Thursday, November 10, 2016

Shooting Stars

Below is an article from Harper's Magazine ©1868. The science during the 19th century was developing regarding stars, meteors and shooting stars. I became interested in the science of that century while researching my novel, Raining Fire. The 1833 Lenoids meteor shower was so profound at that time that it was nicked name "The Night it Rained Fire." This article is over 30 years from that event and the information they learned/observed at that point in time is quite interesting, imho.

By ELIAS LOOMIS, Professor Yale College.

EVERY one has occasionally seen upon a clear evening a small bright object, in appearance very much like a fixed star, move rapidly across the sky and suddenly disappear, as if a star were shot away from its place in the firmament to a distant region of the heavens. This phenomenon is commonly known by the name of " Shooting-star," or "Falling Star." Occasionally the path of a shooting-star is marked by a luminous stream which continues for an appreciable time after the star has vanished. Shooting-stars may occasionally be seen on every clear night, and at times follow each other so rapidly that it is quite impossible to count them.

Ordinary shooting-stars are not accompanied by any audible sound, although they are sometimes seen to break into pieces. Occasionally meteors of extraordinary brilliancy, like globes of fire, presenting an apparent diameter of considerable magnitude, are succeeded by a loud detonation or explosion, followed by a noise like that of musketry or the discharge of cannon These have been called "Detonating Meteors" or "Bolides."

No solid body has been known to reach the earth's surface which could be traced to an ordinary shooting-star; but occasionally solid substances descend to the earth from beyond the earth's atmosphere. These bodies are called "Aerolites." These three classes of bodies are known by the general term of "Meteors." It is convenient to speak of these classes separately, although it is not supposed that they differ from each other essentially either in their character or their origin.
Shooting-stars are not seen with equal frequency at all hours of the night. They generally increase in numbers from the evening twilight throughout the night until the morning twilight; and when the light of day does not interfere, they are generally most numerous about six o'clock in the morning. From a comparison of a vast number of observations it has been ascertained that the average number of shooting-stars which may be seen by a single observer upon a clear night, in the absence of the moon, about the middle of the evening, is four per hour; about midnight it is six per hour; about two o'clock in the morning it is eight per hour; and about four o'clock it is ten per hour.

In order, however, that an individual maysee so large a number he must observe, not from an open window, much less through a pane of glass, but he must stand in the open air where the view of the sky is entirely unobstructed, and he must devote his exclusive attention to a constant watch of the heavens. Upon a cool night such exposure is far from agreeable, and few persons are willing long to persevere in it.

Professor Newton of Yale College has made extensive investigations to determine the relative number of shooting-stars which may he seen in a given period by different numbers of observers. For this purpose twelve observers were stationed upon the top of a tower from which there was an unobstructed view of the heavens, and they were intended to be so arranged as to divide the sky equally amonj; them. Whenever a meteor was seen, each person perceiving it called out his own name, and a secretary entered the names of the observers upon a record. These observations were continued for several hours. From n comparison of these records it has been concluded that four persons, looking toward difl'erent portions of the heavens so ns to divide the' sky symmetrically among them, will see three times as many meteors as the average number seen by them individually; eight persons will see four times as many as one; and fifteen observers will see five times as many as one. The entire number of meteors which might be seen by a sufficient number of observers is about six times as many as would be seen by a single observer. The reason that four persons will not sec four times as many meteors as one person is that two of them will frequently see the same meteor.

Combining these results with those previously stated we conclude that the nverago number of meteors that traverse the ntmosphcre, and that are large enough to be visible to the naked eye, if the sun, moon, and clouds would permit, is forty-two in an hour, or one thousand daih/.

Shooting-stars are not seen with equal frequency at all seasons of the year. From July to December they are more abundant than during the other six months of the year; and they are ordinarily most abundant in the month of August.
If two observers, at a suitable distance from each other, note the npparent altitude and azimuth of a shooting-star at the commencement of its flight, and do the same also for its termination, they have the data for computing the absolute height of beginning and end above the surface, of the earth. The earliest observations of this kind were made in 171)8 by Benzenberg and Brandes in Germany, and since that time similar observations have been made in mamparts of Europe, us well as in the United States. Such observations were made nt New Haven, Hartford, Williamstown, Wolcottville, Albany, etc., on the night of August 10-11, 18(53; at Washington and Philadelphia on the night of November 13-14, 1868; and again on the 13-Hth of November, 1807, such observations were made at Washington, Richmond, NewHaven, and several other places. It has been ascertained that when the base line employed is only three or four miles in length a shooting

star is seen in nearly the same direction at both stations, showing that its altitude is much greater than the length of that base. When the base line is 30 or 40 miles, the average difference of the directions of the star at the two stations is about fifteen degrees. The base line should not be less than 40 or .10 miles in length, and one of 75 or 100 miles would not be too great. Observers at distances of over 150 miles from each other see for the most part different shooting-stars.
The heights of over 500 meteor paths have been computed, and we thus learn that shooting-stars begin to be visible* at elevations of from 40 to 120 miles, and perhaps sometimes 150 miles, or an average height of 74 English statute miles. They disappear at elevations of from 30 to 80 miles, and perhaps sometimes 100 miles or more, giving an average height at disappearance of 52 English statute miles.

The length of the visible path of shootingstars varies from 40 to 100 miles, though in a few cases they have been found to be even 300 nnd 400 miles long—the average length being 28 miles. The time of describing the visible path varies from less than one second to five seconds, and in some rare cases amounts to ten seconds; but their average duration is less than one second. The average duration of meteors whose brightness exceeds that of stars of the first magnitude is estimated at one and a half seconds. Their velocity relative to the earth's surface varies from 10 to 45 miles per second, and the average velocity of the blighter class of shooting-stars amounts to about 30 miles per second.

Shooting-stars are seen to move in all directions through the heavens. Their apparent paths are, however, general!} inclined downward, though sometimes they move upward; and after midnight they come in the greatest numbers from that quarter of the heavens toward which the earth is moving in its annual course around the siin.

The magnitude of shooting-stars is very variable. Some of them have been computed to have a diameter of 100 or 200 feet, and others 1000 up to 5000 or 0000 feet. We must, however, regard this as the diameter of the blaze of light which surrounds the meteor, while the meteor itself before it takes fire may have a diameter of only a few feet, or perhaps only a fraction of an inch. The apparent size of meteors is greatly magnified by irradiation.

l'rofessor Darkness has undertaken an elaborate investigation to estimate the qnantity of matter in shooting-stars by means of the light evolved during their-passage through the atmosphere, and he concludes that the mass of ordinary shooting-stars does not differ greatly from one grain ; that is,_/b«r hundred awl eighty ofthem would weigh ou/y one ounce at the surface of the earth.

Occasionally shooting-stars appear in great splendor, flashing with a brightness nearly equal to that of the full moon, and leaving behind them a train of dazzling light, which lasts for several seconds, and even for whole minutes. Their color is usually white, with a reddish tinge; but occasionally they exhibit a green light, and sometimes a mixture of green and blue or purple. Even quite faint shootingstars Bometimesjeave trains. Fig. 1 represents a remarkable meteor seen in June, 1866.
The path of shooting-stars is frequently curved; sometimes the path consists of two portions inclined to each other at a considerable angle; and at last the meteor sometimes hursts like a rocket into numerous fragments. In such cases the place of explosion is usually indicated by a smoky cloud, which sometimes continues visible for ten minutes. Fig. 2 represents a meteor seen in 1850, which was followed by a long train of light, and which exploded emitting a large number of scintillating radiations.

Observers frequently imagine that they hear a whizzing noise accompanying the passage of a brilliant meteor. It may be easily proved that such impressions are an illusion. When we compute the path of the meteor from which the sound was supposed to proceed, we always find that it was quite distant from the observer, frequently 40 or 50 miles, and sometimes 100 miles. Now sound is known to move with a velocity of 1120 feet per second, or 50 miles in about four minutes. If, then, any noise was caused by the motion of the meteor, the sound could not possibly be heard until a considerable time after the meteor disappeared, viz., two, five, or even ten minutes, according to its distance.

The light of shooting-stars is probably due to the high temperature resulting from the resistance of the atmosphere to the rapid motion of the meteor. Since at the ordinary elevation of shooting-stars the air is exceedingly rare, some have supposed that the resistance would not develop sufficient heat to give meteors their brilliant appearance. The researches of modern philosophers have enabled us to compute the quantity of heat that may be developed by the stoppage of a meteor in the atmosphere. A portion of the living force of the body is expended in setting the air in motion, and a portion in heating the meteor and the air. This living force, and the consequent heat that maybe developed, is proportioned to the mass of the body and to the square of its velocity. The arresting the motion of an iron meteor whose velocity is thirty miles per second would, if the whole living force were changed into heat, be sufficient to raise the temperature of the meteoric body more than four million degrees of Fahrenheit's scale. If even the larger part of this force was expended in giving motion to the air, there would remain enough to furnish a brilliant light and to melt the exterior portion of the meteor, or entirely to disintegrate it. Aerolites, such as will be hereafter described, always present a peculiar appearance upon the exterior, an if the outer crust had suffered partial fusion, and many of them when first discovered have still been quite hot.

The mean distance of shooting-stars from the observer is found to be about 105 miles, and the average height above the earth of the middle points of their paths is 63 miles. Hence the mean horizontal distance of the paths may be regarded as about 90 miles. It is estimated that the number of shooting-stars actually falling within a circle of 90 miles radius is somewhat greater^han the number seen at one place. The area of this circle is contained nearly 8000 times in the entire surface of the globe; whence we conclude that the number of shooting-stars over the whole earth is more than eight thousand times the number visible at one place.

The average daily number of shooting-stars visible to the naked eye at one place has already been stated at 1000. Hence the average number of meteors that traverse the atmosphere daily, and that are large enough to be visible to the naked eye, if the sun, moon, and clouds would permit, must be more than a thousand times eight thousand, or more than eight millions.
The observations of two European astronomers indicate that the number of meteors visible with, a telescope of four inches aperture is about forty times the number visible to the naked eye. A further increase of optical power would doubtless reveal a still larger number of these small bodies. Hence we must conclude that the source from which these meteors come is of immense extent, otherwise it would long since have been exhausted.

The quantity of matter in these bodies is, however, so small, and their distance from each other so great, that they exert no appreciable influence upon the motion of the planets. It is computed that the average distance from each other of shooting-stars, such as under favorable circumstances would be visible to the naked eye, is about three hundred miles.
Having determined the velocity and direction of a meteor's path with reference to the earth, and knowing also the direction and velocity of the earth's motion about the sun, we can compute the direction and velocity of the meteor's motion with reference to the sun. This computation has been made for several different meteors, and has shown that these bodies, before they approached the earth, were revolving about the sun in ellipses of considerable eccentricity. In some instnnces the velocity has been found to be so great as to indicate that the path differed little from a parabola.

It is thus demonstrated that ordinary shooting-stars are small meteoric bodies, moving through space in paths*similar to the comets: and it is probable that they do not differ miiterially from the comets except in their dimensions, and perhaps also in their density.

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