Astronomical Curiosities: Facts and Fallacies
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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 masses of stars, sometimes leave spaces 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 6·5 to 8·5; and a third, farther off, in stars of magnitudes 7·6 to 8. These may be called opera-glass, or field-glass stars.
Stratonoff finds that stars with spectra of the first type (class A, B, C, and D of Harvard) which include the Sirian and Orion stars, are principally situated near the Milky Way, while those of type II. (which includes the solar stars) “are principally 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.”466
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 space, lies in our uncertainty about the amount of loss suffered by the light of the stars on its way to the observer.”467 He says, “There can be little doubt in my opinion, about the existence of absorption in space, and I think that even a good guess as to the order of its amount can be made. For, first we know that space contains an enormous mass 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 0·016 (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 quantity 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 plainWhich when the skies are clear is seen belowAnd mortals, by the name of Milky, know;The groundwork is of stars, through which the roadLies open to great Jupiter’s abode.”468From 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 9½ 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.”469 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 (103), and this great difference is not perhaps improbable.
The constitution of the Milky Way is not the same in all its parts. The bright spot between β and γ 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 λ Aquilæ, is due to stars below magnitude 9½.
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.470 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 space). 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 constitution resembles more that of a spiral nebula.471 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 nebulæ 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 analogy 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 β and γ Cygni, which is very rich in comparatively bright stars, may possibly represent the “central accumulations 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 cluster” 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 sparse 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 cluster 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 cluster, 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.”472 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 title of “The Father of Astronomy.”473
The following is a translation of a Greek passage 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 gods.”474 This was inscribed (in Greek) on a silver loving cup presented to the late Professor C. A. Young, the famous American astronomer.475
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 Cæsar. 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 passage 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.476 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 (6h 36m 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.
A dark shade was seen on the waning moon by Messrs. Hirst and J. C. Russell on October 21, 1878, “as dark as the shadow during an eclipse of the moon.”477 If this observation is correct, it is certainly most difficult to explain. Another curious observation is recorded by Mr. E. Stone Wiggins, who says that a partial eclipse of the sun by a dark body was observed in the State of Michigan (U.S.A.) on May 16, 1884, at 7 p.m. The “moon at that moment was 12 degrees south of the equator and the sun as many degrees north of it.” The existence of a dark satellite of the earth has been suggested, but this seems highly improbable.
The sun’s corona seems to have been first noticed in the total eclipse of the sun which occurred at the death of the Roman emperor Domitian, A.D. 95. Philostratus in his Life of Apollonius says, with reference to this eclipse, “In the heavens there appeared a prodigy of this nature: a certain corona resembling the Iris surrounded the orb of the sun, and obscured its light.”478 In more modern times the corona seems to have been first noticed by Clavius during the total eclipse of April 9, 1567.479 Kepler proved that this eclipse was total, not annular, so that the ring seen by Clavius must have been the corona.
With reference to the visibility of planets and stars during total eclipses of the sun; in the eclipse of May 12, 1706, Venus, Mercury, and Aldebaran, and several other stars were seen. During the totality of the eclipse of May 3, 1715, about twenty stars were seen with the naked eye.480 At the eclipse of May 22, 1724, Venus and Mercury, and a few fixed stars were seen.481 The corona was also noticed. At the eclipse of May 2, 1733, Jupiter, the stars of the “Plough,” Capella, and other stars were visible to the naked eye; and the corona was again seen.[483]
During the total eclipses of February 9, 1766, June 24, 1778, and June 16, 1806, the corona was again noticed. But its true character was then unknown.
At the eclipse of July 8, 1842, it was noticed by observers at Lipesk that the stars Aldebaran and Betelgeuse (α Orionis), which are usually red, “appeared quite white.”482
There will be seven eclipses in the years 1917, 1935, and 1985. In the year 1935 there will be five eclipses of the sun, a rare event; and in 1985 there will be three total eclipses of the moon, a most unusual occurrence.483
Among the ancient Hindoos, the common people believed that eclipses were caused by the interposition of a monstrous demon called Raha. This absurd idea, and others equally ridiculous, were based on declarations in their sacred books, and no pious Hindoo would think of denying it.
The following cases of darkenings of the sun are given by Humboldt: —
According to Plutarch the sun remained pale for a whole year at the death of Julius Cæsar, and gave less than its usual heat.484
A sun-darkening lasting for two hours is recorded on August 22, 358 A.D., before the great earthquake of Nicomedia.
In 360 A.D. there was a sun-darkening from early morn till noon. The description given by the historians of the time corresponds to an eclipse of the sun, but the duration of the obscurity is inexplicable.
In 409 A.D., when Alaric lay siege to Rome, “there was so great a darkness that the stars were seen by day.”
In 536 A.D. the sun is said to have been darkened for a year and two months!
In 626 A.D., according to Abul Farag, half the sun’s disc was darkened for eight months!
In 934 A.D. the sun lost its brightness for two months in Portugal.
In 1090 A.D. the sun was darkened for three hours.
In 1096, sun-spots were seen with the naked eye on March 3.
In 1206 A.D. on the last day of February, “there was complete darkness for six hours, turning the day into night.” This seems to have occurred in Spain.
In 1241 the sun was so darkened that stars could be seen at 3 p.m. on Michaelmas day. This happened in Vienna.485
The sun is said to have been so darkened in the year 1547 A.D. for three days that stars were visible at midday. This occurred about the time of the battle of Mühlbergh.486
Some of these darkenings may possibly have been due to an enormous development of sun-spots; but in some cases the darkness is supposed by Chladni and Schnurrer to have been caused by “the passage of meteoric masses before the sun’s disc.”
The first observer of a transit of Venus was Jeremiah Horrocks, who observed the transit of November 24 (O.S.), 1639. He had previously corrected Kepler’s predicted time of the transit from 8h 8m a.m. at Manchester to 5h 57m p.m. At the end of 1875 a marble scroll was placed on the pedestal of the monument of John Conduitt (nephew of Sir Isaac Newton, and who adopted Horrocks’ theory of lunar motions) at the west end of the nave of Westminster Abbey, bearing this inscription from the pen of Dean Stanley —
“Ad majora avocatusquæ ob hæc parerga negligi non decuit”In Memory ofJEREMIAH HORROCKSCurate of Hoole in LancashireWho died on the 3d of January, 1641, in or near his22d yearHaving in so short a lifeDetected the long inequality in the mean motion ofJupiter and SaturnDiscovered the orbit of the Moon to be an ellipse;Determined the motion of the lunar aspe,Suggested the physical cause of its revolution;And predicted from his own observations, theTransit of VenusWhich was seen by himself and his friendWILLIAM CRABTREEOn Sunday, the 24th November (O.S.) 1639;This Tablet, facing the Monument of NewtonWas raised after the lapse of more than two centuriesDecember 9, 1874.487The transit of Venus which occurred in 1761 was observed on board ship(!) by the famous but unfortunate French astronomer Le Gentil. The ship was the frigate Sylphide, sent to the help of Pondicherry (India) which was then being besieged by the English. Owing to unfavourable winds the Sylphide was tossed about from March 25, 1761, to May 24 of the same year. When, on the later date, off the coast of Malabar, the captain of the frigate learned that Pondicherry had been captured by the English, the vessel returned to the Isle of France, where it arrived on June 23, after touching at Point de Galle on May 30. It was between these two places that Le Gentil made his observations of the transit of Venus under such unfavourable conditions. He had an object-glass of 15 feet (French) focus, and this he mounted in a tube formed of “four pine planks.” This rough instrument was fixed to a small mast set up on the quarter-deck and worked by ropes. The observations made under such curious conditions, were not, as may be imagined, very satisfactory. As another transit was to take place on June 3, 1769, Le Gentil made the heroic resolution of remaining in the southern hemisphere to observe it! This determination was duly carried out, but his devotion to astronomy was not rewarded; for on the day of the long waited for transit the sky at Pondicherry (where he had gone to observe it) was clouded over during the whole phenomenon, “although for many days previous the sky had been cloudless.” To add to his feeling of disappointment he heard that at Manilla, where he had been staying some time previously, the sky was quite clear, and two of his friends there had seen the transit without any difficulty.488 Truly the unfortunate Le Gentil was a martyr to science.
The famous German astronomer Bessel once said “that a practical astronomer could make observations of value if he had only a cart-wheel and a gun barrel”; and Watson said that “the most important part of the instrument is the person at the small end.”489
With reference to Father Hell’s supposed forgery of his observations of the transit of Venus in 1769, and Littrow’s criticism of some of the entries in Hell’s manuscript being corrected with a different coloured ink, Professor Newcomb ascertained from Weiss that Littrow was colour blind, and could not distinguish between the colour of Aldebaran and the whitest star. Newcomb adds, “For half a century the astronomical world had based an impression on the innocent but mistaken evidence of a colour-blind man respecting the tint of ink in a manuscript.”
It is recorded that on February 26, B.C. 2012, the moon, Mercury, Venus, Jupiter, and Saturn, were in the same constellation, and within 14 degrees of each other. On September 14, 1186 A.D., the sun, moon, and all the planets then known, are said to have been situated in Libra.490
In the Sanscrit epic poem, “The Ramaya,” it is stated that at the birth of Rama, the moon was in Cancer, the sun in Aries, Mercury in Taurus, Venus in Pisces, Mars in Capricornus, Jupiter in Cancer, and Saturn in Libra. From these data, Mr. Walter R. Old has computed that Rama was born on February 10, 1761 B.C.491
A close conjunction of Mars and Saturn was observed by Denning on September 29, 1889, the bright star Regulus (α Leonis) being at the time only 47′ distant from the planets.492