« السابقةمتابعة »
the German, by E. C. OTTÉ. In 5 vols. New York : Harper & Brothers. 1869.
2. History of Physical Astronomy, from the earliest ages to the
middle of the nineteenth century. By ROBERT GRANT, F.R.A.S. 1 vol., 8vo. London. 1852.
3. Annuaire Publié par le Bureau des Longitudes pour l'Année.
AMONG all the recurring phenomena of nature, there are none, perhaps, that have attracted more general attention than solar and lunar eclipses. In the infant state of science, among all nations, they seem rather to have been regarded as the indication of the wrath of an offended Deity, than as the result of the operation of the immutable laws of nature. Indeed, what could be more appalling to the ignorant and superstitious mind, than the intercepting of the sun's light by an unknown object, without the least warning! Not only so, but to the astronomer who is well acquainted with the laws which produce them, an eclipse of the sun or moon is an evidence of the existence of the unspent forces operating in the celestial regions, that teach him how feeble is man's power when put in comparison with them. The scene presented by a total eclipse of the sun, is rendered still more impressive by the circumstances attending so remarkable an occurrence. All pature seems hushed into silence, as if night had suddenly fallen upon the earth in the midst of day. “The heavens assume an unnatural aspect, which excites a feeling of horror in the spectator; a livid hue is diffused over all terrestrial objects; the plants close up their leaves, as on the approach of night; the fowls betake themselves to their resting places; the warbling of the grove is hushed in profound silence. Universal nature seems to have relaxed her energies, as if the pulse which stimulated her mighty movements had, all at once, stood still."
An eclipse, in general terms, is caused by the interposition of an opaque body between a luminous object and the body, or part of the body illuminated. In astronomy, it is tech
nically the occasional passage of the moon between the sun and the earth, which produces a solar eclipse; and the falling of the moon into the earth's shadow, thereby producing a lunar eclipse. The first recorded eclipses, so far as yet known, are those of the moon, and they were observed by the Chaldeans, in the years 720 and 719, B.C.* We are informed by Simplicius, in his Commentary on the second book of Aristotle's work, De Coelo, that, after Alexander's conquest of Babylon, Callisthenes sent to Aristotle a catalogue of eclipses, which, it was said, had been observed at that place during a period of 1903 years antecedent to that time. The catalogue is now lost; but the above statement shows how attentively such celestial phenomena were watched for ages, probably, before man was able to predict them. But man's desire to penetrate into the future, doubtless stimulated him to great efforts to foretell when such interesting phenomena would occur.
A careful observation of the motions of the sun and moon, and their relative positions during eclipses, extending through many years, would probably be required, to enable the ancient astronomer to satisfy himself as to the true cause of solar and lunar eclipses ; and, after this was done, they could not be calculated until solar and lunar tables of the motions of those bodies were formed. It would be interesting to know who was the first astronomer that succeeded in solving this great problem. According to Diodorus Siculus, the explanation of eclipses of the sun, given by the Chaldeans, was so defective, that they were unable to predict them; but there is some evidence that they were in possession of means of calculating lunar eclipses.t
We are informed by Herodotus I that Thales, the celebrated Grecian philosopher, predicted an eclipse of the sun, which put an end to an engagement between the Medes and Lydians, and induced them to listen to propositions of peace, which were made by Syennesis, of Cilicia, and Labynetus, of Babylon. Herodotus does not enter into particulars ;
* Origin and Progress of Astronomy, by John Xarrien, p. 71. + Bibliothecce Historico, Lib. II.
* Clio., sect. 74
and we are uncertain as to the accuracy with which the time of the eclipse was foretold by Thales; and also with respect to the means employed in arriving at his conclusions. The cycle of eighteen years, known as the saros, a knowledge of which Thales might have obtained from the priests while he was travelling in Egypt, was sufficient to enable him, as observed by Delambre, to predict the time of the phenomenon within a month. Two other eclipses are said to have been predicted by the ancient Greek astronomers; one by Endemus, who wrote a history of astronomy, now supposed to be lost; and the other by Helicon, of Cyzicene, who announced to King Dionysius the time of its occurrence, and it is said to have happened conformably to the prediction.*
The saros is a Chaldean period of about eighteen years, which was discovered by that people many centuries before the beginning of our era, by means of a comparison of the eclipses observed and recorded during many years; and it was used by them and by other nations for predicting such phenomena. It consists in this : 223 mean lunations consist of 6,585,321 days, and the moon's nodes return to the same position, with respect to the sun, in 6,585,772 days, giving only the small difference 0,451 of a day, or not quite twelve hours. We thus see that the relative position of the sun, moon, and node is nearly the same at the end of this period, as at the beginning; and any eclipse happening at the commencement of a period will be nearly repeated at the beginning of the succeeding one. There is too great a variation, however, to count on the recurrence of a total solar eclipse at the same place.
Eclipses of the sun, it is well known, depend for their occurrence on the relative position of the sun, moon, and node of the lunar orbit. It has been found, by calculation, that for a solar eclipse to be possible, the distance of the centres of the sun and moon from the moon's node, must not exceed 18° 36'; and the greatest possible distance between the centres of the sun and moon, at the time of contact, is 1° 34' 28"* These are the solar ecliptic limits. Out of seventy eclipses which annually occur within one cycle of eclipses, or saros, the average number of solar eclipses is forty-one, and of lunar twenty-nine. We learn, from theory, that seven eclipses may, and that two must take place within every year. When the number of eclipses is least, they are both of the sun; and when the number of eclipses in a year is greatest, five may be solar and two lunar, or three solar and four lunar. The average number of eclipses is four. Although it is usually stated that, in any given long period, the number of solar eclipses exceeds the number of lunar, yet, if we take into consideration the penumbral lunar eclipses, as we do the penumbral solar eclipses, there will be more eclipses of the moon, by a small number, than of the sun.
* Aristotle's De Colo, Lib. II., cap. vi.
† 18y., 10d., 7h., 43m.
Although a solar eclipse may last several hours, counting from the beginning to the end of the observation, yet the duration of a total or annular eclipse is very limited. Other things being equal, the duration of a total solar eclipse varies with the latitude of the place of observation, being greatest at the equator. According to the calculations of Du Sejour, the greatest possible duration of a total eclipse of the sun, is, under the equator, 7' 58", and 6' 10" in the latitude of Paris. $ This takes place when the moon is in perigee, of the sun in apogee, when the difference in the diameters is 2' 2";S but since the sun's apparent motion is least, and that of the moon greatest, at these points, the duration of totality is necessarily quite limited. The duration of an annular eclipse is greatest when the sun is in perigee and the moon in apogee; and since the apparent motion of the sun is greatest, and that of the moon least, and the difference of their apparent diameters equal to 3' 28'| at these points, every circumstance conspires to increase the duration of such an eclipse. Du Sejour found, by actual
Bartlett's Spherical Astronomy, p. 333.
# Mémoires Acad. des Sciences, 1777, p. 318. $ Bartlett's Spher. Ast., p. 333.
| P. 333.
calculation, that the utmost possible duration of an annular eclipse is, at the equator, 12' 24'* and in the latitude of Paris, 8' 56''+.
The great use which the astronomer makes of eclipses, not to mention the public interest generally excited by their prediction and appearance, called the attention of the astronomer at an early period, into the field of investigation, and several methods of calculation have been adopted by different authors, at various periods. Among all the methods, the plan of orthographic projection, for the general circumstances which take place on the earth, seems to have been the most popular, though it only affords a close approximation. In actual calculations, however, its convenience seems to have inclined astronomers, hitherto, almost exclusively, to use it, when the greatest accuracy was not required. I
The circumstances of an eclipse for a particular place, calculated by the “Method of the nonogesimal,” which refers the bodies to the ecliptic, were discussed analytically by Lagrange, in the “ Astronomischi Jahrbuch" for 1782; and since then the celebrated Bessel has made many important additions to the theory.ş In the English “ Nautical Almanac" for 1836, Mr. W. S. B. Woolhouse has given an elaborate paper on the calculation of eclipses, discussing all the various circumstances; and then he has applied his formula to the annular eclipse of May the 15th, 1836.
Eclipses have ever been regarded as very important phenomena in the systems of the world, since their accurate observation enables the astronomer to test the accuracy of his theory, and of his solar and lunar tables. They are very important, also, and especially eclipses of the sun, for the determination of terrestrial longitude. Eclipses are also of the first importance in the science of chronology, for the purpose of fixing dates as landmarks, or mile-stones, in the
* Mémoires Acad. des Sciences, 1777, p. 317. f Ibid., p. 316. 1 The reader will find this method treated in the astronomical treatises of M. De la Lande, J. M. Delambre, and more recently in the Elementa Eclipsium of Hallaschka, Prague, 1816. See also, Mémoires sur l'Astronomie Pratie y M. J. Monteiro Da Rocha, Paris, 1808.
& See Astron. Nachrichten, No. 151.