IF YOU should be driving along a back road one moonlit night, you might encounter the following scene. About every 500 feet for a mile or so you would see someone looking through small telescope pointed at the moon. I you stopped to inquire, you would discover that the observers were timing the disappearance and reappearance of stars eclipsed by the moon as it move across the sky. The people engaged in this task will inform you that they are observing lunar occultations. They are participating in an activity, combining the pleasures of observational astronomy with the opportunity of gathering useful scientific data, that is attracting the interest of a growing number of amateurs. One of them is Trudy E. Bell formerly a history of science major at the University of California at Santa Cruz and now a member of the editorial staff of this magazine. She discusses her avocation as follows:

"Everyone knows that the moon an the stars rise in the east and set in the west, an apparent motion caused by the eastward rotation of the earth. Those who watch the night sky carefully also observe that the stars move westward faster than the moon does. A star that rises with the moon disappears below the western horizon several minutes before the moon does. In other words, the moon appears to drift slowly eastward through the pattern of fixed stars. The relative motion is not apparent at a glance because the moon drifts eastward at a rate of about one lunar diameter per hour.

"As the moon proceeds across the sky it occasionally encounters a fixed star that lies in its path. The star disappears behind the eastern edge of the moon and eventually emerges from the western edge. During the encounter the moon intercepts rays of light emitted by the star, creating a shadow that sweeps across the earth.

"If the star is the sun, the result is a spectacular eclipse. Twenty solar eclipses will be seen somewhere on the earth during the remainder of this century. Lunar shadows cast in the light of more distant stars sweep the earth somewhere every night, so that the phenomenon can be seen many times each year. These lunar occultations are less dramatic than solar eclipses because the shadows can rarely be detected by the unaided eye. Nonetheless, the eclipses can be eerily beautiful, particularly when the edge of the moon just grazes the star.

"To observe an occultation you look directly at a bright star that lies in the path of the moon. The star will wink out when it is covered by the advancing edge of the moon. It will reappear in less than two hours. Not all stars that are occulted can be seen by the unaided eye. Some are too dim. The occultation of many dim stars, however, can be observed with a small telescope.

"The shadow of the moon, as cast in the parallel light rays of a distant star, turns out to be quite interesting and useful even though it cannot be seen by the unaided eye. It is an exact silhouette of the moon, distorted only by the curvature of the earth Its jagged edges reproduce the profile of the moon's mountains, valleys and plains. In effect the silhouette enables the observer to examine part of the moon's topography without making a voyage into space. To examine the profile of the lunar mountains, however, the observer must station himself at a strategic point on the earth.

"Assume that the shadow sweeps across the earth from west to east. All observers in its path may see a star wink out or another star suddenly pop from behind the moon. In either case the observer is seeing a total occultation.

"The few observers who are fortunate enough to be stationed at the northern and southern edges of the shadow, which is 2,000 miles wide, get a special treat. They see what is termed a grazing occultation. At these locations the jagged edge of the moon causes the star to blink off and on successively as lunar crags and valleys pass in review. An observe who is just outside the edge of the shadow sees a near-miss: one edge of the moon comes extremely close to the star but does not quite touch it. An observer who is a few miles away but within the shadow sees the star wink off and five minutes later reappear slightly west of the moon.

"Observers at intermediate points see the best show. Some catch the starlight at the bottom of the lunar valleys. Observers who are stationed progressively closer to the edge of the shadow see the lunar mountains at intermediate elevations. The observer who is barely inside the edge of the shadow sees a momentary blink as the summit of the highest lunar crag drifts across the ray of starlight. Observers only 50 or 100 feet apart may see different details on the lunar features, thereby obtaining a high accuracy in locating the features. The point at which the ray of starlight is tangent to the edge of the moon can also be identified. This point can be used for determining the position of the moon itself. During total occultations the observer cannot easily identify the exact points on the moon's edge at which the star disappears and reappears.

"Astronomers can mathematically predict the approximate position of the moon, in relation to the earth and the fixed stars, at any time in the future. They would like to be able to predict its future position exactly. Although such precision may be unattainable, several projects have been undertaken in recent years to reduce the uncertainty. For example, astronauts have placed reflectors on the moon so that a laser beam from the earth can be reflected back along the same path. This scheme yields an improved measure of the distance between the moon and the earth.

"In addition a program of observing grazing occultations on a worldwide basis has been established to serve as a check on the accuracy of the laser observations. The program may also substantially reduce present uncertainties in the position of the moon with respect to the fixed stars. Responsibility for organizing and conducting this program has been accepted by Her Majesty's Nautical Almanac Office at Herstmonceux Castle in England. Astronomers there routinely predict as accurately as possible the paths where grazing occultations of certain stars can be seen. The office encourages teams of amateurs to use the predictions for making timed observations of grazes. The resulting observations are collected by the office for computing improved predictions, which amateurs can use for making more observations that lead to still better predictions and also to improved mathematical procedures for making predictions.

"The observation of grazing occultations can generate other useful information. For example, although the rough terrain at the edge of the moon has been mapped as carefully as possible from the earth, data collected by observing grazing occultations can establish accurate points of reference for the study of systematic errors in the charts and thus can lead to their improvement. Moreover, since grazing occultations are often visible over intercontinental distances, observations of them at widely separated locations can be used to establish the relations between independent surveying networks. In general, then, by knowing exactly where the moon is at any time one has a reference point for determining the relative location of other points in the universe, including points on the earth's surface.

"The technique of observing grazes can be easily mastered by laymen of all ages. The bulk of the work is currently being done by amateur astronomers. Professional astronomers record total occultations, but most of the observations of grazes are made by laymen because the requirements of the work are ideally suited to the resources of the amateur.

"Observers of grazing occultations work with fairly simple apparatus. Each observer should have a small telescope with an objective lens or a mirror at least two and a half inches in diameter. Many observers use six-inch reflecting telescopes that they make at home. The optimum size of the instrument, in terms of its light-gathering power, depends on the brightness of the star to be observed and the extent to which the moon is illuminated by the sun. A bright, fourthmagnitude star occulted on the dark side of a lunar crescent that is 40 percent illuminated can be observed successfully with a good pair of binoculars if they are rigidly mounted. At the other extreme an eight-inch reflecting telescope would be barely adequate for observing a star of the eighth magnitude if the star appeared near the cusp of a gibbous moon. Glare from the illuminated portion of the moon increases the difficulty of making observations. For this reason instruments of large focal ratio (f/8 or more) are preferred. A small refracting telescope is somewhat preferable to a larger reflector. Clean optics are important.

"A clock drive, which compensates for the rotation of the earth and keeps the telescope trained on a selected star, is convenient but not essential. If a clock drive is used, it must be of the type that operates from a battery or some other portable source of power. Many sites from which grazing occultations are visible lie along back roads where electric power is not conveniently available.

"The primary purpose of observing a grazing occultation is to obtain accurate timings of the momentary disappearances and reappearances of the star behind the lunar crags. There are two main methods of recording the events. One is for the observer to use a portable tape recorder and a shortwave radio receiver. The radio brings in accurate time signals broadcast either by station WWV (2.5, 5, 10, 15, 20 and 25 megacycles), which is operated by the National Bureau of Standards at Fort Collins, Colo., or by the equivalent Canadian station CHU (best heard on the East Coast at 3.33, 7.335 and 14.67 megacycles). These time signals are recorded on tape along with the observer's shouts of the events he sees through the telescope. Batterypowered equipment is easiest to set up and entails the least cost. Depending on the ingenuity and mechanical inclination of the individual, an observer can make valuable contributions to astronomy for less than $100.

"In the second method timings from many observers are recorded on one tape with a long cable that connects all the stations. Each observer has a small oscillator that generates a tone of distinctive pitch. Sections of telephone wire typically 500 feet long are joined to form the cable, and an oscillator is connected to each of the junctions. When the observer sees the star disappear or reappear at the edge of the moon, he presses a button on the oscillator in a certain sequence. The inputs from all the oscillators are recorded on the tape at a central station, together with time signals from WWV or CHU. At least one group in the U.S. acquired a paper-chart recorder with multiple pens. Inputs from the various cable stations are thus recorded on paper instead of on magnetic tape.

"Observing teams operate under the direction of an expedition leader who is usually an experienced amateur. He is responsible for selecting the site for observing the graze; if a cable is used, he supervises the installation and testing of the apparatus. At the conclusion of the expedition he analyzes the data and forwards them to Her Majesty's Nautical Almanac Office.

"The leader receives information about future grazes from any one of half a dozen individuals, called computors, scattered over the U.S. These people generate the predictions with the aid of large electronic computers and special programs that take into account lunar theory, star positions and the motion of the moon to yield the predicted limit of a grazing occultation: the path traveled by the extreme northern or southern edge of the shadow of the moon cast by the occulted star. Depicted on a map of the earth, the predicted limit is an arc that can be several thousand miles long. Groups of observers set up their instruments at right angles to the arc at any convenient point along it [see illustration at right]. Grazes can be observed within a mile or two of the limit. Any number of groups can observe the graze sequentially by setting up their equipment along the predicted limit.

"Such necessary information as the position of the limit, the brightness of the star and the date and time of graze is sent to the expedition leader on computer printout sheets; he then chooses the particular observing site for his group. He must be familiar with the geography of the area, since he is constrained by a number of factors. The ideal graze site should be not more than an hour's drive away, out along flat country roads with easily identified landmarks and few interfering lights. The leader relies on special large-scale maps published by the U.S. Geological Survey and the Canada Department of Energy, Mines and Resources to determine the best location.

"After the expedition leader selects the site he must decide where to put the observers. What he is interested in is the profile of the moon's edge at the time of central graze and the place on the moon where the graze occurs. The shadow of the moon cast by the occulted star is a silhouette of the lunar limb. Fortunately there are contour charts of the moon's limb, drawn from photographs made between 1927 and 1956 by Chester B. Watts at the U.S. Naval Observatory in Washington and at two other observatories. With Watts's charts the expedition leader, or another person designated as a profile plotter, can draw the profile. Knowing the geography of the moon at the point of graze, the leader can tell which locations north and south of the predicted limit will yield the most information about the lunar limb.

"The expedition leader generally arrives at the graze site several hours before the observers do-often in daylight- and surveys the area to make sure that the station locations selected from the maps are satisfactory. If a cable is to be used, it is unrolled along the road. Usually each length of cable is stored on its own spool and distances are measured along the ground, so that when all the lengths are simultaneously unrolled they fit together evenly. In this way any number of people can assist in laying the entire cable. For an experienced team the operation may take less than an hour.

"The expedition leader and how they affect the steadiness of the image of the star, so that during the graze a temporary dimming will not be mistaken for a disappearance. The seeing conditions are recorded on paper or tape. During this preliminary period observers also select the magnification that yields the clearest image of the star. Optimum magnification usually ranges between 90 and 150 diameters.

"As the moment of the graze approaches, all observers keep their eye glued to the eyepiece of their telescope, and anyone at a station who is not observing should maintain strict silence. If tape recorders are used at individual stations, each observer calls out a clear, sharp 'D' into the microphone when the star disappears behind the moon's edge and a sharp 'R' when it reappears. For short disappearances (those on the order of a tenth of a second) it is customary to call 'Blink.' A similarly short reappearance is recorded by calling 'Flash.' When oscillators are being used, the button is pressed once to signal a disappearance and twice in rapid succession to signal a reappearance. Some groups prefer to signal the events with the toy noisemaker known as a cricket, which emits a click when a metal tab is depressed. The cricket has the advantage of emitting a sharp sound that is easier to time than a fuzzy-edged voice syllable.

"Following the graze the group breaks camp. The event can occur at any hour of the night. It may be followed by coffee, a night's sleep or breakfast. Tapes or data that have been recorded by hand must be analyzed; the useful information from them is transcribed on report forms that are available from the expedition leader. If individual recorders are used at each station and are of the type having a capstan to drive the tape, the chances are good that the sounds can be reproduced and successfully timed with a stopwatch. Then the analysis can be made after the observer has returned home. All pertinent data are sent to the leader as soon as possible, but never later than 10 days after the expedition.

"Having turned in his observations to the expedition leader, the observer can relax until the date of the next graze. He is spared the difficulty of employing his data to improve the body of mathematics known as lunar theory. This work is performed at Her Majesty's Nautical Almanac Office with the aid of computers.

"A number of astronomical societies around the country have established grazing-occulation programs and welcome amateurs who would like to participate in the expeditions. An active group may go 'grazing' on an average of once a month. The accompanying map [above] shows some of the brighter grazes that will be visible in the U.S. between January and March of this year. They are numbered in order of their occurrence. Each predicted limit path starts in the west at the date and Greenwich Mean Time given beside the path. G.M.T. is used in astronomy to give a single time for an event. Subtract five hours for Eastern Standard Time. N and S indicate whether the graze is on the northern or the southern limb of the moon. The other numbers and letters are the names of the stars.

"Those who are interested in joining a group can get a foretaste of the fun b observing a few total occultations. These events can be observed anywhere, eve from a rooftop or a backyard. Whereas grazing occultations can be seen only along a narrow strip a few miles wide, total occultations appear everywhere within the 2,000-mile width of the shadow. Train your telescope on the moon for a few evenings. Carefully examine the star pattern at the eastern edge of the moon. Eventually you will see one or more stars directly in the lunar path and within a diameter or so of the approaching disk. You can estimate when the moon will overtake the star by bearing in mind that the moon drifts a distance equal to its diameter in about an hour. By the same technique you can estimate when the occulted star will reappear, taking into account the width of the lunar disk at the point where the star disappeared.

"Beginners can also get a feel for occultation observing, without the responsibility of actually timing the events, by accompanying an expedition and just watching a graze. It is a beautiful spectacle and one of the few astronomical phenomena that can be viewed from start to finish within a reasonable period of time. Those who decide to plunge in can get more information about this avocation, together with the addresses of nearby occultation observers, from the man who started it all: David W. Dunham, Apartment C, 4771 South Spring Street, St. Louis, Mo. 63116."

Bibliography

SOLAR ECLIPSES AND OCCULTATIONS. Ivan I. Mueller in Spherical and Practical Astronomy as Applied to Geodesy. Frederick Ungar Publishing Co., 1969.

THE 1972 OCCULTATION SUPPLEMENT. Sky and Telescope, Dept. SA, 49-50-51 Bay State Road, Cambridge, Mass. 02138.

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