[footnote" *The following remarkable pa.s.sage on the radiation of heat from the fixed stars, and on their low combustion and vitality -- one of Kepler"s many aspirations -- occurs in the "Paralipom. in Vitell. Astron.
parsOpticqa", 1604, Propos. x.x.xii., p. 25: "Luciis proprium est calor, sydera omnia calefaciunt. De syderum luce claritatis ratio testatur, calorem universorum in minori esse proportione ad calorem unius solis, quam ut ab homine, cujus est certa caloris mensura, utrque simul percipi et judicari possit. De cincindularum lucula tenuissima negare non potes, quin c.u.m calore sit. Vivunt enim et moventur, hoc auten non sine calefactione perficitur. Sic neque putrescentium lignorum lux sui calore dest.i.tuitur; nam ipsa puetredo quidam lentus ignis est. Inest et stirpibus suus calor."
(Compare Kepler, "Epit. Astron. Copernican", 1618, t. i., lib. i., p. 35.)
Another and different kind of cosmical, or, rather, material mode of contact is, however, opened to us, if we admit falling stars and meteoric stones to be planetary asteroids. They not only act upon us merely from a distance by the excitement of luminous or calorific vibrations, or in obedience to the laws of mutual attraction, but they acquire an actual material existence for us, reaching our atmosphere from the remoter regions of universal s.p.a.ce, and remaining on the earth itself. Meteoric stones are the only means by which we can be brought in possible contact with that which is foreign to our own planet. Accustomed to gain our knowledge of what is not telluric solely through measurement, calculations, and the deductions of reason, we experience a sentiment of astonishment at finding that we may examine, weigh, and a.n.a.lyze bodies that appertain p 137 to the outer world. This awakens, by the power of the imagination, a meditative, spiritual train of thought, where the untutored mind perceives only scintillations of light in the firmament, and sees in the blackened stone that falls from the exploded cloud nothing beyond the rough product of a powerful natural force.
Although the asteroid-swarms, on which we have been led, from special predilection, to dwell somewhat at length, approximate to a certain degree, in their inconsiderable ma.s.s and the diversity of their orbits, to comets, they present this essential difference from the latter bodies, that our knowledge of their existence is almost entirely limited to the moment of their destruction, that is, to the period when, drawn within the sphere of the Earth"s attraction they become luminous and ignite.
In order to complete our view of all that we have learned to consider as appertaining to our solar system, which now, since the discovery of the small planets, of the interior comets of short revolutions, and of the meteoric asteroids, is so rich and complicated in its form, it remains for us to speak of the ring of Zodiacal light, to which we have already alluded.
Those who have lived for many years in the zone of palms must retain a pleasing impression of the mild radiance with which the zodiacal light, shooting pyramidally upward, illumines a part of the uniform length of tropical nights. I have seen it shine with an intensity of light equal to the milky way in Sagittarius, and that not only in the rare and dry atmosphere of the summits of the Andes, at an elevation of from thirteen to fifteen thousand feet, but even on the boundless gra.s.sy plains, the Illanos of Venezuela, and on the sea-sh.o.r.e, beneath the ever-clear sky of c.u.mana.
This phenomenon was often rendered especially beautiful by the pa.s.sage of light, fleecy clouds, which stood out in picturesque and bold relief from the luminous back-ground. A notice of this a?rial spectacle is contained in a pa.s.sage in my journal, while I was on the voyage from Lima to the western coasts of Mexico: "For three or four nights (between 10degrees and 14degrees north lat.i.tude) the zodiacal light has appeared in greater splendor than I have ever observed it. The transparency of the atmosphere must be remarkably great in this part of the Southern Ocean, to judge by the radiance of the stars and nebulous spots. From the 14th to the 19th of March a regular interval of three quarters of an hour occurred between the disappearance of the sun"s disk in the ocean and the first manifestation of the zodiacal p 138 light, although the night was already perfectly dark. an hour after sunset it was seen in great briliancy between Aldebaran and the Pleiades; and on the 18th of March it attained an alt.i.tude of 39degrees5"minutes. Narrow elongated clouds are scattered over the beautiful deep azure of the distant horizon, flitting past the zodiacal light as before a golden curtain. Above these, other clouds are from time to time reflecting the most brightly variegated colors. It seems a second sunset. On this side of the vault of heaven the lightness of the night appears to increase almost as much as at the first quarter of the moon. Toward 10 o"clock the zodiacal light generally becomes very faint in this part of the Southern Ocean, and at midnight I have scarcely been able to trace a vestige of it. On the 16th of March, when most strongly luminous a faint reflection was visible in the east." In our gloomy so-called "temperate" northern zone, the zodiacal light is only distinctly visible in the beginning of Spring, after the evening twilight, in the western part of the sky, and at the close of Autumn, before the dawn of day, above the eastern horizon.
It is difficult to understand how so striking a natural phenomenon should have failed to attract the attention of physicists and astronomers until the middle of the seventeenth century, or how it could have escaped the observation of the Atabian natural philosophers in ancient Bactria, on the euphrates, and in the south of Spain. Almost equal surprise is excited by the tardiness of observation of the nebulous spots in Andromeda and Orion, first described by Simon Marius and Huygens. The earliest explicit descriptions of the zodiacal light occurs in Childrey"s "Britannia Baconica",* in the year 1661.
p 139
[footnote] *"There is another thing which I recommend to the observation of mathematical men, which is that in February, and for a little before and a little after that month (as I have observed several years together), about six in the evening, when the twilight hath almost deserted the horizon, you shall see a plainly discernible way of the twilight striking up toward the Pleiades, and seeming almost to touch them. It is so observed any clear night, but it is best illac nocte. There is no such way to be observed at any other time of the year (that I can perceive), nor any other way at that time to be perceived darting up elsewhere; and I believe it hath been, and will be constantly visible at that time of the year; but what the cause of it in nature should be, I can not yet imagine, but leave it to future inquiry." (Childrey, "Britannia Baconica", 1661, p. 183.) This is the first view and a simple description of the phenomenon. (Ca.s.sini, "D?couverte de la Lumi dfd ?leste qui paro?t dans le Zodiaque", in the "M?m. de l"Acad.", t. viii., 1730, p 276. Mairan, "Trait?Phys de l"Aurore Bor?ale", 1754, 0. 16.) In this remarkable work by Childrey there are to be found (p. 91) very clear accounts of the epochs of maxima and minima diurnal and annual temperatures, and of the r.e.t.a.r.dation of the extremes of the effects in meteorological processes. It is, however, to be regretted that our Baconian-philosophy-loving author, who was Lord Henry Somerset"s chaplain, fell into the same error as Bernardin de St. Pierre, and regarded the Earth as elongated at the poles (see p. 148). At the first he believes that the Earth was spherical, but supposes that the uninterrupted and increasing addition of layers of ice at both poles has changed its figure; and that as the ice is formed from water, the quant.i.ty of that liquid is every where diminishing.
The first observation of the phenomenon may have been made two or three years prior to this period; but, notwithstanding, the merit of having (in the spring of 1683) been the first to investigate the phenomenon in all its relations in s.p.a.ce is incontestably due to Dominicus Ca.s.sini. The light which he saw at Bologna in 1668, and which was observed at the same time in Persia by the celebrated traveler Chardin (the court astrologers of Ispahan called this light, which had never before been observed, "nyzek", a small lance), was not the zodiacal light, as has often been a.s.serted,* but the p 140 enormous tail of a comet, whose head was concealed in the vapory mist of the horizon, and which, from its length and appearance, presented much similarity to the great comet of 1843.
[footnote] *Dominicus Ca.s.sini ("M?m. de l"Acad.", t. viii., 1730, p. 188), and Mairan ("Aurore Bor.", p. 16), have even maintained that the phenomenon observed in Persia in 1668 was the zodiacal light. Delambre ("Hist. de l"Astron. Moderne", t. ii., p. 742), in very decided trms ascribes the discovery of this light to the celebrated traveler Chardin; but in the "Couronnement de Soliman", and in several pa.s.sages of the narrative of his travels (?d. de Langls. t. iv., p. 326; t. x., p. 97), he only applies the term niazouk (nyzek), or "pet.i.te lance," to "the great and famous comet which appeared over nearly the whole world in 1668, and whose head was so hidden in the wewst that it could not be perceived in the horizon of Ispahan" ("Atlas du Voyage de Chardin", Tab. iv.; from the observations at Schiraz). The head or nucleus of the comet was, however, visible in the Brazils and in India (Pingr?, "Com?togr.", t. ii., p. 22). Regarding the conjectured ident.i.ty of the last great comet of March, 1843, with this, which Ca.s.sini mistook for the zodiacal light, see Schum., "Astr. Nachr.", 1843, No. 476 and 480. In Persian, the term "nizehi ?tesch?n"(fiery spears or lances) is also applied to the rays of the rising or setting sun, in the same way as "nay?zik," according to Freytag"s Arabic Lexicon, signifies "stell cadentes." The comparison of comets to lances and swords was, however, in the Middle Ages, very common in all languages. The great comet of 1500, which was visible from April to June, was always termed by the Italian writers of that time "il Signor Astone" (see my "Examen Critique de l"Hist. de la G?ographie", t. v., p. 80). All the hypotheses that have been advanced to show that Descartes (Ca.s.sini, p. 230; Mairan, p. 16), and even Kepler (Delambre, t. i., p. 601), were acquainted with the zodiacal light, appear to me altogether untenable. Descartes ("Principes", iii., art. 136, 137) is very obscure in his remarks on comets, observing that their tails are formed "by oblique rays, which, falling on different parts of the planetary orbs, strike the eye laterally by extraordinary refraction," and that they might be seen morning and evening, "like a long beam," when the Sun is between the comet and the Earth. This pa.s.sage no more refers to the zodiacal light than those in which Kepler ("Epit. Astron.
Copernican", t. i., p. 57, and t. ii., p. 893) speaks of the existence of a solar atmosphere (limbus circa solem, coma lucida), which, in eclipses of the Sun, prevents it "from being quite night:" and even more uncertain, or indeed erroneous, is the a.s.sumption that the "trabes quas [Greek word]
vocant" (Plin., ii., 26 and 27) had reference to the tongue-shaped rising zodiacal light, as Ca.s.sini (p. 231, art. x.x.xi.) and Mairan (p. 15) have maintained. Every where among the ancients the trabes are a.s.sociated with the bolides (ardores et faces) and other fiery meteors, and even with long-barbed comets. (Regarding [Greek words] . see Sch?fer, "Schol. Par.
ad Apoll. Rhod.", 1813, t. ii., p. 206; Pseudo-Aristot., "de Mundo, 2, 9; "Comment. Alex. Joh. Philop. et Olymp. in Aristot. Meteor.", lib. i., cap.
vii., 3, p. 195, Ideler; Seneca, "Nat. Qust.", i., 1.)
We may conjecture, with much probability, that the remarkable light on the elevated plains of Mexico, seen for forty nights consecutively i8n 1509, and observed in the eastern horizon rising pyramidally from the earth, was the zodiacal light. I found a notice of this phenomenon in an ancient Aztec MS., the "CodexTelleriano-Remensis",* preserved in the Royal Library at Paris.
[footnote] *Humboldt, "Monumens des Peuples Indignes de l"Am?rique", t.
ii., p. 301. The rare ma.n.u.script which belonged to the Archbishop of Rheims, Le Tellier, contains various kinds of extracts from an Aztec ritual, an astrological calendar, and historical annals, extending from 1197 to 1549, and embracing a notice of different natural phenomena, epochs of earthquakes and comets (as, for instance, those of 1490 and 1529), and of (which are important in relation to Mexican chronology) solar eclipses. In Camargo"s ma.n.u.script "Historia de Tlascala", the light rising in the east almost to the zenith is, singularly enough, described as "sparkling, and as if sown with stars." The description of this phenomenon, which lasted forty days, can not in any way apply to volcanic eruptions of Popcatepetl, which lies very near, in the southeastery direction. (Prescott, "History of the Conquest of Mesico", vol. i., p. 284.) Later commentators have confounded this phenomenon, which Montezuma regarded as a warning of his misfortunes, with the "estrella que humeava" (literally, "which spring forth"; Mexican "choloa, to leap or spring forth"). With respect to the connection of this vapor with the star Citlal Choloha (Venus) and with "the mountain of the star" (Citialtepetl, the volcano of Orizaba), see my "Monumens", t. ii., p.
303.
This phenomenon, whose primordial antiquity can scarcely be doubted, and which was first noticed in Europe by Childrey and Dominicus Ca.s.sini, is not the luminous solar atmosphere itself, since this can not, in accordance with mechanical laws, be more compressed than in the relation of 2 to 3, and consequently can not be diffused beyond 9/20ths of Mercury"s heliocentric distance. These same laws teach us that the alt.i.tude of the extreme boundaries of the atmosphere of a cosmical p 141 body above its equator, that is to say, the point at which gravity and centrifugal force are in equilibrium, must be the same as the alt.i.tude at which a satellite would rotate round the central body simultaneously with the diurnal revolution of the latter.*
[footnote] *Laplace, "Expos. du Syst. du Monde", p. 270; "M?canique C?leste", t. ii., p. 169 and 171; Schubert, "Astr.", bd. iii., 206.
This limitation of the solar atmosphere in its present concentrated condition is especially remarkable when we compare the central body of our system with the nucleus of other nebulous stars. Herschel has discovered several, in which the radius of the nebulous matter surrounding the star appeared at an angle of 150". On the a.s.sumption that the parallax is not fully equal to 1", we find that the outermost nebulous layer of such a star must be 150 times further from the central body than our Earth is from the Sun. If, therefore, the nebulous star were to occupy the place of our Sun, its atmosphere would not only include the orbit of Ura.n.u.s, but even extend eight times beyond it.
[footnote] *Arago, in the "Annuaire", 1842, p. 408. Compare Sir John Herschel"s considerations on the volume and faintness of light of planetary nebul, in Mary Somerville"s "Connection of the Physical Sciences", 1835, p. 108. The opinion that the Sun is a nebulous star, whose atmosphere presents the phenomenon of zodiacal light, did not originate with Dominicus Ca.s.sini, but was first promulgated by Mairan in 1730 ("Trait? de l"Aurore Bor.", p. 47 and 263; Arago, in the "Annuaire", 1842, p. 412). It is a renewal of Kepler"s views.
Considering the narrow limitation of the Sun"s atmosphere, which we have just described, we may with much probability regard the existence of a very compressed annulus of nebulous matter,* revolving freely in s.p.a.ce between the orbits of Venus and Mars, as the material cause of the zodiacal light.
[footnote] *Cominicus Ca.s.sini was the first to a.s.sume, as did subsequently Laplace, Schubert, and Poisson, the hypothesis of a separate ring to explain the form of the zodiacal light. He says distinctly, "If the orbits of Mercury and Venus were visible (throughout their whole extent), we should invariably observe them with the same figure and in the same position with regard to the Sun, and at the same time of the year with the zodiacal light." ("M?m. de l"Acad.", t. viii., 1730, p. 218, and Biot, in the "Comptes Rendus", 1836, t. iii., p. 666.) Ca.s.sini believed that the nebulous ring of zodiacal light consisted of innumerable small planetary bodies revolving round the Sun. He even went so far as to believe that the fall of fire-b.a.l.l.s might be connected with the pa.s.sage of the Earth through the zodiacal nebulous ring. Olmsted, and especially Biot (op. cit., p.
673), have attempted to establish its connection with the November phenomenon -- a connection which Olbers doubts. (Schum., "Jahrb.", 1837, s.
281.) Regarding the question whether the place of the zodiacal light perfectly coincides with that of the Sun"s equator, see Houzeau, in Schum., "Astr. Nachr.", 1843, No. 492, s. 190.
As p 142 yet we certainly know nothing definite regarding its actual material dimensions; its augmentation* by emanations from the tails of myriads of comets that come within the Sun"s vicinity; the singular changes affecting its expansion, since it sometimes does not apper to extend beyond our Earth"s...o...b..t; or, lastly, regarding its conjectural intimate connection with the more condensed cosmical vapor in the vicinity of the Sun.
[footnote] *Sir John Herschel, "Astron.", 487.
The nebulous particles composing this ring, and revolving round the sun in accordance with planetary laws, may either be self-luminous or receive light from that luminary. Even in the case of a terrestrial mist (and this fact is very remarkable), which occurred at the time of the new moon at midnight in 1743, the phosph.o.r.escence was so intense that objects could be distinctly recognized at a distance of more than 600 feet.
I have occasionally been astonished in the tropical climates of south america, to observe the variable intensity of the zodiacal light. As i pa.s.sed the nights, during many months, in the open air, on the sh.o.r.es of rivers and on ilanos, i enjoyed ample opportunities of carefully examining this phenomenon. When the zodiacal light had been most intense, i have observed that it would be perceptibly weakened for a few minutes, until it again suddenly shone forth in full brilliancy. In some few instances i have thought that i could perceive -- not exactly a reddish coloration, nor the lower portion darkened in an arc-like form, nor even a scintillation, as mairan affirms he has observed -- but a kind of flickering and wavering of the light.*
[footnote] *Arago, in the "Annuaire", 1832, p. 246. Several physical facts appear to indicate that, in a mechanical separation of matter into its smallest particles, if the ma.s.s be very small in relation to the surface, the electrical tension may increase sufficiently for the production of light and heat. Experiments with a large concave mirror have not hitherto given any positive evidence of the presence of radiant heat in the zodiacal light.
(Lettre de M. Matthiessen ? M. Arago, in the "Comptes Rendus", t. xvi., 1843, Avril, p. 687.)
Must we suppose that changes are actually in progress in the nebulous ring?
or is it not more probable that, although I could not, by my meteorological instruments, detect any change of heat or moisture near the ground, and small stars of the fifth and sixth magnitudes appeared to shine with equally undiminished intensity of light, processes of condensation may be going on in the uppermost strata of the air, by means of which the transparency, or rather, the reflection of light, may be modified in some peculiar and unknown manner?
p 143 An a.s.sumption of the existence of such meteorological causes on the confines of our atmosphere is strengthened by the "sudden flash and pulsation of light," which, according to the acute observations of Olbers, vibrated for several seconds through the tail of a comet, which appeared during the continuance of the pulsations of light to be lengthened by several degrees, and then again contracted.*
[footnote] *"What you tell me of the changes of light in the zodiacal light, and of the causes to which you ascribe such changes within the tropics, is of the greatr interest to me, since I have been for a long time past particularly attentive, every spring, to this phenomenon in our northern lat.i.tudes. I, too, have always believed that the zodiacal light rotated; but I a.s.sumed (contrary to Poisson"s opinion, which you have communicated to me) that it completely extended to the Sun, with considerably augmenting brightness. The light circle which, in total solar eclipses, is seen surrounding the darkened Sun, I have regarded as the brightest portion of the zodiacal light. I have convinced my self that this light is very different in different years, often for several successive years being very bright and diffused, while in othr years it is scarcely perceptible. I tyhink that I find the first trace of an allusion to the zodiacal light in a letter from Rothmann to Tycho, in which he mentions that in the spring he has observed the twilight did not close until the sun was 24degrees below the horizon. Rothmann must certainly have confounded the disappearance of the setting zodiacal light in the vapors of the western horizon with the actual cessation of twilight. I have failed to observe the pulsations of the light, probably on account of the faintness with which it appears in these countries. You are, however, certainly right in ascribing those rapid variations in the light of the heavenly bodies, which you have perceived in tropical climates, to our own atmosphere, and especially to its higher regions. This is especially in the clearest weather, that these tails exhibit pulsations, commencing from the head, as being the lowest part, and vibrating in one or two seconds through the entire tail, which thus appears rapidly to become some degrees longer, but again as rapidly contracts. That these undulations, which were formerly noticed with attention by Robert Hooke, and in more recent times by Schr?ter and Chladni, "do not actually occur in the tails of the comets", but are produced by our atmosphere, is obvious when we recollect that the individual parts of those tails (which are many millions of miles in length) lie "at very different distances" from us, and that the light from their extreme points can only reach us at intervals of time which differ several minutes from one another. Whether what you saw on the Orinoco, not at intervals of seconds, but of minutes, were actual coruscations of the zodiacal light, or whether they belonged exclusively to the upper strata of our atmosphere, I will not attempt to decide; neither can I explain the remarkable "lightness of whole nights", nor the anomalous augmentation and prolongation of the twilight in the year 1831, particularly if, as has been remarked, the lightest part of these singular twilights did not coincide with the Sun"s place below the horizon." (From a lettr written by Dr. Olbers to myself, and dated Bremen, Marth 26th, 1833.)
As, however, the separate particles of a comet"s tail, measuring millions of miles, p 144 are very unequally distant from earth, it is not possible, according to the laws of the velocity and transmission of light, that we should be able, in so short a period of time, to perceive any actual changes in a cosmical body of such vast extent. There considerations in no way exclude the realith of the changes that have been observed in the emanations from the more condensed envelopes around the nucleus of a comet, nor that of the sudden irradiation of the zodiacal light, from internal molecular motion, nor of the increased or diminished reflection of light in the cosmical vapor of the luminous ring, but should simply be the means of drawing our attention to the differences existing between that which appertains to the air of heaven (the realms of universal s.p.a.ce) and that which belongs to the strata of our terrestrial atmosphere. It is not possible, as well-attested facts prove, perfectly to explain the operations at work in the much-contested upper boundaries of our atmosphere. The extraordinary lightness of whole nights in the year 1831, during which small print might be read at midnight in the lat.i.tudes of Italy and the north of Germany is a fact directly at variance with all that we know, according to the most recent and acute researches on the crepuscular theory, and of the height of the atmosphere.*
[footnote] *Biot, "Trait? d"Astron. Physique", 3me ?d., 1841, t. i., p.
171, 238 and 312.
The phenomena of light depend upon conditions still less understood, and their variability at twilight, as well as in the zodiacal light, excite our astonishment.
We have hitherto considered that which belongs to our solare system -- that world of material forms governed by the Sun -- which includes the primary and secondary planets, comets of short and long periods of revolution, meteoric asteroids, which move thronged together in streams, either sporadically or in closed rings, and finally a luminous nebulous ring, that revolves round the Sun in the vicinity of the Earth, and for which, owing to its position, we may retain the name of zodiacal light. Every where the law of periodicity governs the motions of these bodies, however different may be the amount of tangential velocity, or the quant.i.ty of their agglomerated material parts; the meteoric asteroids which enter our atmosphere from the external regions of universal s.p.a.ce are alone arrested in the course of their planetary revolution, and retained within the sphere of a larger planet. In the solar system, whose boundaries determine the attractive force of the central body, comets are made to revolve in their elliptical p 145 orbits at a distance 44 times greater than that of Ura.n.u.s; may, in those comets whose nucleus appears to us, from its inconsiderable ma.s.s, like a mere pa.s.sing cosmical cloud, the Sun exercises its attractive force on the outermost parts of the emanations radiating from the tail over a s.p.a.ce of many millions of miles. Central forces, therefore, at once const.i.tute and maintain the system.
Our Sun may be considered as at rest when compared to all the large and small, dense and almost vaporous cosmical bodies tht appertain to and revolve around it; but it actually rotates around the common center of gravity of the whole system, which occasionally falls within itself, that is to say, remains within the material circ.u.mference of the Sun, whatever changes may be a.s.sumed by the position of the planets. A very different phenomenon is that presented by the translatory motion of the Sun, that is, the progressive motion of the center of gravity of the whole solar system in universal s.p.a.ce. Its velocity is such* that, according to Bessel, the relative motion of the Sun, and that of 61 Cygni, is not less in one day than 3,336,000 geographical miles.
[footnote] *Bessel, in Schum., "Jahrb. f?r" 1839, s. 51; probably four millions of miles daily, in a relative velocity of at the least 3,336,000 miles, or more than couble the velocity of revolution of the Earth in her orbit round the Sun.
This change of the entire solar system would remain unknown to us, if the admirable exactness of our astronomical instruments of measurement, and the advancement recently made in the art of observing, did not cause our advance toward remote stars to be perceptible, like an approximation to the objects of a distant sh.o.r.e in apparent motion. The proper motion of the star 61 Cygni, for instance, is so considerable, that it has amounted to a whole degree in the course of 700 years.