Astronomy of To-day Part 6

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Of very great influence in the growth of our knowledge with regard to the sun, is the remarkable piece of good fortune by which the countries around the Mediterranean, so easy of access, have been favoured with a comparatively large number of total eclipses during the past sixty years. Tracks of totality have, for instance, traversed the Spanish peninsula on no less than five occasions during that period. Two of these are among the most notable eclipses of recent years, namely, those of May 28, 1900, and of August 30, 1905. In the former the track of totality stretched from the western seaboard of Mexico, through the Southern States of America, and across the Atlantic Ocean, after which it pa.s.sed over Portugal and Spain into North Africa. The total phase lasted for about a minute and a half, and the eclipse was well observed from a great many points along the line. A representation of the corona, as it appeared on this occasion, will be found on Plate VII. (B), p.

142.

The track of the other eclipse to which we have alluded, _i.e._ that of August 30, 1905, crossed Spain about 200 miles to the northward of that of 1900. It stretched from Winnipeg in Canada, through Labrador, and over the Atlantic; then traversing Spain, it pa.s.sed across the Balearic Islands, North Africa, and Egypt, and ended in Arabia (see Fig. 6, p.

81). Much was to be expected from a comparison between the photographs taken in Labrador and Egypt on the question as to whether the corona would show any alteration in shape during the time that the shadow was traversing the intervening s.p.a.ce--some 6000 miles. The duration of the total phase in this eclipse was nearly four minutes. Bad weather, however, interfered a good deal with the observations. It was not possible, for instance, to do anything at all in Labrador. In Spain the weather conditions were by no means favourable; though at Burgos, where an immense number of people had a.s.sembled, the total phase was, fortunately, well seen. On the whole, the best results were obtained at Guelma in Algeria. The corona on the occasion of this eclipse was a very fine one, and some magnificent groups of prominences were plainly visible to the naked eye (see the Frontispiece).

The next total eclipse after that of 1905 was one which occurred on January 14, 1907. It pa.s.sed across Central Asia and Siberia, and had a totality lasting two and a half minutes at most; but it was not observed as the weather was extremely bad, a circ.u.mstance not surprising with regard to those regions at that time of year.

The eclipse of January 3, 1908, pa.s.sed across the Pacific Ocean. Only two small coral islands--Hull Island in the Phoenix Group, and Flint Island about 400 miles north of Tahiti--lay in the track. Two expeditions set out to observe it, _i.e._ a combined American party from the Lick Observatory and the Smithsonian Inst.i.tution of Was.h.i.+ngton, and a private one from England under Mr. F.K. McClean. As Hull Island afforded few facilities, both parties installed their instruments on Flint Island, although it was very little better. The duration of the total phase was fairly long--about four minutes, and the sun very favourably placed, being nearly overhead. Heavy rain and clouds, however, marred observation during the first minute of totality, but the remaining three minutes were successfully utilised, good photographs of the corona being obtained.

The next few years to come are unfortunately by no means favourable from the point of view of the eclipse observer. An eclipse will take place on June 17, 1909, the track stretching from Greenland across the North Polar regions into Siberia. The geographical situation is, however, a very awkward one, and totality will be extremely short--only six seconds in Greenland and twenty-three seconds in Siberia.

The eclipse of May 9, 1910, will be visible in Tasmania. Totality will last so long as four minutes, but the sun will be at the time much too low in the sky for good observation.

The eclipse of the following year, April 28, 1911, will also be confined, roughly speaking, to the same quarter of the earth, the track pa.s.sing across the old convict settlement of Norfolk Island, and then out into the Pacific.

The eclipse of April 17, 1912, will stretch from Portugal, through France and Belgium into North Germany. It will, however, be of practically no service to astronomy. Totality, for instance, will last for only three seconds in Portugal; and, though Paris lies in the central track, the eclipse, which begins as barely total, will have changed into an _annular_ one by the time it pa.s.ses over that city.

The first really favourable eclipse in the near future will be that of August 21, 1914. Its track will stretch from Greenland across Norway, Sweden, and Russia. This eclipse is a return, after one saros, of the eclipse of August 9, 1896.

The last solar eclipse which we will touch upon is that predicted for June 29, 1927. It has been already alluded to as the first of those in the future to be _total_ in England. The central line will stretch from Wales in a north-easterly direction. Stonyhurst Observatory, in Lancas.h.i.+re, will lie in the track; but totality there will be very short, only about twenty seconds in duration.

[6] _Knowledge_, vol. xx. p. 9, January 1897.

[7] The _first photographic representation of the corona_ had, however, been made during the eclipse of 1851. This was a daguerreotype taken by Dr. Busch at Konigsberg in Prussia.

CHAPTER IX

FAMOUS ECLIPSES OF THE MOON

The earliest lunar eclipse, of which we have any trustworthy information, was a total one which took place on the 19th March, 721 B.C., and was observed from Babylon. For our knowledge of this eclipse we are indebted to Ptolemy, the astronomer, who copied it, along with two others, from the records of the reign of the Chaldean king, Merodach-Baladan.

The next eclipse of the moon worth noting was a total one, which took place some three hundred years later, namely, in 425 B.C. This eclipse was observed at Athens, and is mentioned by Aristophanes in his play, _The Clouds_.

Plutarch relates that a total eclipse of the moon, which occurred in 413 B.C., so greatly frightened Nicias, the general of the Athenians, then warring in Sicily, as to cause a delay in his retreat from Syracuse which led to the destruction of his whole army.

Seven years later--namely, in 406 B.C., the twenty-sixth year of the Peloponnesian War--there took place another total lunar eclipse of which mention is made by Xenophon.

Omitting a number of other eclipses alluded to by ancient writers, we come to one recorded by Josephus as having occurred a little before the death of Herod the Great. It is probable that the eclipse in question was the total lunar one, which calculation shows to have taken place on the 15th September 5 B.C., and to have been visible in Western Asia.

This is very important, for we are thus enabled to fix that year as the date of the birth of Christ, for Herod is known to have died in the early part of the year following the Nativity.

In those accounts of total lunar eclipses, which have come down to us from the Dark and Middle Ages, the colour of the moon is nearly always likened to "blood." On the other hand, in an account of the eclipse of January 23, A.D. 753, our satellite is described as "covered with a horrid black s.h.i.+eld." We thus have examples of the two distinct appearances alluded to in Chapter VII., _i.e._ when the moon appears of a coppery-red colour, and when it is entirely darkened.

It appears, indeed, that, in the majority of lunar eclipses on record, the moon has appeared of a ruddy, or rather of a coppery hue, and the details on its surface have been thus rendered visible. One of the best examples of a _bright_ eclipse of this kind is that of the 19th March 1848, when the illumination of our satellite was so great that many persons could not believe that an eclipse was actually taking place. A certain Mr. Foster, who observed this eclipse from Bruges, states that the markings on the lunar disc were almost as visible as on an "ordinary dull moonlight night." He goes on to say that the British Consul at Ghent, not knowing that there had been any eclipse, wrote to him for an explanation of the red colour of the moon on that evening.

Out of the _dark_ eclipses recorded, perhaps the best example is that of May 18, 1761, observed by Wargentin at Stockholm. On this occasion the lunar disc is said to have disappeared so completely, that it could not be discovered even with the telescope. Another such instance is the eclipse of June 10, 1816, observed from London. The summer of that year was particularly wet--a point worthy of notice in connection with the theory that these different appearances are due to the varying state of our earth's atmosphere.

Sometimes, indeed, it has happened that an eclipse of the moon has partaken of both appearances, part of the disc being visible and part invisible. An instance of this occurred in the eclipse of July 12, 1870, when the late Rev. S.J. Johnson, one of the leading authorities on eclipses, who observed it, states that he found one-half the moon's surface quite invisible, both with the naked eye and with the telescope.

In addition to the examples given above, there are three total lunar eclipses which deserve especial mention.

1. A.D. 755, November 23. During the progress of this eclipse the moon occulted the star Aldebaran in the constellation of Taurus.

2. A.D. 1493, April 2. This is the celebrated eclipse which is said to have so well served the purposes of Christopher Columbus. Certain natives having refused to supply him with provisions when in sore straits, he announced to them that the moon would be darkened as a sign of the anger of heaven. When the event duly came to pa.s.s, the savages were so terrified that they brought him provisions as much as he needed.

3. A.D. 1610, July 6. The eclipse in question is notable as having been seen through the telescope, then a recent invention. It was without doubt the first so observed, but unfortunately the name of the observer has not come down to us.

CHAPTER X

THE GROWTH OF OBSERVATION

The earliest astronomical observations must have been made in the Dawn of Historic Time by the men who tended their flocks upon the great plains. As they watched the clear night sky they no doubt soon noticed that, with the exception of the moon and those brilliant wandering objects known to us as the planets, the individual stars in the heaven remained apparently fixed with reference to each other. These seemingly changeless points of light came in time to be regarded as sign-posts to guide the wanderer across the trackless desert, or the voyager upon the wide sea.

Just as when looking into the red coals of a fire, or when watching the clouds, our imagination conjures up strange and grotesque forms, so did the men of old see in the grouping of the stars the outlines of weird and curious shapes. Fed with mythological lore, they imagined these to be rough representations of ancient heroes and fabled beasts, whom they supposed to have been elevated to the heavens as a reward for great deeds done upon the earth. We know these groupings of stars to-day under the name of the Constellations. Looking up at them we find it extremely difficult to fit in the majority with the figures which the ancients believed them to represent. Nevertheless, astronomy has accepted the arrangement, for want of a better method of fixing the leading stars in the memory.

Our early ancestors lived the greater part of their lives in the open air, and so came to pay more attention in general to the heavenly orbs than we do. Their clock and their calendar was, so to speak, in the celestial vault. They regulated their hours, their days, and their nights by the changing positions of the sun, the moon, and the stars; and recognised the periods of seed-time and harvest, of calm and stormy weather, by the rising or setting of certain well-known constellations.

Students of the cla.s.sics will recall many allusions to this, especially in the Odes of Horace.

As time went on and civilisation progressed, men soon devised measuring instruments, by means of which they could note the positions of the celestial bodies in the sky with respect to each other; and, from observations thus made, they constructed charts of the stars. The earliest complete survey of this kind, of which we have a record, is the great Catalogue of stars which was made, in the second century B.C., by the celebrated Greek astronomer, Hipparchus, and in which he is said to have noted down about 1080 stars.

It is unnecessary to follow in detail the tedious progress of astronomical discovery prior to the advent of the telescope. Certain it is that, as time went on, the measuring instruments to which we have alluded had become greatly improved; but, had they even been perfect, they would have been utterly inadequate to reveal those minute displacements, from which we have learned the actual distance of the nearest of the celestial orbs. From the early times, therefore, until the mediaeval period of our own era, astronomy grew up upon a faulty basis, for the earth ever seemed so much the largest body in the universe, that it continued from century to century to be regarded as the very centre of things.

To the Arabians is due the credit of having kept alive the study of the stars during the dark ages of European history. They erected some fine observatories, notably in Spain and in the neighbourhood of Bagdad.

Following them, some of the Oriental peoples embraced the science in earnest; Ulugh Beigh, grandson of the famous Tamerlane, founding, for instance, a great observatory at Samarcand in Central Asia. The Mongol emperors of India also established large astronomical instruments in the chief cities of their empire. When the revival of learning took place in the West, the Europeans came to the front once more in science, and rapidly forged ahead of those who had so a.s.siduously kept alight the lamp of knowledge through the long centuries.

The dethronement of the older theories by the Copernican system, in which the earth was relegated to its true place, was fortunately soon followed by an invention of immense import, the invention of the Telescope. It is to this instrument, indeed, that we are indebted for our knowledge of the actual scale of the celestial distances. It penetrated the depths of s.p.a.ce; it brought the distant orbs so near, that men could note the detail on the planets, or measure the small changes in their positions in the sky which resulted from the movement of our own globe.

It was in the year 1609 that the telescope was first constructed. A year or so previous to this a spectacle-maker of Middleburgh in Holland, one Hans Lippershey, had, it appears, hit upon the fact that distant objects, when viewed through certain gla.s.s lenses suitably arranged, looked nearer.[8] News of this discovery reached the ears of Galileo Galilei, of Florence, the foremost philosopher of the day, and he at once applied his great scientific attainments to the construction of an instrument based upon this principle. The result was what was called an "optick tube," which magnified distant objects some few times. It was not much larger than what we nowadays contemptuously refer to as a "spy-gla.s.s," yet its employment upon the leading celestial objects instantly sent astronomical science onward with a bound. In rapid succession Galileo announced world-moving discoveries; large spots upon the face of the sun; crater-like mountains upon the moon; four subordinate bodies, or satellites, circling around the planet Jupiter; and a strange appearance in connection with Saturn, which later telescopic observers found to be a broad flat ring encircling that planet. And more important still, the magnified image of Venus showed itself in the telescope at certain periods in crescent and other forms; a result which Copernicus is said to have announced should of necessity follow if his system were the true one.

The discoveries made with the telescope produced, as time went on, a great alteration in the notions of men with regard to the universe at large. It must have been, indeed, a revelation to find that those points of light which they called the planets, were, after all, globes of a size comparable with the earth, and peopled perchance with sentient beings. Even to us, who have been accustomed since our early youth to such an idea, it still requires a certain stretch of imagination to enlarge, say, the Bright Star of Eve, into a body similar in size to our earth. The reader will perhaps recollect Tennyson's allusion to this in _Locksley Hall, Sixty Years After_:--

"Hesper--Venus--were we native to that splendour or in Mars, We should see the Globe we groan in, fairest of their evening stars.

"Could we dream of wars and carnage, craft and madness, l.u.s.t and spite, Roaring London, raving Paris, in that point of peaceful light?"

The form of instrument as devised by Galileo is called the Refracting Telescope, or "Refractor." As we know it to-day it is the same in principle as his "optick tube," but it is not quite the same in construction. The early _object-gla.s.s_, or large gla.s.s at the end, was a single convex lens (see Fig. 8, p. 113, "Galilean"); the modern one is, on the other hand, composed of two lenses fitted together. The attempts to construct large telescopes of the Galilean type met in course of time with a great difficulty. The magnified image of the object observed was not quite pure; its edges, indeed, were fringed with rainbow-like colours. This defect was found to be aggravated with increase in the size of object-gla.s.ses. A method was, however, discovered of diminis.h.i.+ng this colouration, or _chromatic aberration_ as it is called from the Greek word [chroma] (_chroma_), which means colour, viz. by making telescopes of great length and only a few inches in width. But the remedy was, in a way, worse than the disease; for telescopes thus became of such huge proportions as to be too unwieldy for use. Attempts were made to evade this unwieldiness by constructing them with skeleton tubes (see Plate II., p. 110), or, indeed, even without tubes at all; the object-gla.s.s in the tubeless or "aerial" telescope being fixed at the top of a high post, and the _eye-piece_, that small lens or combination of lenses, which the eye looks directly into, being kept in line with it by means of a string and manoeuvred about near the ground (Plate III., p. 112). The idea of a telescope without a tube may appear a contradiction in terms; but it is not really so, for the tube adds nothing to the magnifying power of the instrument, and is, in fact, no more than a mere device for keeping the object-gla.s.s and eye-piece in a straight line, and for preventing the observer from being hindered by stray lights in his neighbourhood. It goes without saying, of course, that the image of a celestial object will be more clear and defined when examined in the darkness of a tube.

The ancients, though they knew nothing of telescopes, had, however, found out the merit of a tube in this respect; for they employed simple tubes, blackened on the inside, in order to obtain a clearer view of distant objects. It is said that Julius Caesar, before crossing the Channel, surveyed the opposite coast of Britain through a tube of this kind.