful attempts to find the returning Comet, it was finally sighted on October 16, 1982. It was about a billion miles away from the sun, beyond the orbital distance of the planet Saturn from the sun. The chunk of frozen gas and dust was a very faint object - equivalent in brightness to an ordinary candle viewed at a distance of 27,000 miles! To record such a faint object required the 200-inch (diameter) telescope at Mount Palomar (California) using an electronic camera to process the faint light. But the object was successfully photographed to assure us of its 75-76 year unmistakable return. And so the drama associated with sighting Comet Halley had begun.
Do not expect to see something go "flashing" by like a meteor (or so called "shooting" or "falling" star). That will not happen. If one observes with the naked-eye, a person should have three chances to see the comet: (1) in the evening sky in late December and January; (2) in the morning sky before sunrise between March and early April; and (3) again in the evening sky in mid to late April. Let us consider the situation in more detail.
FIGURE 4. The relative positions of the planes of orbit of Comet Halley and planet Earth. The line N1 to N2 (called the line of nodes) is the line of intersection of the two orbital planes. The angle of intersection of the two planes is 18° (or 162° when specified as the "inclination," i, since the comet is moving in a clockwise fashion in its orbit). The distance E1 to C1 is 0.67 AU (the pre-perihelion close approach); this will occur November 27, 1985. Positions E2 and C2 represent relative positions for January 1, 1986. The perihelion position for the Comet, C3 will occur February 9 when the comet is 0.59 AU from the sun. The E4 and C4 positions represent post-perihelion close approach when the two will be 0.42 AU apart on April 11, 1986. "VE" represents the "vernal equinox," the sun's projected position as seen from Earth on the first day of spring.
Why do we have these three observing opportunities? The answer is provided by the sketches of Earth's orbit and Comet Halley's orbit as shown in Figure 4. Note that the Comet moves from orbital positions C1 to C3. C3 is the Comet's closest distance to the sun, which will occur on February 9, 1986. Earth will move from E1 to E3 during the time that the Comet moves from C1 to C3. During this time interval the Comet will be seen in the evening sky setting after sunset. See Figure 5. But as the Comet approaches C3 and the Earth approaches E3, the Comet will be positioned so that it is nearly in line with the sun. At that time the Comet and sun will rise and set together, and the Comet will not be seen. A few on either side of February 9 we will not be able to observe the Comet from Earth, which is unfortunate. Why? Because when the Comet is nearest the sun, its tail should be the longest. But the tail will be pointing away from the sun and us. And the light of the sun will be too bright to let us see the Comet anyway.
FIGURE 5. The early evening January, 1986, sky for an observer at 40° N latitude. The Comet's position relative to the southern and western horizon directions is portrayed at a time about one-and-one-half hours after sunset (the end of evening twilight). The numbers in parentheses are predicted magnitudes. (Since the numbers are less then 6.0, these would all be magnitudes of visible objects.)
The numerical values on the horizon represent degrees of angular measure relative to east and south. (They are values of "azimuth.") The numbers on the vertical edge represent degrees of altitude. (You may approximate angular measures by comparison to the angular distances between stars of the bowl of the Big Dipper - See End Note 8. Look at the Big Dipper. Hold your hand at arm's length and approximate the angle formed by your clenched fist, or the width of your hand, or across your knuckles. You will need to have one eye closed while doing this.)
Figure adapted from The Comet Halley Handbook, p.6.
So our next chance to observe the Comet will be as a morning sky object between early March and early April. In early March it should be hovering toward the southeast before sunrise. See Figure 6. On successive days it will be positioned further to the south. Finally, by early April it should be seen nearly due south low to the horizon before sunrise. The reason it is seen at that time and shifting in direction along the horizon is provided by Figure 4. The Comet will move in its orbit from C3 to C4 between February 9 and April 11, and during the same time the Earth will move from E3 to E4. When the objects are at C4 and E4, the Comet will be too close to the sun to be seen and we will have to wait again a few days to see the Comet.
Our last opportunity to view the Comet may be in mid to late April. See Figure 7. Again we must observe toward the south-southeast an hour and a half after sunset. On successive nights the Comet will be higher in the sky, but it should be dimming gradually in brightness. It will appear to slowly disappear among the stars.
Table II summarize observational circumstances and changes anticipated for Comet Halley.
FIGURE 6. The early March to early April, 1986, morning sky at about one-and-one-half hours prior to sunrise (beginning of morning twilight). The observer is assumed to be at 40° N latitude. For observers further north, the Comet will be lower in the sky at this time; for observers to the south of this location. the Comet will be higher in the sky. See Figure 5 caption for additional details. Adapted from The Comet Halley Handbook, p.6.
SUMMARY OF VIEWING CONDITIONS FOR HALLEY'S COMET, 1985-85
|September & October||After midnight!||If you know constellations, look with binoculars in vicinity of feet of GEMINI, or club region of ORION - see "End Note" 9 and Figure 8.||Most favorable dates would be near new moon (or a few days before) - Sept. 15, Oct. 14, Nov. 13, and Dec. 12.||The Comet will be nearly motionless among the stars from nigh to night. It should be brightening.|
|November 10||After evening twilight and after constellation TAURUS has risen.||If you know constellations, look with binoculars about 5 1/2 degrees (one hand width) north from star Aldebaran in TAURUS (the Bull).||Apparent magnitude (m) or brightness of about 8.0. (Smaller number = brighter)||The magnitude limit for naked-eye seeing is m=6.0.|
|November 16||After evening twilight and after constellation TAURUS has risen.||If you know constellations, look with binoculars about 2 degrees S (four moon diameters) of the Pleiades ("Seven Sisters.")||Possibly brighten to m=7.3 as total brightness.|
|Late December through January||After evening twilight (1 1/2 hours after sunset).||To west-southwest, at altitude of 30 degrees (about 5-6 hand widths) in early January. See Figure 5.||Increasing brightness from m=6.0 to m=4.5(?). Tail develops (?).||Decreasing altitude on successive nights. Should be visible by mid January with naked-eye.|
|Early March to early April||Morning sky before twilight (2 hours before sunrise).||Early March in east-southeast; progressing toward south by early April. See Figure 6.||Increasing brightness from m=4.5 to 4.0. Tail away from horizon direction.||Five to ten degrees above horizon. Impressive naked-eye object.|
|Mid to late April||Evening sky after twilight.||Toward south-southeast. See Figure 7.||Decrease in brightness on successive nights such that fades from naked-eye view.||Seen higher in sky on successive nights in SSE.|
FIGURE 7. Expected appearance of Comet Halley in evening sky mid to late April, 1986. The assumed location is 40° N latitude; the time is one-and-one-half hours after sunset (end of evening twilight), Ideal sky conditions for observing are assumed. See Figure 5 and 6 captions for additional details. Adapted from The Comet Halley Handbook, p. 6.
Plan to share in the excitement of observing the Comet. But also be wary that the Comet might not develop as expected. (Remember Comet Kohoutek?) In fact, circumstances for observing this return of Comet Halley are not so favorable as some past returns. Because of the orbital orientation of Comet Halley to Earth's orbit, observers near the equator are in a much more favorable position than those in the mid-latitudes of the Northern Hemisphere. Also, the tail of a cornet is often most impressive when the comet is nearest the sun. Unfortunately, the Comet will be in line with the sun when it is at perihelion (on February 9).
Because a portion of the nucleus of a comet vaporizes on each return passage by the sun, we would expect any periodic comet to diminish on each successive solar passage. The solid debris "lost" from the nucleus tends to become distributed through the comet's orbit. Earth's orbit intersects near Comet Halley's orbit twice during the year - in early May and late October. At those times of the year planet Earth encounters some of the cometary debris and we observe predictable meteor showers. The early May showers are called the Eta Aquarids; the late October are known as the Orionids. At some future time these meteor showers may be the only reminders of Comet Halley!
FIGURE 8. Comet Halley traced against the star background. Dates are shown for the Halley path through the stars. November 10 and 16 would be particularly good dates for a first attempt to observe - See Table II.
ADDITIONAL OBSERVING COMMENTS
Observing without the use of a binocular or telescope can be very satisfying. At least try to get several glimpses of the Comet in January, early to late March, or again in mid to late April. Try to make some observations of tail length and changes of brightness. "End Note" 8 provides some observing suggestions.
In many ways binoculars may offer the most rewarding opportunities for viewing Comet Halley. The binocular has an important characteristic not shared by telescopes - a relatively large "field of view." The field of view of the ordinary binocular may be 5 or more degrees. (The moon's angular diameter is 0.5 degrees; the separation between the "pointer stars" of the Big Dipper is 5 degrees.)
Don't necessarily purchase binoculars for the purpose of viewing Halley if one is readily available. But if you should decide to purchase binoculars for astronomical purposes, a 7x50 is especially good. ("7x" means it possesses a 7 power magnification; the 50 refers to the fact that the large, light-gathering lens is 50 millimeters in diameter.) Any larger power binocular will require a tripod to hold it sufficiently steady during use.
PHOTOGRAPHING. If the Comet develops into a splendid naked-eye object, capture the image with a photograph. The following would be
needed: a camera with a variable shutter speed, a tripod, and a cable release (optional). No special lens is needed; the "standard" focal length of 50 to 55 mm. is satisfactory. Here are some additional tips for stationary camera photography for the Comet: (1) Use a relatively "fast" film: an ISO (formerly ASA) 1000 speed slide or print daylight film. (2) Photograph something else first in daylight; this allows the film processor to establish "cut" lines between frames on the film at the time of film processing. (3) Place the camera on a sturdy tripod, and attach the cable release (if one is available) to control the exposure time. (4) Set the lens focus at "infinity," and "open" the lens as far as
possible (to the smallest f-stop number, e.g. f/1.4). (5) An exposure time of 30-seconds or less will record bright stars and plants, and show a bright comet as a "fuzzy" ball with a tail. The cover picture is a stationary camera photograph of Comet West taken in the manner previously described. Take several photographs, each with a different exposure time. If the exposure time is more than about 30 seconds, the image may be blurred and "trails" will be developed due to the rotation of the Earth.(10)
OTHER PLANS FOR HALLEY'S COMET IN 1986
The response to Comet Halley will be quite different than on previous returns. Oh, there will be hucksters or people out to make a buck who will try to sell "comet pills," or gas masks, or other souvenirs to play on our superstitions or fears, or to just let us have a way of commemorating this return of Halley. But unlike past generations who may have cowered in fear as they contemplated what evil might occur, several nations are prepared to greet this return of the "hairy star" with a number of spacecraft.
During March of 1986 five spacecraft will fly close to the Comet. Two are Soviet probes (Vega 1 and 2); two are Japanese (MS-T5 and Planet-A); and one craft (called Giotto-see Table I) is a joint effort of several (11) nations under the guidance of the European Space Agency. The U.S. has in orbit the International Cometary Explorer (ICE) which has been redirected to fly near Comet Halley in March of 1986 after passing through the tail of Giacobini-Zinner in September, 1985. Furthermore, a NASA payload will be carried aboard a shuttle to do some telescopic studies of the Comet, also in March of 1986. These craft have been programmed to "flyby" after the Comet has passed nearest to the sun and, hence, after the Comet has had an opportunity to develop a full coma and tail. These flybys should provide additional insight of the structure and composition of comets.
Through the coordinated effort of several nations with several space probes, some of our curiosity about the nature of comets should be satisfied. And if we recall that comets are considered to be very old, and relatively unaltered remnants of the early solar system and its formation, we should also have the opportunity to confirm some of our thinking about our ancient heritage as Earthlings. Or, we may be forced to reshape some of our thinking. Those outcomes can be thought of as the ultimate scientific goal. But, for most, the ultimate excitement may be the opportunity to glimpse this return of Comet Halley.
ACKNOWLEDGEMENT: The author wishes to acknowledge the many hours which Sandi Fowler spent at the keyboard of a word processor, and the drafting work which was done by Mike Law. Both are E.S.U. students.
1 After the spectroscopic detection of gases such as cyanogen, a poison, fears were aroused that if planet Earth were to be "swept" by the tail of a comet, Earth life might be poisoned. (Gas masks were sold in 1910 at the time that Earth was to pass through the tail of Halley.)
2 The term "meteorite" refers to objects found on the surface of the Earth and of extra-terrestrial origin. A chunk of stony, stony-metallic, or metallic debris in space is called a "meteoroid." If a "meteoroid" is gravitationally attracted to the Earth, and heats to the point that it glows visibly because of friction between it and the Earth's atmosphere, then the phenomenon is called a "meteor" or "shooting star." If a meteoroid survives as a meteor and a chunk strikes the Earth, the chunk is called a "meteorite."
5 Comets may be classified as "periodic" or "nonperiodic" comets. A "periodic" comet has a closed orbit in the shape of an ellipse. (An ellipse is often compared to an oval or flattened circle.) Periodic comets visit the sun's vicinity many times; this is because they are members of the solar system orbitting the sun. "Non-periodic" comets have open orbits; the "orbit" is in the shape of a hyperbola or a parabola. If the "orbit" is open, the comet will only visit the sun's vicinity once before leaving the solar system and traveling through space until the gravitational influence of another star changes its course. Periodic comets, such as Halley, are often designated as follows: P/Halley.
6 A "nebula" is a mass of gas and dust in space. Most (80% plus) of a typical nebula is composed of hydrogen, the most abundant element in the universe. Hence, the composition of stars (and the sun) is predominantly hydrogen. The next most abundant constituent is helium.
7 It may seem bothersome that a variable period (75-76 years) is suggested for Halley. The variation is a result of gravitational effects that the major planets and other objects in the solar system have on the "flimsy" comets. These gravitational perturbations alter the orbit. The shortest period between successive returns of Comet Halley is 74.42 years (1835-1910), and the longest is 79,25 years (451-530). (The Comet Halley Handbook, An Observer's Guide. Second Edition. Donald K. Yeomans. May, 1983. U.S. Government Printing Office, Washington, D.C. p.19.) Another factor which alters comet orbits is a "jet" action produced when some comet nuclei break apart during passages near the sun.
LOOKING NORTH, JANUARY, ABOUT 1 1/2 HOURS AFTER SUNSET
Also, we can note the following:
a. The North Star may be found by drawing a line from the "pointer stars" of the Big Dipper, away from the open part of its bowl, to the first star seen;
b. The North Star for those at latitudes near those of Emporia will be 38° above the northern horizon; the point on the horizon below the North Star will be the direction of north;
c. Note that the stars of the bowl of the Little Dipper are numbered. The numbers refers to the apparent magnitude or observed brightness of those stars. The North Star is also a second magnitude, or m = 2, star.
9 If one is familiar with Right Ascension (RA) and Declination (D), and has a star chart with coordinates, plotting the following data may assist to find Comet Halley with a binocular or small telescope as it approaches naked-eye visibility:
|1985 NOV 1||05h 24m||+21°50'||8.8|
|NOV 10||04h 38m||+22°19'||7.9||near Aldebaran|
|NOV 15||04h 00m||+22°02'||7.4||near Pleiades|
|DEC 1||01h 07m||+13°51'||6.4|
|DEC 15||23h 18m||+03°49'||6.2|
|1986 JAN 1||22h 17m||-02°23'||5.8||near naked-eye visibility|
Since m = 5.8 for Jan. 1, the Comet should have approached seeing with the naked-eye. (Recall that the larger the magnitude number, the fainter the object.) All data from Yeomans, Donald K. The Comet Halley Handbook, An Observer's Guide. Second Edition. pages 34-36.
10 For additional information see magazines such as "Astronomy" or "Sky and Telescope" (August, 1985, for example), or books such as Outer Space Photography by Henery E. Paul for tips on guided astrophotography or through-the-telescope photography.
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