“In autumn, that is to say in the months of August and September, a star of the first magnitude was seen, very red, and accompanied by a great tail which extended towards the top of the sky in the form of a cone extremely pointed. It appeared to be very near the earth. It was observed (at first?) near the place of the setting sun in the month of December.”
“In autumn, that is to say in the months of August and September, a star of the first magnitude was seen, very red, and accompanied by a great tail which extended towards the top of the sky in the form of a cone extremely pointed. It appeared to be very near the earth. It was observed (at first?) near the place of the setting sun in the month of December.”
With reference to its appearance in the year 1456, when it was of “vivid brightness,” and had a tail of 60° in length, Admiral Smyth says,[204]“To its malign influence were imputed the rapid successes of Mahomet II., which then threatened all Christendom. The general alarm was greatly aggravated by the conduct of Pope Callixtus III., who, though otherwise a man of abilities, was a poor astronomer; for that pontiff daily ordered the church bells to be rung at noon-tide, extraAve-Mariasto be repeated, and a special protest and excommunication was composed, exorcising equally the Devil, the Turks, and the comet.” With reference to this story, Mr. G. F. Chambers points out[205]that it is probably based on a passage in Platina’sVitæ Pontificum. But in this passage there is no mention made of excommunication or exorcism, so that the story, which has long been current, is probably mythical. In confirmation of this view, the Rev. W. F. Rigge has shown conclusively[206]that no bull was ever issuedby Pope Callixtus III. containing a reference toanycomet. The story would therefore seem to be absolutely without foundation, and should be consigned to the limbo of all such baseless myths.
With reference to the appearance of Halley’s comet, at his last return in 1835, Sir John Herschel, who observed it at the Cape of Good Hope, says—
“Among the innumerable stars of all magnitudes, from the ninth downwards, which at various times were seen through it, and some extremely near to the nucleus (though notexactly on it) there never appeared the least ground for presuming any extinction of their light in traversing it. Very minute stars indeed, on entering its brightest portions, were obliterated, as they would have been by an equal illumination of the field of view; but stars which before their entry appeared bright enough to bear that degree of illumination, were in no case, so far as I could judge, affected to a greater extent than they would have been by so much lamp-light artificially introduced.”[207]
“Among the innumerable stars of all magnitudes, from the ninth downwards, which at various times were seen through it, and some extremely near to the nucleus (though notexactly on it) there never appeared the least ground for presuming any extinction of their light in traversing it. Very minute stars indeed, on entering its brightest portions, were obliterated, as they would have been by an equal illumination of the field of view; but stars which before their entry appeared bright enough to bear that degree of illumination, were in no case, so far as I could judge, affected to a greater extent than they would have been by so much lamp-light artificially introduced.”[207]
It is computed by Prof. J. Holetschak that, early in October, 1909, Halley’s comet should have the brightness of a star of about 14½ magnitude.[208]It should then—if not detected before—be discoverable with some of the large telescopes now available.
According to the computations of Messrs. Cowell and Crommelin, the comet should enter Piscesfrom Aries in January, 1910. “Travelling westward towards the star γ Piscium until the beginning of May, and then turning eastward again, it will travel back through the constellations Cetus, Orion, Monoceros, Hydra, and Sextans.” From this it seems that observers in the southern hemisphere will have a better view of the comet than those in northern latitudes. The computed brightness varies from 1 on January 2, 1910, to 1112 on May 10. But the actual brightness of a comet does not always agree with theory. It is sometimes brighter than calculation would indicate.
According to Prof. O. C. Wendell, Halley’s comet will, on May 12, 1910, approach the earth’s orbit within 4·6 millions of miles; and he thinks that possibly the earth may “encounter some meteors,” which are presumably connected with the comet. He has computed the “radiant point” of these meteors (that is, the point from which they appear to come), and finds its position to be R.A. 22h42m·9, Decl. N. 1° 18′. This point lies a little south-west of the star β Piscium.
According to Dr. Smart, the comet will, on June 2, “cross the Equator thirteen degrees south of Regulus, and will then move slowly in the direction of φ Leonis. The comet will be at its descending node on the ecliptic in the morning of May 16, and the earth will pass through the node on the comet’s orbit about two and a halfdays later. The comet’s orbit at the node is about 13 million miles within that of the earth. Matter repelled from the comet’s nucleus by the sun with a velocity of about 216,000 miles per hour, would just meet the earth when crossing the comet’s orbit plane. Matter expelled with a velocity of 80,000 miles per hour, as in the case of Comet Morehouse, would require seven days for the journey. Cometary matter is said to have acquired greater velocities than this, for (according to Webb, who quotes Chacornac) Comet II., 1862, shot luminous matter towards the sun, with a velocity of nearly 2200 miles per second. It is therefore possible that matter thrown off by the comet at the node may enter our atmosphere, in which case we must hope that cyanogen, which so often appears in cometary spectra, may not be inconveniently in evidence.”[209]
Cyanogen is, of course, a poisonous gas, but cometary matter is so rarefied that injurious effects on the earth need not be feared.
If we can believe the accounts which have been handed down to us, some very wonderful comets were visible in ancient times. The following may be mentioned:—
B.C. 165. The sun is said to have been “seen for several hours in the night.” If this was a comet it must have been one of extraordinary brilliancy.[210]
B.C. 146. “After the death of Demetrius, king of Syria, the father of Demetrius and Antiochus, a little before the war in Achaia, there appeared a comet as large as the sun. Its disc was first red, and like fire, spreading sufficient light to dissipate the darkness of night; after a little while its size diminished, its brilliancy became weakened, and at length it entirely disappeared.”[211]
B.C. 134. It is recorded that at the birth of Mithridates a great comet appeared which “occupied the fourth part of the sky, and its brilliancy was superior to that of the sun.” (?)[212]
B.C. 75. A comet is described as equal in size to the moon, and giving as much light as the sun on a cloudy day. (!)[213]
A.D. 531. In this year a great comet was observed in Europe and China. It is described as “a very large and fearful comet,” and was visible in the west for three weeks. Hind thinks that this was an appearance of Halley’s comet,[214]and this has been confirmed by Mr. Crommelin.
A.D. 813, August 4. A comet is said to have appeared on this date, of which the following curious description is given: “It resembled two moons joined together; they separated, and having taken different forms, at length appeared like a man without a head.” (!)[215]
A.D. 893. A great comet is said to have appeared in this year with a tail 100° in length, which afterwards increased to 200°![216]
A.D. 1402. A comet appeared in February of this year, which was visible in daylight for eight days. “On Palm Sunday, March 19, its size was prodigious.” Another comet, visible in the daytime, was seen from June to September of the same year.
When the orbit of the comet known as 1889 V was computed, it was found that it had passed through Jupiter’s system in 1886 (July 18-21). The calculations show that it must have passed within a distance of 112,300 miles of the planet itself—or less than half the moon’s distance from the earth—and “its centre may possibly have grazed the surface of Jupiter.”[217]
Sir John Herschel thought that the great comet of 1861 was by far the brightest comet he had ever seen, those of 1811 and 1858 (Donati’s) not excepted.[218]Prof. Kreutz found its period of revolution round the sun to be about 409 years, with the plane of the orbit nearly at right angles to the plane of the ecliptic.
On November 9, 1795, Sir William Herschel saw the comet of that year pass centrally overa small double star of the 11th and 12th magnitudes, and the fainter of the two components remained distinctly visible during the comet’s transit over the star. This comet was an appearance of the comet now known as Encke’s.[219]Struve saw a star of the 10th magnitude through nearly the brightest part of Encke’s comet on November 7, 1828, but the star’s light was not dimmed by the comet.
Sir John Herschel saw a cluster of stars of the 16th or 17th magnitude through Biela’s comet, although the interposed cometary matter must have been at least 50,000 miles in thickness.[220]
Bessel found that on September 29, 1835, a star of the 10th magnitude shone with undimmed lustre through the tail of Halley’s comet within 8 seconds of arc of the central point of the head. At Dorpat (Russia) Struve saw the same star “in conjunction only 2″·2 from the brightest point of the comet. The star remained continuously visible, and its light was not perceptibly diminished whilst the nucleus of the comet seemed to be almost extinguished before the radiance of the small star of the 9th or 10th magnitude.”[221]
Webb says—
“Donati saw a 7 mg. star enlarged so as to show a sensible disc, when the nucleus of comet III.,1860, passed very near it. Stars are said to have started, or become tremulous, during occultations by comets. Birmingham observed the comet of Encke illuminated by a star over which it passed, August 23, 1868; and Klein, in 1861, remarked an exceptional twinkling in 5 mg. stars involved in the tail.”[222]
“Donati saw a 7 mg. star enlarged so as to show a sensible disc, when the nucleus of comet III.,1860, passed very near it. Stars are said to have started, or become tremulous, during occultations by comets. Birmingham observed the comet of Encke illuminated by a star over which it passed, August 23, 1868; and Klein, in 1861, remarked an exceptional twinkling in 5 mg. stars involved in the tail.”[222]
The comet of 1729 had the greatest perihelion distance of any known comet;[223]that is, when nearest to the sun, it did not approach the central luminary within four times the earth’s distance from the sun!
Barnard’s comet, 1889 I., although it never became visible to the naked eye, was visible with a telescope from September 2, 1888, to August 18, 1890, or 715 days—the longest period of visibility of any comet on record. When last seen it was 6¼ times the earth’s distance from the sun, or about 580 millions of miles,[224]or beyond the orbit of Jupiter!
Messier, who was called “the comet ferret,” discovered “all his comets with a small 2-foot telescope of 2¼ inches aperture, magnifying 5 times, and with a field of 4°.”[225]
It is a very curious fact that Sir William Herschel, “during all his star-gaugings and sweeps for nebulæ, never discovered a comet;”[226]that isan object which was afterwardsprovedto be a comet. Possibly, however, some of his nebulæ which are now missing, may have been really comets.
Sir William Herschel found the diameter of the head of the great comet of 1811 to be 127,000 miles. The surrounding envelope he estimated to be at least 643,000 miles, or about three-fourths of the sun’s diameter.
On a drawing of the tails of the great comet of 1744 given in a little book printed in Berlin in that year, no less than 12 tails are shown! These vary in length and brightness. A copy of this drawing is given inCopernicus.[227]The observations were made by “einen geschichten Frauenzimmer,” who Dr. Dreyer identifies with Christian Kirch, or one of her two sisters, daughters of the famous Gottfried and Maria Margaretta Kirch (Idem, p. 107). Dr. Dreyer thinks that the drawing “seems to have been carefully made, and not to be a mere rough sketch as I had at first supposed” (Idem, p. 185).
The tails of some comets were of immense length. That of the comet of 1769 had an absolute length of 38 millions of miles. That of 1680, 96 million of miles, or more than the sun’s distance from the earth. According to Sir William Herschel, the tail of the great comet of 1811 was over 100 millions of miles in length. That of thegreat comet of 1843—one of the finest in history—is supposed to have reached a length of 150 millions of miles![228]
In width the tails of comets were in some cases enormous. According to Sir William Herschel, the tail of the comet of 1811 had a diameter of 15 millions of miles! Its volume was, therefore, far greater than that of the sun![228]
According to Hevelius the comet of 1652 was of such a magnitude that it “resembled the moon when half full; only it shone with a pale and dismal light.”[229]
Halley’s comet at its next appearance will be examined with the spectroscope for the first time in its history. At its last return in 1835, the spectroscope had not been invented.
For the great comet of 1811, Arago computed a period of 3065 years; and Encke found a period of 8800 years for the great comet of 1680.[230]
The variation in the orbital velocity of some comets is enormous. The velocity of the comet of 1680 when passing round the sun (perihelion) was about 212 miles a second! Whereas at its greatest distance from the sun (aphelion) the velocity is reduced to about 10 feet a second!
Meteors
Mr. Denningthinks that the meteor shower of the month of May, known as the Aquarids, is probably connected with Halley’s comet. The meteors should be looked for after 1 a.m. during the first week in May, and may possibly show an enhanced display in May, 1910, when Halley’s comet will be near the sun and earth.[231]
On November 29, 1905, Sir David Gill observed a fireball with an apparent diameter equal to that of the moon, which remained visible for 5 minutes and disappeared in a hazy sky. Observed from another place, Mr. Fuller found that the meteor was visible 2 hours later! Sir David Gill stated that he does not know of any similar phenomenon.[232]
Mr. Denning finds that swiftly moving meteors become visible at a greater height above the earth’s surface than the slower ones. Thus, for the Leonids and Perseids, which are both swift,it has been found that the Leonids appear at an average height of 84 miles, and disappear at a height of 56 miles; and the Perseids at 80 and 54 miles respectively. “On the other hand, the mean height of the very slow meteors average about 65 miles at the beginning and 38 miles at the end of their appearance.”[233]
During the night of July 21-22, 1896, Mr. William Brooks, the well-known astronomer, and director of the Smith Observatory at Geneva (New York), saw a round dark body pass slowly across the moon’s bright disc, the moon being nearly full at the time. The apparent diameter of the object was about one minute of arc, and the duration of the transit 3 or 4 seconds, the direction of motion being from east to west. On August 22 of the same year, Mr. Gathman (an American observer) saw a meteor crossing thesun’sdisc, the transit lasting about 8 seconds.[234]
A meteor which appeared in Italy on July 7, 1892, was shown by Prof. von Niessl to have had anascendingpath towards the latter end of its course! The length of its path was computed to be 683 miles. When first seen, its height above the earth was about 42 miles, and when it disappeared its height had increased to about 98 miles, showing that its motion was directed upwards![235]
In the case of the fall of meteoric stones, which occasionally occur, it has sometimes been noticed that the sound caused by the explosion of the meteorite, or its passage through the air, is heard before the meteorite is seen to fall. This has been explained by the fact that owing to the resistance of the air to a body moving at first with a high velocity its speed is so reduced that it strikes the earth with a velocity less than that of sound. Hence the sound reaches the earth before the body strikes the ground.[236]
The largest meteoric stone preserved in a museum is that known as the Anighita, which weighs 36½ tons, and was found at Cape York in Greenland. It was brought to the American Museum of Natural History by Commander R. E. Peary, the Arctic explorer.
The second largest known is that of Bacubirito in Mexico, the weight of which is estimated at 27½ tons.
The third largest is that known as the Williamette, which was found in 1902 near the town of that name in Western Oregon (U.S.A.). It is composed of metallic nickel-iron, and weighs about 13½ tons. It is now in the American Museum of Natural History.
A large meteorite was actually seen, from the deck of the steamerAfrican Prince, to fall into the Atlantic Ocean, on October 7, 1906! The captainof the vessel, Captain Anderson, describes it as having a train of light resembling “an immense broad electric-coloured band, gradually turning to orange, and then to the colour of molten metal. When the meteor came into the denser atmosphere close to the earth, it appeared, as nearly as is possible to describe it, like a molten mass of metal being poured out. It entered the water with a hissing noise close to the ship.”[237]This was a very curious and perhaps unique phenomenon, and it would seem that the vessel had a narrow escape from destruction.
In Central Arizona (U.S.A.) there is a hill called Coon Butte, or Coon Mountain. This so-called “mountain” rises to a height of only 130 to 160 feet above the surrounding plain, and has on its top a crater of 530 to 560 feet deep; the bottom of the crater—which is dry—being thus 400 feet below the level of the surrounding country. This so-called “crater” is almost circular and nearly three-quarters of a mile in diameter. It has been suggested that this “crater” was formed by the fall of an enormous iron meteorite, or small asteroid. The “crater” has been carefully examined by a geologist and a physicist. From the evidence and facts found, the geologist (Mr. Barringer) states that “they do not leave, in my mind, a scintilla of doubt that this mountain and its crater were produced by the impactof a huge meteorite or small asteroid.” The physicist (Mr. Tilghmann) says that he “is justified, under due reserve as to subsequently developed facts, in announcing that the formation at this locality is due to the impact of a meteor of enormous and unprecedented size.” There are numerous masses of meteoric iron in the vicinity of the “crater.” The so-called Canyon Diabolo meteorite was found in a canyon of that name about 2½ miles from the Coon Mountain. The investigators estimate that the great meteoric fall took place “not more than 5000 years ago, perhaps much less.” Cedar trees about 700 years old are now growing on the rim of the mountain. From the results of artillery experiments, Mr. Gilbert finds that “a spherical projectile striking solid limestone with a velocity of 1800 feet a second will penetrate to a depth of something less than two diameters,” and from this Mr. L. Fletcher concludes “that a meteorite of large size would not be prevented by the earth’s atmosphere from having a penetration effect sufficient for the production of such a crater.”[238]
The meteoric origin of this remarkable “crater” is strongly favoured by Mr. G. P. Merrill, Head Curator of Geology, U.S. National Museum.
The Canyon Diabolo meteorite above referred to was found to contain diamonds! some black, others transparent. So some have said that “thediamond is a gift from Heaven,” conveyed to earth in meteoric showers.[239]But diamond-bearing meteorites would seem to be rather a freak of nature. It does not follow thatalldiamonds had their origin in meteoric stones. The mineral known as periodot is frequently found in meteoric stones, but it is also a constituent of terrestrial rocks.
In the year 1882 it was stated by Dr. Halm and Dr. Weinhand that they had found fossil sponges, corals, and crinoids in meteoric stones! Dr. Weinhand thought he had actually determined three genera![240]But this startling result was flatly contradicted by Carl Vogt, who stated that the supposed fossils are merely crystalline conformations.[241]
Some meteorites contain a large quantity of occluded gases, hydrogen, helium, and carbon oxides. It is stated that Dr. Odling once “lighted up the theatre of the Royal Institution with gas brought down from interstellar space by meteorites”![242]
On February 10, 1896, a large meteorite burst over Madrid with a loud report. The concussion was so great that many windows in the city were broken, and some partitions in houses were shaken down![243]
A very brilliant meteor or fireball was seen in daylight on June 9, 1900, at 2h55mp.m. from various places in Surrey, Sussex, and near London. Calculations showed that “the meteor began 59 miles in height over a point 10 miles east of Valognes, near Cherbourg, France. Meteor ended 23 miles in height, over Calais, France. Length of path 175 miles. Radiant point, 280°, 12°.”[244]
It was decided some years ago “in the American Supreme Court that a meteorite, though a stone fallen from heaven, belongs to the owner of the freehold interest in the land on which it falls, and not to the tenant.”[245]
With reference to the fall of meteoric matter on the earth, Mr. Proctor says, “It is calculated by Dr. Kleiber of St. Petersburgh that 4250 lbs. of meteoric dust fall on the earth every hour—that is, 59 tons a day, and more than 11,435 tons a year. I believe this to be considerably short of the truth. It sounds like a large annual growth, and the downfall of such an enormous mass of meteoric matter seems suggestive of some degree of danger. But in reality, Dr. Kleiber’s estimate gives only about 25 millions of pounds annually, which is less than 2 ounces annually to each square mile of the earth’s surface,”[246]a quantity which is, of course, quite insignificant.
According to Humboldt, Chladni states that a Franciscan monk was killed by the fall of an aërolite at Milan in the year 1660.[247]Humboldt also mentions the death by meteoric stones of a monk at Crema on September 4, 1511, and two Swedish sailors on board ship in 1674.[248]
It is a curious fact that, according to Olbers, “no fossil meteoric stones” have ever been discovered.[249]Considering the number which are supposed to have fallen to the earth in the course of ages, this fact seems a remarkable one.
On May 10, 1879, a shower of meteorites fell at Eitherville, Iowa (U.S.A.). Some of the fragments found weighed 437, 170, 92½, 28, 10½, 4 and 2 lbs. in weight. In the following year (1880) when the prairie grass had been consumed by a fire, about “5000 pieces were found from the size of a pin to a pound in weight.”[250]
According to Prof. Silvestria of Catania, a shower of meteoric dust mixed with rain fell on the night of March 29, 1880. The dust contained a large proportion of iron in the metallic state. In size the particles varied from a tenth to a hundredth of a millimetre.[251]
It is sometimes stated that the average mass of a “shooting star” is only a few grains. But fromcomparisons with an electric arc light, Prof. W. H. Pickering concludes that a meteor as bright as a third magnitude star, composed of iron or stone, would probably have a diameter of 6 or 7 inches. An average bright fireball would perhaps measure 5 or 6 feet in diameter.[252]
In the Book of Joshua we are told “that theLordcast down great stones from heaven upon them unto Azekah, and they died” (Joshua x. 11). In the latter portion of the verse “hailstones” are mentioned, but as the original Hebrew word means stones in general (not hailstones), it seems very probable that the stones referred to were aërolites.[253]
The stone mentioned in the Acts of the Apostles, from which was found “theimagewhich fell down from Jupiter” (Acts xix. 35), was evidently a meteoric stone.[253]
The famous stone in the Caaba at Mecca, is probably a stone of meteoric origin.[253]
I“Stones from Heaven! Can you wonder,You who scrutinize the Earth,At the love and venerationThey received before the birthOf our scientific methods?II“Stones from Heaven! we can handleFragments fallen from realms of Space;Oh! the marvel and the mystery,Could we understand their placeIn the scheme of things created!III“Stones from Heaven! With a mightyComet whirling formed they part?Fell they from their lofty stationLike a brilliant fiery dart,Hurl’d from starry fields of Night?”[254]
The Zodiacal Light and Gegenschein
Accordingto Gruson and Brugsch, the Zodiacal Light was known in ancient times, and was even worshipped by the Egyptians. Strabo does not mention it; but Diodorus Siculus seems to refer to it (B.C.373), and he probably obtained his information from some Greek writers before his time, possibly from Zenophon, who lived in the sixth centuryB.C.[255]Coming to the Christian era, it was noticed by Nicephorus, about 410B.C.In the Koran, it is called the “false Aurora”; and it is supposed to be referred to in the “Rubáiyát” of Omar Khayyam, the Persian astronomical poet, in the second stanza of that poem (Edward Fitzgerald’s translation)—
“Dreaming when Dawn’s Left Hand was in the Sky,[256]I heard a voice within the Tavern cry,Awake, my Little ones, and fill the Cup,Before Life’s Liquor in its Cup be dry.”
It was observed by Cassini in 1668,[257]and byHooke in 1705. A short description of its appearance will be found in Childrey’sBritannia Baconica(1661), p. 183.
The finest displays of this curious light seem to occur between the middle of January and the middle of February. In February, 1856, Secchi found it brighter than he had ever seen it before. It was yellowish towards the axis of the cone, and it seemed to be brighter than the Milky Way in Cygnus. He described it as “un grande spectacle.” In the middle of February, 1866, Mr. Lassell, during his last residence in Malta, saw a remarkable display of the Zodiacal Light. He found it at least twice as bright as the brightest part of the Milky Way, and much brighter than he had previously seen it. He found that the character of its light differed considerably from that of the Milky Way. It was of a much redder hue than the Galaxy. In 1874 very remarkable displays were seen in the neighbourhood of London in January and February of that year; and in 1875 on January 24, 25, and 30. On January 24 it was noticed that the “light” was distinctly reddish and much excelled in brightness any portion of the Milky Way.
Humboldt, who observed it from Andes (at a height of 13,000 to 15,000 feet), from Venezuela and from Cumana, tells us that he has seen the Zodiacal Light equal in brightness to the Milky Way in Sagittarius.
As probably many people have never seen the “light,” a caution may be given to those who care to look for it. It is defined by the Rev. George Jones, Chaplain to the “United States’ Japan Expedition” (1853-55), as “a brightness that appears in the western sky after sunset, and in the east before sunrise; following nearly or quite the line of the ecliptic in the heavens, and stretching upwards to various elevations according to the season of the year.” From the description some might suppose that the light is visibleimmediatelyafter sunset. But this is not so; it never appears until twilight is over and “the night has fully set in.”
The “light” is usually seen after sunset or before sunrise. But attempts have recently been made by Prof. Simon Newcomb to observe it north of the sun. To avoid the effects of twilight the sun must be only slightly more than 18° below the horizon (that is, a little before or after the longest day). This condition limits the place of observation to latitudes not much south of 46°; and to reduce atmospheric absorption the observing station should be as high as possible above the level of the sea. Prof. Newcomb, observing from the Brienzer Rothorn in Switzerland (latitude 46° 47′ N., longitude 8° 3′ E.), succeeded in tracing the “light” to a distance of 35° north of the sun. It would seem, therefore, that the Zodiacal Light envelops the sun on allsides, but has a greater extension in the direction of the ecliptic.[258]From observations at the Lick Observatory, Mr. E. A. Fath found an extension of 46° north of the sun.[259]
From observations of the “light” made by Prof. Barnard at the Yerkes Observatory during the summer of 1906, he finds that it extends to at least 65° north of the sun, a considerably higher value than that found by Prof. Newcomb.[260]The difference may perhaps be explained by actual variation of the meteoric matter producing the light. Prof. J. H. Poynting thinks that possibly the Zodiacal Light is due to the “dust of long dead comets.”[261]
From careful observations of the “light,” Mr. Gavin J. Burns finds that its luminosity is “some 40 or 50 per cent. brighter than the background of the sky. Prof. Newcomb has made a precisely similar remark about the luminosity of the Milky Way, viz. that it is surprisingly small.” This agrees with my own observations during many years. It is only on the finest and clearest nights that the Milky Way forms a conspicuous object in the night sky. And this only in the country. The lights of a city almost entirely obliterate it. Mr. Burns finds that the Zodiacal Lightappears “to be of a yellowish tint; or if we call it white, then the Milky Way is comparatively of a bluish tint.” During my residence in the Punjab the Zodiacal Light seemed to me constantly visible in the evening sky in the spring months. In the west of Ireland I have seen it nearly as bright as the brightest portions of the Milky Way visible in this country (February 20, 1890). The “meteoric theory” of the “light” seems to be the one now generally accepted by astronomers, and in this opinion I fully concur.
From observations made in Jamaica in the years 1899 and 1901, Mr. Maxwell Hall arrived at the conclusion that “the Zodiacal Light is caused by reflection of sunlight from masses of meteoric matter still contained in the invariable plane, which may be considered the original plane of the solar system.”[262]According to Humboldt, Cassini believed that the Zodiacal Light “consisted of innumerably small planetary bodies revolving round the sun.”[263]
The Gegenschein, orCounter-glow.—This is a faint patch of light seen opposite the sun’s place in the sky, that is on the meridian at midnight. It is usually elliptical in shape, with its longer axis lying nearly in the plane of the ecliptic. It seems to have been first detected by Brorsen (the discoverer of the short-period cometof 1846) about the middle of the nineteenth century. But it is not easy to see, for the famous Heis of Münster, who had very keen eyesight, did not succeed in seeing it for several years after Brorsen’s announcement.[264]It was afterwards independently discovered by Backhouse, and Barnard.
Prof. Barnard’s earlier observations seemed to show that the Gegenschein does not lie exactly opposite to the sun, but very nearly so. He found its longitude is within one degree of 180°, and its latitude about 1°·3 north of the ecliptic.[265]But from subsequent observations he came to the conclusion that the differences in longitude and apparent latitude are due to atmospheric absorption, and that the object really lies in the ecliptic andexactlyopposite to the sun.[266]
Barnard finds that the Gegenschein is not so faint as is generally supposed. He says “it is best seen by averted vision, the face being turned 60° or 70° to the right or left, and the eyes alone turned towards it.” It is invisible in June and December, while in September it is round, with a diameter of 20°, and very distinct. No satisfactory theory has yet been advanced to account for this curious phenomenon. Prof. Arthur Searle of Harvard attributes it to a number ofasteroids too small to be seen individually. When in “opposition” to the sun these would be fully illuminated and nearest to the earth. Its distance from the earth probably exceeds that of the moon. Dr. Johnson Stoney thinks that the Gegenschein may possibly be due to a “tail” of hydrogen and helium gases repelled from the earth by solar action; this “tail” being visible to us by reflected sunlight.
It was observed under favourable circumstances in January and February, 1903, by the French astronomer, M. F. Quénisset. He found that it was better seen when the atmosphere was less clear, contrary to his experience of the Zodiacal Light. Prof. Barnard’s experience confirms this. M. Quénisset notes that—as in the case of the Zodiacal Light—the southern border of the Gegenschein is sharper than the northern. He found that its brightness is less than that of the Milky Way between Betelgeuse and γ Geminorum; and thinks that it is merely a strengthening of the Zodiacal Light.[267]
A meteoritic theory of the Gegenschein has been advanced by Prof. F. R. Moulton, which explains it by light reflected from a swarm of meteorites revolving round the sun at a distance of 930,240 miles outside the earth’s orbit.
Both the Zodiacal Light and Gegenschein were observed by Herr Leo Brenner on the evening ofMarch 4, 1896. He found the Zodiacal Light on this evening to be “perhaps eight times brighterthan the Milky Way in Perseus.” The “Gegenschein distinctly visibleas a round, bright, cloud-like nebula below Leo (Virgo), and about twice the brightness of the Milky Way in Monoceros between Canis Major and Canis Minor.”[268]
Humboldt thought that the fluctuations in the brilliancy of the Zodiacal Light were probably due to a real variation in the intensity of the phenomenon rather than to the elevated position of the observer.[269]He says that he was “astonished in the tropical climates of South America, to observe the variable intensity of the light.”
The Stars
Plinysays that Hipparchus “ventured to count the stars, a work arduous even for the Deity.” But this was quite a mistaken idea. Those visible to the naked eye are comparatively few in number, and the enumeration of those visible in an opera-glass—which of course far exceed those which can be seen by unaided vision—is a matter of no great difficulty. Those visible in a small telescope of 2¾ inches aperture have all been observed and catalogued; and even those shown on photographs taken with large telescopes can be easily counted. The present writer has made an attempt in this direction, and taking an average of a large number of counts in various parts of the sky, as shown on stellar photographs, he finds a total of about 64 millions for the whole sky in both hemispheres.[270]Probably the total number will not exceed 100 millions. But this is a comparatively smallnumber, even when compared with the human population of our little globe.
With reference to the charts made by photography in the International scheme commenced some years ago, it has now been estimated that the charts will probably contain a total of about 9,854,000 stars down to about the 14th magnitude (13·7). The “catalogue plates” (taken with a shorter exposure) will, it is expected, include about 2,676,500 stars down to 11½ magnitude. These numbers may, however, be somewhat increased when the work has been completed.[271]If this estimate proves to be correct, the number of stars visible down to the 14th magnitude will be considerably less than former estimates have made it.
Prof. E. C. Pickering estimates that the total number of stars visible on photographs down to the 16th magnitude (about the faintest visible in the great Lick telescope) will be about 50 millions.[272]In the present writer’s enumeration, above referred to, many stars fainter than the 16th magnitude were included.
Admiral Smyth says, with reference to Sir William Herschel—perhaps the greatest observer that ever lived—“As to Sir William himself, he could unhesitatingly call every star down to the 6th magnitude, by its name, letter, or number.”[273]This shows great powers of observation, and a wonderful memory.
On a photographic plate of the Pleiades taken with the Bruce telescope and an exposure of 6 hours, Prof. Bailey of Harvard has counted “3972 stars within an area 2° square, having Alcyone at its centre.”[274]This would give a total of about 41 millions for the whole sky, if of the same richness.
With an exposure of 16 hours, Prof. H. C. Wilson finds on an area of less that 110′ square a total of 4621 stars. He thinks, “That all of these stars belong to the Pleiades group is not at all probable. The great majority of them probably lie at immense distances beyond the group, and simply appear in it by projection.”[274]He adds, “It has been found, however, by very careful measurements made during the last 75 years at the Königsbergh and Yale Observatories, that of the sixty-nine brighter stars, including those down to the 9th magnitude, only eight show any certain movement with reference to Alcyone. Since Alcyone has a proper motion or drift of 6″ per century, this means that all the brightest stars except the eight mentioned are drifting with Alcyone and so form a true cluster, at approximately the same distance from the earth. Six of the eight stars which show relative drift are moving in the opposite direction to themovement of Alcyone, and at nearly the same rate, so that their motion is only apparent. They are really stationary, while Alcyone and the rest of the cluster are moving past them.”[275]This tends to show that the faint stars are reallybehindthe cluster, and are unconnected with it.
It is a popular idea with some people that the Pole Star is the nearest of all the stars to the celestial pole. But photographs show that there are many faint stars nearer to the pole than the Pole Star. The Pole Star is at present at a distance of 1° 13′ from the real pole of the heavens, but it is slowly approaching it. The minimum distance will be reached in the year 2104. From photographs taken by M. Flammarion at the Juvisy Observatory, he finds that there are at least 128 stars nearer to the pole than the Pole Star! The nearest star to the pole was, in the year 1902, a small star of about 12½ magnitude, which was distant about 4 minutes of arc from the pole.[276]The estimated magnitude shows that the Pole Star is nearly 10,000 times brighter than this faint star!
It has been found that Sirius is bright enough to cast a shadow under favourable conditions. On March 22, 1903, the distinguished French astronomer Touchet succeeded in photographingthe shadow of a brooch cast by this brilliant star. The exposure was 1h5m.
Martinus Hortensius seems to have been the first to see stars in daylight, perhaps early in the seventeenth century. He mentions the fact in a letter to Gassendi dated October 12, 1636, but does not give the date of his observation. Schickard saw Arcturus in broad daylight early in 1632. Morin saw the same bright star half an hour after sunset in March, 1635.
Some interesting observations were made by Professors Payne and H. C. Wilson, in the summer of 1904, at Midvale, Montana (U.S.A.), at a height of 4790 feet above sea-level. At this height they found the air very clear and transparent. “Many more stars were visible at a glance, and the familiar stars appeared more brilliant.... In the great bright cloud of the Milky Way, between β and γ Cygni, one could count easily sixteen or seventeen stars, besides the bright ones η and χ,[277]while at Northfield it is difficult to distinctly see eight or nine with the naked eye.” Some nebulæ and star fields were photographed with good results by the aid of a 2½-inch Darlot lens and 3 hours’ exposure.[278]
Prof. Barnard has taken some good stellar photographs with a lens of only 1½ inches in diameter,and 4 or 5 inches focus belonging to an ordinary “magic lantern”! He says that these “photographs with the small lens show us in the most striking manner how the most valuable and important information may be obtained with the simplest means.”[279]
With reference to the rising and setting of the stars due to the earth’s rotation on its axis, the late Sir George B. Airy, Astronomer Royal of England, once said to a schoolmaster, “I should like to know how far your pupils go into the first practical points for which reading is scarcely necessary. Do they know that the stars rise and set? Very few people in England know it. I once had a correspondence with a literary man of the highest rank on a point of Greek astronomy, and found that he did not know it!”[280]
Admiral Smyth says, “I have been struck with the beautiful blue tint of the smallest stars visible in my telescope. This, however, may be attributed to some optical peculiarity.” This bluish colour of small stars agrees with the conclusion arrived at by Prof. Pickering in recent years, that the majority of faint stars in the Milky Way have spectra of the Sirian type and, like that brilliant star, are of a bluish white colour. Sir William Herschel saw many stars of a redder tinge than other observers have noticed. Admiral Smythsays, “This may be owing to the effect of his metallic mirror or to some peculiarity of vision, or perhaps both.”[281]
The ancient astronomers do not mention any coloured stars except white and red. Among the latter they only speak of Arcturus, Aldebaran, Pollux, Antares, and Betelgeuse as of a striking red colour. To these Al-Sufi adds Alphard (α Hydræ).
Sir William Herschel remarked that no decidedly green or blue star “has ever been noticed unassociated with a companion brighter than itself.” An exception to Herschel’s rule seems to be found in the case of the star β Libræ, which Admiral Smyth called “pale emerald.” Mr. George Knott observed it on May 19, 1852, as “beautiful pale green” (3·7 inches achromatic, power 80), and on May 9, 1872, as “fine pale green” (5·5 inches achromatic, power 65).
The motion of stars in the line of sight, as shown by the spectroscope—should theoretically alter their brightness in the course of time; those approaching the earth becoming gradually brighter, while those receding should become fainter. But the distance of the stars is so enormous that even with very high velocities the change would not become perceptible for ages. Prof. Oudemans found that to change the brightness of a star by only one-tenth of a magnitude—a quantity barelyperceptible to the eye-a number of years would be necessary, which is represented by the formula
for a star approaching the earth, and for a receding star
This is in geographical miles, 1 geographical mile being equal to 4·61 English miles.
Reducing the above to English miles, and taking an average for both approaching and receding stars, we have
where p = parallax in seconds of arc, and m = radial velocity in English miles per second.
Prof. Oudemans found that the only star which could have changed in brightness by one-tenth of a magnitude since the time of Hipparchus is Aldebaran. This is taking its parallax as 0″·52. But assuming the more reliable parallax 0″·12 found by Dr. Elkin, this period is 4⅓ times longer. For Procyon, the period would be 5500 years.[282]The above calculation shows how absurd it is to suppose that any star could have gained or lost in brightness by motion in the line of sight during historical times. The “secular variation” of starsis quite another thing. This is due to physical changes in the stars themselves.
The famous astronomer Halley, the second Astronomer Royal at Greenwich, says (Phil. Trans., 1796), “Supposing the number of 1st magnitude stars to be 13, at twice the distance from the sun there may be placed four times as many, or 52; which with the same allowance would nearly represent the star we find to be of the 2nd magnitude. So 9 × 13, or 117, for those at three times the distance; and at ten times the distance 100 × 13, or 1300 stars; of which distance may probably diminish the light of any of the stars of the 1st magnitude to that of the 6th, it being but the hundredth part of what, at their present distance, they appear with.” This agrees with the now generally accepted “light ratio” of 2·512 for each magnitude, which makes a first magnitude star 100 times the light of a 6th magnitude.
On the 4th of March, 1796,[283]the famous French astronomer Lalande observed on the meridian a star of small 6th magnitude, the exact position of which he determined. On the 15th of the same month he again observed the star, and the places found for 1800 refer to numbers 16292-3 of the reduced catalogue. In the observation of March 4 he attached the curious remark, “Étoile singulière” (the observation of March 15 is withoutnote). This remark of Lalande has puzzled observers who failed to find any peculiarity about the star. Indeed, “the remark is a strange one for the observer of so many thousands of stars to attach unless there was really something singular in the star’s aspect at the time.” On the evening of April 18, 1887, the star was examined by the present writer, and the following is the record in his observing book, “Lalande’s étoile singulière (16292-3) about half a magnitude less than η Cancri. With the binocular I see two streams of small stars branching out from it, north preceding like the tails of comet.” This may perhaps have something to do with Lalande’s curious remark.
The star numbered 1647 in Baily’sFlamsteed Catalogueis now known to have been an observation of the planet Uranus.[284]
Prof. Pickering states that the fainter stars photographed with the 8-inch telescope at Cambridge (U.S.A.) are invisible to the eye in the 15-inch telescope.[285]
Sir Norman Lockyer finds that the lines of sulphur are present in the spectrum of the bright star Rigel (β Orionis).[286]
About 8½° south of the bright star Regulus (α Leonis) is a faint nebula (H I, 4 Sextantis). On or near this spot the Capuchin monk De Rheita fancied he saw, in the year 1643, agroup of stars representing the napkin of S. Veronica—“sudarium Veronicæ sive faciem Domini maxima similitudina in astris expressum.” And he gave a picture of the napkin and star group. But all subsequent observers have failed to find any trace of the star group referred to by De Rheita![287]
The Bible story of the star of the Magi is also told in connection with the birth of the sun-gods Osiris, Horus, Mithra, Serapis, etc.[288]The present writer has also heard it suggested that the phenomenon may have been an apparition of Halley’s comet! But as this famous comet is known to have appeared in the yearB.C.11, and as the date of the Nativity was probably not earlier thanB.C.5, the hypothesis seems for this (and other reasons) to be inadmissible. It has also been suggested that the phenomenon might have been an appearance of Tycho Brahé’s temporary star of 1572, known as the “Pilgrim star”; but there seems to be no real foundation for such an hypothesis. There is no reason to think that “temporary” or new stars ever appear a second time.
Admiral Smyth has well said, “It checks one’s pride to recollect that if our sun with the whole system of planets, asteroids, and moons, and comets were to be removed from the spectatorto the distance of the nearest fixed star, not one of them would be visible, except the sun, which would then appear but as a star of perhaps the 2nd magnitude. Nay, more, were the whole system of which our globe forms an insignificant member, with its central luminary, suddenly annihilated, no effect would be produced on those unconnected and remote bodies; and the only annunciation of such a catastrophe in the Sidereal “Times” would be that a small star once seen in a distant quarter of the sky had ceased to shine.”[289]
Prof. George C. Comstock finds that the average parallax of 67 selected stars ranging in brightness between the 9th and the 12th magnitude, is of the value of 0″·0051.[290]This gives a distance representing a journey for light of about 639 years!
Mr. Henry Norris Russell thinks that nearly all the bright stars in the constellation of Orion are practically at the same distance from the earth. His reasons for this opinion are: (1) the stars are similar in their spectra and proper motions, (2) their proper motions are small, which suggests a small parallax, and therefore a great distance from the earth. Mr. Russell thinks that the average parallax of these stars may perhaps be 0″·005, which gives a distance of about 650 “light years.”[291]
According to Sir Norman Lockyer’s classification of the stars, the order ofincreasingtemperature is represented by the following, beginning with those in the earliest stage of stellar evolution:—Nebulæ, Antares, Aldebaran, Polaris, α Cygni, Rigel, ε Tauri, β Crucis. Then we have the hottest stars represented by ε Puppis, γ Argus, and Alnitam (ε Orionis).Decreasingtemperature is represented by (in order), Achernar, Algol, Markab, Sirius, Procyon, Arcturus, 19 Piscium, and the “Dark Stars.”[292]But other astronomers do not agree with this classification. Antares and Aldebaran are by some authorities considered to becoolingsuns.
According to Ritter’s views of the Constitution of the Celestial Bodies, if we “divide the stars into three classes according to age corresponding to these three stages of development, we shall assign to the first class, A, those stars still in the nebular phase of development; to the second class, B, those in the transient stage of greatest brilliancy; and to the class C, those stars which have already entered into the long period of slow extinction. It should be noted in this classification that we refer to relative and not absolute age, since a star of slight mass passes through the successive phases of its development more rapidly than the star of greater mass.”[293]Rittercomes to the conclusion that “the duration of the period in which the sun as a star had a greater brightness than at present was very short in comparison with the period in which it had and will continue to have a brightness differing only slightly from its present value.”[294]
In a valuable and interesting paper on “The Evolution of Solar Stars,”[295]Prof. Schuster says that “measurements by E. F. Nichols on the heat of Vega and Arcturus indicated a lower temperature for Arcturus, and confirms the conclusion arrived at on other grounds, that the hydrogen stars have a higher temperature than the solar stars.” “An inspection of the ultraviolet region of the spectrum gives the same result. These different lines of argument, all leading to the same result, justify us in saying that the surface temperature of the hydrogen stars is higher than that of the solar stars. An extension of the same reasoning leads to the belief that the helium stars have a temperature which is higher still.” Hence we have Schuster, Hale, and Sir William Huggins in agreement that the Sirian stars are hotter than the solar stars; and personally I agree with these high authorities. The late Dr. W. E. Wilson, however, held the opinion that the sun is hotter that Sirius!
Schuster thinks that Lane’s law does not applyto the temperature of the photosphere and the absorbing layers of the sun and stars, but only to the portions between the photosphere and the centre, which probably act like a perfect gas. On this view he says the interior might become “hotter and hotter until the condensation had reached a point at which the laws of gaseous condensation no longer hold.”
With reference to the stars having spectra of the 3rd and 4th type (usually orange and red in colour), Schuster says—