Astronomy of To-day: A Popular Introduction in Non-Technical Language. Cecil Goodrich Julius Dolmage

and so the fourth recurrence will duplicate the one which preceded it, three saros returns, or 54 years and 1 month before. This duplication, as we have already seen, will, however, be situated in a latitude to the south or north of its predecessor, according as the eclipse series is progressing in a southerly or northerly direction.

      Lastly, every eclipse series, after working its way across the earth, will return again to go through the same process after some 12,000 years; so that, at the end of that great lapse of time, the entire "life" of every eclipse should repeat itself, provided that the conditions of the solar system have not altered appreciably during the interval.

      We are now in a position to consider this gradual southerly or northerly progress of eclipse recurrences in its application to the case of eclipses of the moon. It should be evident that, just as in solar eclipses the lunar shadow is lowered or raised (as the case may be) each time it strikes the terrestrial surface, so in lunar eclipses will the body of the moon shift its place at each recurrence relatively to the position of the earth's shadow. Every lunar eclipse, therefore, will commence on our satellite's disc as a partial eclipse at the northern or southern extremity, as the case may be. Let us take, as an example, an imaginary series of eclipses of the moon progressing from north to south. At each recurrence the partial phase will grow greater, its boundary encroaching more and more to the southward, until eventually the whole disc is enveloped by the shadow, and the eclipse becomes total. It will then repeat itself as total during a number of recurrences, until the entire breadth of the shadow has been passed through, and the northern edge of the moon at length springs out into sunlight. This illuminated portion will grow more and more extensive at each succeeding return, the edge of the shadow appearing to recede from it until it finally passes off at the south. Similarly, when a lunar eclipse commences as partial at the south of the moon, the edge of the shadow at each subsequent recurrence finds itself more and more to the northward. In due course the total phase will supervene, and will persist during a number of recurrences until the southerly trend of the moon results in the uncovering of the lunar surface at the south. Thus, as the boundary of the shadow is left more and more to the northward, the illuminated portion on the southern side of the moon becomes at each recurrence greater and the darkened portion on the northern side less, until the shadow eventually passes off at the north.

      The "life" of an eclipse of the moon happens, for certain reasons, to be much shorter than that of an eclipse of the sun. It lasts during only about 860 years, or 48 saros returns.

      Fig. 6, p. 81, is a map of the world on Mercator's Projection, showing a portion of the march of the total solar eclipse of August 30, 1905, across the surface of the earth. The projection in question has been employed because it is the one with which people are most familiar. This eclipse began by striking the neighbourhood of the North Pole in the guise of a partial eclipse during the latter part of the reign of Queen Elizabeth, and became total on the earth for the first time on the 24th of June 1797. Its next appearance was on the 6th of July 1815. It has not been possible to show the tracks of totality of these two early visitations on account of the distortion of the polar regions consequent on the fiction of Mercator's Projection. It is therefore made to commence with the track of its third appearance, viz. on July 17, 1833. In consequence of those variations in the apparent sizes of the sun and moon, which result, as we have seen, from the variations in their distances from the earth, this eclipse will change from a total into an annular eclipse towards the end of the twenty-first century. By that time the track will have passed to the southern side of the equator. The track will eventually leave the earth near the South Pole about the beginning of the twenty-sixth century, and the rear portion of the partial shadow will in its turn be clear of the terrestrial surface by about 2700 A.D., when the series comes to an end.

      

Fig. 6.—Map of the World on Mercator's Projection, showing a portion of the progress of the Total Solar Eclipse of August 30, 1905, across the surface of the earth.

[4] Astronomical Essays (p. 40), London, 1907.

[5] In some cases the periods between the dates of the corresponding eclipses appear to include a greater number of days than ten; but this is easily explained when allowance is made for intervening leap years (in each of which an extra day has of course been added), and also for variations in local time.

CHAPTER VIII

       Table of Contents

      FAMOUS ECLIPSES OF THE SUN

      What is thought to be the earliest reference to an eclipse comes down to us from the ancient Chinese records, and is over four thousand years old. The eclipse in question was a solar one, and occurred, so far as can be ascertained, during the twenty-second century B.C. The story runs that the two state astronomers, Ho and Hi by name, being exceedingly intoxicated, were unable to perform their required duties, which consisted in superintending the customary rites of beating drums, shooting arrows, and the like, in order to frighten away the mighty dragon which it was believed was about to swallow up the Lord of Day. This eclipse seems to have been only partial; nevertheless a great turmoil ensued, and the two astronomers were put to death, no doubt with the usual celestial cruelty.

      The next eclipse mentioned in the Chinese annals is also a solar eclipse, and appears to have taken place more than a thousand years later, namely in 776 B.C. Records of similar eclipses follow from the same source; but as they are mere notes of the events, and do not enter into any detail, they are of little interest. Curiously enough the Chinese have taken practically no notice of eclipses of the moon, but have left us a comparatively careful record of comets, which has been of value to modern astronomy.

      The earliest mention of a total eclipse of the sun (for it should be noted that the ancient Chinese eclipse above-mentioned was merely partial) was deciphered in 1905, on a very ancient Babylonian tablet, by Mr. L.W. King of the British Museum. This eclipse took place in the year 1063 B.C.

      Assyrian tablets record three solar eclipses which occurred between three and four hundred years later than this. The first of these was in 763 B.C.; the total phase being visible near Nineveh.

      The next record of an eclipse of the sun comes to us from a Grecian source. This eclipse took place in 585 B.C., and has been the subject of much investigation. Herodotus, to whom we are indebted for the account, tells us that it occurred during a battle in a war which had been waging for some years between the Lydians and Medes. The sudden coming on of darkness led to a termination of the contest, and peace was afterwards made between the combatants. The historian goes on to state that the eclipse had been foretold by Thales, who is looked upon as the Founder of Grecian astronomy. This eclipse is in consequence known as the "Eclipse of Thales." It would seem as if that philosopher were acquainted with the Chaldean saros.

      The next solar eclipse worthy of note was an annular one, and occurred in 431 B.C., the first year of the Peloponnesian War. Plutarch relates that the pilot of the ship, which was about to convey Pericles to the Peloponnesus, was very much frightened by it; but Pericles calmed him by holding up a cloak before his eyes, and saying that the only difference between this and the eclipse was that something larger than the cloak prevented his seeing the sun for the time being.

      An eclipse of great historical interest is that known as the "Eclipse of Agathocles," which occurred on the morning of the 15th of August, 310 B.C. Agathocles, Tyrant of Syracuse, had been blockaded in the harbour of that town by the Carthaginian fleet, but effected the escape of his squadron under cover of night, and sailed for Africa in order to invade the enemy's territory. During the following day he and his vessels experienced a total eclipse, in which "day wholly put on the appearance of night, and the stars were seen in all parts of the sky."

      A few solar eclipses are supposed to be