Astronomical Curiosities Part 21

Cygni, he says, "The stars here also are remarkably uniform in size."[459]

Eastman's results for parallax seem to show that "the fainter rather than the brighter stars are nearest to our system." But this apparent paradox is considered by Mr. Monck to be very misleading;[460] and the present writer holds the same opinion.

Prof. Kapteyn finds "that stars whose proper motions exceed 0"05 are not more numerous in the Milky Way than in other parts of the sky; or, in other words, if only the stars having proper motions of 0"05 or upwards were mapped, there would be no aggregation of stars showing the existence of the Milky Way."[461]

With reference to the number of stars visible on photographs, the late Dr.

Isaac Roberts says--

"So far as I am able at present to judge, under the atmospheric conditions prevalent in this country, the limit of the photographic method of delineation will be reached at stellar, or nebular, light of the feebleness of about 18th-magnitude stars. The reason for this inference is that the general illumination of the atmosphere by starlight concentrated upon a film by the instrument will mask the light of objects that are fainter than about 18th-magnitude stars."[462]

With reference to blank s.p.a.ces in the sky, the late Mr. Norman Pogson remarked--

"Near S Ophiuchi we find one of the most remarkable vacuities in this hemisphere--an elliptic s.p.a.ce of about 65' in length in the direction of R.A., and 40' in width, in which there exists _no_ star larger than the 13th magnitude ... it is impossible to turn a large telescope in that direction and, if I may so express it, view such black darkness, without a feeling that we are here searching into the remote regions of s.p.a.ce, far beyond the limits of our own sidereal system."[463]

Prof. Barnard describes some regions in the constellation Taurus containing "dark lanes" in a groundwork of faint nebulosity. He gives two beautiful photographs of the regions referred to, and says that the dark holes and lanes are apparently darker than the sky in the immediate vicinity. He says, "A very singular feature in this connection is that the stars also are absent in general from the lanes." A close examination of these photographs has given the present writer the impression that the dark lanes and spots are _in_ the nebulosity, and that the nebulosity is mixed up with the stars. This would account for the fact that the stars are in general absent from the dark lanes. For if there is an intimate relation between the stars and the nebulosity, it would follow that where there is no nebulosity in this particular region there would be no stars.

Prof. Barnard adds that the nebulosity is easily visible in a 12-inch telescope.[464]

With reference to the life of the universe, Prof. F. R. Moulton well says--

"The lifetime of a man seems fairly long, and the epoch when Troy was besieged, or when the Pharaohs piled up the pyramids in the valley of the Nile, or when our ancestors separated on the high plateaux of Asia, seems extremely remote, but these intervals are only moments compared to the immense periods required for geological evolutions and the enormously greater ones consumed in the developement of worlds from widely extended nebulous ma.s.ses. We recognize the existence of only those forces whose immediate consequences are appreciable, and it may be that those whose effects are yet unseen are really of the highest importance. A little creature whose life extended over only two or three hours of a summer's day might be led, if he were sufficiently endowed with intelligence, to infer that pa.s.sing clouds were the chief influence at work in changing the climate instead of perceiving that the sun's slow motion across the sky would bring on the night and its southward motion the winter."[465]

In a review of my book _Astronomical Essays_ in _The Observatory_, September, 1907, the following words occur. They seem to form a good and sufficient answer to people who ask, What is there beyond our visible universe? "If the stellar universe is contained in a sphere of say 1000 stellar units radius, what is there beyond? To this the astronomer will reply that theories and hypotheses are put forward for the purpose of explaining observed facts; when there are no facts to be explained, no theory is required. As there are no observed facts as to what exists beyond the farthest stars, the mind of the astronomer is a complete blank on the subject. Popular imagination can fill up the blank as it pleases."

With these remarks I fully concur.

In his address to the British a.s.sociation, Prof. G. H. Darwin (now Sir George Darwin) said--

"Man is but a microscopic being relatively to astronomical s.p.a.ce, and he lives on a puny planet circling round a star of inferior rank. Does it not, then, seem futile to imagine that he can discover the origin and tendency of the Universe as to expect a housefly to instruct us as to the theory of the motions of the planets? And yet, so long as he shall last, he will pursue his search, and will no doubt discover many wonderful things which are still hidden. We may indeed be amazed at all that man has been able to find out, but the immeasurable magnitude of the undiscovered will throughout all time remain to humble his pride. Our children's children will still be gazing and marvelling at the starry heavens, but the riddle will never be read."

The ancient philosopher Lucretius said--

"Globed from the atoms falling slow or swift I see the suns, I see the systems lift Their forms; and even the system and the suns Shall go back slowly to the eternal drift."[466]

But it has been well said that the structure of the universe "has a fascination of its own for most readers quite apart from any real progress which may be made towards its solution."[467]

The Milky Way itself, Mr. Stratonoff considers to be an agglomeration of immense condensations, or stellar clouds, which are scattered round the region of the galactic equator. These clouds, or ma.s.ses of stars, sometimes leave s.p.a.ces between them, and sometimes they overlap, and in this way he accounts for the great rifts, like the Coal Sack, which allow us to see through this great circle of light. He finds other condensations of stars; the nearest is one of which our sun is a member, chiefly composed of stars of the higher magnitudes which "thin out rapidly as the Milky Way is approached." There are other condensations: one in stars of magnitudes 65 to 85; and a third, farther off, in stars of magnitudes 76 to 8. These may be called opera-gla.s.s, or field-gla.s.s stars.

Stratonoff finds that stars with spectra of the first type (cla.s.s A, B, C, and D of Harvard) which include the Sirian and Orion stars, are princ.i.p.ally situated near the Milky Way, while those of type II. (which includes the solar stars) "are princ.i.p.ally condensed in a region coinciding roughly with the terrestrial pole, and only show a slight increase, as compared with other stars, as the galaxy is approached."[468]

Prof. Kapteyn thinks that "undoubtedly one of the greatest difficulties, if not the greatest of all, in the way of obtaining an understanding of the real distribution of the stars in s.p.a.ce, lies in our uncertainty about the amount of loss suffered by the light of the stars on its way to the observer."[469] He says, "There can be little doubt in my opinion, about the existence of absorption in s.p.a.ce, and I think that even a good guess as to the order of its amount can be made. For, first we know that s.p.a.ce contains an enormous ma.s.s of meteoric matter. This matter must necessarily intercept some part of the star-light."

This absorption, however, seems to be comparatively small. Kapteyn finds a value of 0016 (about 1/60th) of a magnitude for a star at a distance corresponding to a parallax of one-tenth of a second (about 33 "light years"). This is a quant.i.ty almost imperceptible in the most delicate photometer. But for very great distances--such as 3000 "light years"--the absorption would evidently become very considerable, and would account satisfactorily for the gradual "thinning out" of the fainter stars. If this were fully proved, we should have to consider the fainter stars of the Milky Way to be in all probability fairly large suns, the light of which is reduced by absorption.

That some of the ancients knew that the Milky Way is composed of stars is shown by the following lines translated from Ovid:--

"A way there is in heaven's extended plain Which when the skies are clear is seen below And mortals, by the name of Milky, know; The groundwork is of stars, through which the road Lies open to great Jupiter's abode."[470]

From an examination of the distribution of the faint stars composing the Milky Way, and those shown in Argelander's charts of stars down to the 9 magnitude, Easton finds that there is "a real connection between the distribution of 9th and 10th magnitude stars, and that of the faint stars of the Milky Way, and that consequently the faint or very faint stars of the galactic zone are at a distance which does not greatly exceed that of the 9th and 10th magnitude stars."[471] A similar conclusion was, I think, arrived at by Proctor many years ago. Now let us consider the meaning of this result. Taking stars of the 15th magnitude, if their faintness were merely due to greater distance, their actual brightness--if of the same size--would imply that they are at 10 times the distance of stars of the 10th magnitude. But if at the same distance from us, a 10th magnitude star would be 100 times brighter than a 15th magnitude star, and if of the same density and "intrinsic brightness" (or luminosity of surface) the 10th magnitude would have 10 times the diameter of the fainter star, and hence its volume would be 1000 times greater (10{3}), and this great difference is not perhaps improbable.

The const.i.tution of the Milky Way is not the same in all its parts. The bright spot between and ? Cygni is due to relatively bright stars.

Others equally dense but fainter regions in Auriga and Monoceros are only evident in stars of the 8th and 9th magnitude, and the light of the well-known luminous spot in "Sobieski's Shield," closely south of ?

Aquilae, is due to stars below magnitude 9.

The correspondence in distribution between the stars of Argelander's charts and the fainter stars of the Milky Way shows, as Easton points out, that Herschel's hypothesis of a uniform distribution of stars of approximately equal size is quite untenable.

It has been suggested that the Milky Way may perhaps form a ring of stars with the sun placed nearly, but not exactly, in the centre of the ring.

But were it really a ring of uniform width with the sun eccentrically placed within it, we should expect to find the Milky Way wider at its nearest part, and gradually narrowing towards the opposite point. Now, Herschel's "gages" and Celoria's counts show that the Galaxy is wider in Aquila than in Monoceros. This is confirmed by Easton, who says, "_for the faint stars taken as a whole, the Milky Way is widest in its brightest part_" (the italics are Easton's). From this we should conclude that the Milky Way is nearer to us in the direction of Aquila than in that of Monoceros. Sir John Herschel suggested that the southern parts of the galactic zone are nearer to us on account of their greater _brightness_ in those regions.[472] But greater width is a safer test of distance than relative brightness. For it may be easily shown than the _intrinsic_ brightness of an area containing a large number of stars would be the same for _all_ distances (neglecting the supposed absorption of light in s.p.a.ce). For suppose any given area crowded with stars to be removed to a greater distance. The light of each star would be diminished inversely as the square of the distance. But the given area would also be diminished _directly_ as the square of the distance, so we should have a diminished amount of light on an equally diminished area, and hence the intrinsic brightness, or luminosity of the area per unit of surface, would remain unaltered. The increased brightness of the Milky Way in Aquila is accounted for by the fact that Herschel's "gages" show an increased number of stars, and hence the brightness in Aquila and Sagittarius does not necessarily imply that the Milky Way is nearer to us in those parts, but that it is richer in small stars than in other regions.

Easton is of opinion that the annular hypothesis of the Milky Way is inconsistent with our present knowledge of the galactic phenomena, and he suggests that its actual const.i.tution resembles more that of a spiral nebula.[473] On this hypothesis the increase in the number of stars in the regions above referred to may be due to our seeing one branch of the supposed "two-branched spiral" projected on another branch of the same spiral. This seems supported by Sir John Herschel's observations in the southern hemisphere, where he found in some places "a tissue as it were of large stars spread over another of very small ones, the immediate magnitudes being wanting." Again, portions of the spiral branches may be richer than others, as photographs of spiral nebulae seem to indicate.

Celoria, rejecting the hypothesis of a single ring, suggests the existence of _two_ galactic rings inclined to each other at an angle of about 20, one of these including the brighter stars, and the other the fainter. But this seems to be a more artificial arrangement then the hypothesis of a spiral. Further, the complicated structure of the Milky Way cannot be well explained by Celoria's hypothesis of two distinct rings one inside the other. From a.n.a.logy the spiral hypothesis seems much more probable.

Considering the Milky Way to represent a colossal spiral nebula viewed from a point not far removed from the centre of the spiral branches, Easton suggests that the bright region between and ? Cygni, which is very rich in comparatively bright stars, may possibly represent the "_central acc.u.mulations of the Milky Way_," that is, the portion corresponding to the nucleus of a spiral nebula. If this be so, this portion of the Milky Way should be nearer to us than others. Easton also thinks that the so-called "solar cl.u.s.ter" of Gould, Kapteyn, and Schiaparelli may perhaps be "the expression of the central condensation of the galactic system itself, composed of the most part of suns comparable with our own, and which would thus embrace most of the bright stars to the 9th or 10th magnitude. The distance of the galactic streams and convolutions would thus be comparable with the distances of these stars." He thinks that the sun lies within a gigantic spiral, "in a comparatively spa.r.s.e region between the central nucleus and Orion."

Scheiner thinks that "the irregularities of the Milky Way, especially in streams, can be quite well accounted for, as Easton has attempted to do, if they are regarded as a system of spirals, and not as a ring system."

Evidence in favour of the spiral hypothesis of the Milky Way, as advocated by Easton and Scheiner, may be found in Kapteyn's researches on the proper motions of the stars. This eminent astronomer finds that stars with measurable proper motions--and therefore in all probability relatively near the earth--have mostly spectra of the solar type, and seem to cl.u.s.ter round "a point adjacent to the sun, in total disregard to the position of the Milky Way," and that stars with little or no proper motion collect round the galactic plain. He is also of opinion that the Milky Way resembles the Andromeda nebula, "the globular nucleus representing the solar cl.u.s.ter, and the far spreading wings or whorls the compressed layer of stars enclosed by the rings of the remote Galaxy."

With reference to the plurality of inhabited worlds, it has been well said by the ancient writer Metrodorus (third century B.C.), "The idea that there is but a single world in all infinitude would be as absurd as to suppose that a vast field had been formed to produce a single blade of wheat."[474] With this opinion the present writer fully concurs.

CHAPTER XXI

General

The achievements of Hipparchus in astronomy were very remarkable, considering the age in which he lived. He found the amount of the apparent motion of the stars due to the precession of the equinoxes (of which he was the discoverer) to be 59" per annum. The correct amount is about 50".

He measured the length of the year to within 9 minutes of its true value.

He found the inclination of the ecliptic to the plane of the equator to be 23 51'. It was then 23 46'--as we now know by modern calculations--so that Hipparchus' estimation was a wonderfully close approximation to the truth. He computed the moon's parallax to be 57', which is about its correct value. He found the eccentricity of the sun's apparent orbit round the earth to be one twenty-fourth, the real value being then about one-thirteenth. He determined other motions connected with the earth and moon; and formed a catalogue of 1080 stars. All this work has earned for him the well-merited t.i.tle of "The Father of Astronomy."[475]

The following is a translation of a Greek pa.s.sage ascribed to Ptolemy: "I know that I am mortal and the creature of a day, but when I search out the many rolling circles of the stars, my feet touch the earth no longer, but with Zeus himself I take my fill of ambrosia, the food of the G.o.ds."[476]

This was inscribed (in Greek) on a silver loving cup presented to the late Professor C. A. Young, the famous American astronomer.[477]

Some curious and interesting phenomena are recorded in the old Chinese Annals, which go back to a great antiquity. In 687 B.C. "a night" is mentioned "without clouds and without stars" (!) This may perhaps refer to a total eclipse of the sun; but if so, the eclipse is not mentioned in the Chinese list of eclipses. In the year 141 B.C., it is stated that the sun and moon appeared of a deep red colour during 5 days, a phenomenon which caused great terror among the people. In 74 B.C., it is related that a star as large as the moon appeared, and was followed in its motion by several stars of ordinary size. This probably refers to an unusually large "bolide" or "fireball." In 38 B.C., a fall of meteoric stones is recorded "of the size of a walnut." In A.D. 88, another fall of stones is mentioned. In A.D. 321, sun-spots were visible to the naked eye.

Homer speaks of a curious darkness which occurred during one of the great battles in the last year of the Trojan war. Mr. Stockwell identifies this with an eclipse of the sun which took place on August 28, 1184 B.C. An eclipse referred to by Thucydides as having occurred during the first year of the Peloponnesian War, when the darkness was so great that some stars were seen, is identified by Stockwell with a total eclipse of the sun, which took place on August 2, 430 B.C.

A great eclipse of the sun is supposed to have occurred in the year 43 or 44 B.C., soon after the death of Julius Caesar. Baron de Zach and Arago mention it as the first annular eclipse on record. But calculations show that no solar eclipse whatever, visible in Italy, occurred in either of these years. The phenomenon referred to must therefore have been of atmospherical origin, and indeed this is suggested by a pa.s.sage in Suetonius, one of the authors quoted on the subject.

M. Guillaume thinks that the ninth Egyptian plague, the thick "darkness"

(Exodus x. 21-23), may perhaps be explained by a total eclipse of the sun which occurred in 1332 B.C. It is true that the account states that the darkness lasted "three days," but this, M. Guillaume thinks, may be due to an error in the translation.[478] This explanation, however, seems very improbable.

According to Hind, the moon was eclipsed on the generally received date of the Crucifixion, A.D. 33, April 3. He says, "I find she had emerged from the earth's dark shadow a quarter of an hour before she rose at Jerusalem (6{h} 36{m} p.m.); but the penumbra continued upon her disc for an hour afterwards." An eclipse could not have had anything to do with the "darkness over all the land" during the Crucifixion. For this lasted for three hours, and the totality of a solar eclipse can only last a few minutes at the most. As a matter of fact the "eclipse of Phlegon," a partial one (A.D. 29, November 24) was "the only solar eclipse that could have been visible in Jerusalem during the period usually fixed for the ministry of Christ."

It is mentioned in the Anglo-Saxon Chronicle that a total eclipse of the sun took place in the year after King Alfred's great battle with the Danes. Now, calculation shows that this eclipse occurred on October 29, 878 A.D. King Alfred's victory over the Danes must, therefore, have taken place in 877 A.D., and his death probably occurred in 899 A.D. This solar eclipse is also mentioned in the Annals of Ulster. From this it will be seen that in some cases the dates of historical events can be accurately fixed by astronomical phenomena.

It is stated by some historians that an eclipse of the sun took place on the morning of the battle of Crecy, August 26, 1346. But calculation shows that there was no eclipse of the sun visible in England in that year. At the time of the famous battle the moon had just entered on her first quarter, and she was partially eclipsed six days afterwards--that is on the 1st of September. The mistake seems to have arisen from a mistranslation of the old French word _esclistre_, which means lightning.

This was mistaken for _esclipse_. The account seems to indicate that there was a heavy thunderstorm on the morning of the battle.