Part 21 (2/2)
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.
<script>