See also:

• COMET (Gr. Koµieiis, long-haired)

COMET (Gr. Koµieiis, See also:

• LONG, GEORGE (1800-1879)
• LONG, JOHN DAVIS (1838– )

long-haired)  , in See also:

• ASTRONOMY (from Gr. &arpov, a star, and v ,usiv, to classify or arrange)

astronomy, one of a class; of seemingly nebulous bodies, moving under the See also:

• INFLUENCE (Late Lat. influentia, from influere, to flow in)

influence of the See also:

• SUN
• SUN (0. Eng. sonne, Ger. sonne. Fr. soleil, Lat. sol, Gr. ijXior, from which comes helio- in various English compounds)

sun's attraction in very See also:

• ECCENTRIC (from Gr. F,c, out of, and Kivrpov, centre)

eccentric orbits . A See also:

• COMET (Gr. Koµieiis, long-haired)

comet is visible only in a small arc of its See also:

• ORBIT (from Lat. orbita, a track, orbis, a wheel)

orbit near See also:

• PERIHELION (Gr. reg., near, titXLor, sun)

perihelion, differing but slightly from the arc of a See also:

• PARABOLA

parabola . An obvious but not See also:

• SHARP, GRANVILLE (1735-1813)
• SHARP, JAMES (1618-1679)
• SHARP, JOHN (1645-1714)
• SHARP, RICHARD (1759-1835)
• SHARP, WILLIAM (1749-1824)
• SHARP, WILLIAM (1856-1905)

sharp See also:

• CLASSIFICATION (Lat. classis, a class, probably from the root cal-, cla-, as in Gr. icaMce, clamor)

classification of comets is into See also:

• BRIGHT, JOHN (1811-188g)
• BRIGHT, SIR CHARLES TILSTON

bright comets visible to the naked See also:

• EYE
• EYE (O. Eng. edge, Ger. Auge; derived from an Indo-European root also seen in Lat. oc-ulus, the organ of vision (q.v.)

eye, and telescopic comets which can be seen only with a See also:

• TELESCOPE

telescope . The telescopic class is much the more numerous of the two, only from 20 to 30 bright comets usually appearing in any one See also:

• CENTURY (from Lat. centuria, a division of a hundred men)

century, while several telescopic comets, frequently 6 or 8, are generally observed in the course of a See also:

• YEAR

year . A bright comet consists of (1) a See also:

• STAR

star-like See also:

• NUCLEUS (Lat. for the kernal of a nut, nux, the stone of fruit)

nucleus; (2) a nebulous haze, called the See also:

• COMA (Gr. K(.7)

coma, surrounding this nucleus, the latter fading into the haze by insensible gradations; (3) a tail or luminous stream flowing from the coma in a direction opposite to that of the sun . The nuclei and comae of different comets exhibit few peculiarities to the unaided See also:

• VISION (from Lat. videre, to see), or SIGHT

vision except in respect to brightness; but the tails of comets differ widely, both in brightness and in extent . They range from a barely visible See also:

• BRUSH (from Fr. brosse, which, like the English word, means both the undergrowth of a wood and the instrument; if the word in both these meanings is ultimately the same, then the origin is from a bundle of brushwood used as a brush or broom, but this is h
• BRUSH, GEORGE DE FOREST (1855– )

brush or See also:

• FEATHER (0. Eng. (ether, Ger. Feder, from an Indo-European root seen also in Gr. lrrepov, and aEreo-Oat, to fly)

feather of See also:

• LIGHT

light to a phenomenon extending over a considerable arc of the heavens, which, comparatively bright near the See also:

• HEAD (in 0. Eng. heafad; the word is common to Teutonic languages; cf. Dutch hoofd, Ger. Haupt, generally taken to be in origin connected with Lat. caput, Gr. KerbOvi7)
• HEAD, SIR EDMUND WALKER, BART
• HEAD, SIR FRANCIS BOND, BART

head of the comet, becomes gradually fainter and more diffuse towards its end, fading out by gradations so insensible that a precise length cannot be assigned to it . When a telescopic comet is first discovered the nucleus is frequently invisible, the See also:

• OBJECT

object presenting the See also:

• APPEARANCE (from Lat. apparere, to appear)

appearance of a faint nebulous haze, scarcely distinguishable in aspect from a nebula . When the nucleus appears it may at first be only a comparatively faint condensation, and may or may not develop into a point of light as the comet approaches the sun . A tail also is generally not seen at See also:

• GREAT

great distances from the sun, but gradually develops as the comet approaches perihelion, to fade away again as the comet recedes from the sun . A few comets are known to revolve in orbits with a See also:

• REGULAR

regular See also:

• PERIOD
• PERIOD (Gr. irepioSos, a going or way round, circuit, aepi, round, and &Sis, way, road)

period, while, in the See also:

• CASE
• CASE, JOHN (d. 1600)

case of others, no See also:

• EVIDENCE (Lat. evidentia, evideri, to appear clearly)

evidence is afforded by observation that the orbit deviates from a parabola . Were the orbit a parabola or See also:

• HYPERBOLA

hyperbola the comet would never return (see ORBIT) .

Periodicity may be recognized in two ways: observations during the apparition may show that the See also:

• MOTION (Lat. motio, from movere, to move)
• MOTION, LAWS OF

motion is in an elliptic and not in a parabolic orbit; or a comet may have been observed at more than one return . In the latter case the comet is recognized as distinctly periodic, and therefore a member of the See also:

• SOLAR, SOLLER (Lat. solarium, Fr.• galetas, Ital. solaio)

solar See also:

• SYSTEM

system . The shortest periods range between 3 and 10 years . The See also:

• MAJORITY (Fr. majorite; Med. Lat. majoritas; Lat. major, greater)

majority of comets which have been observed are shown by observation to be periodic; the period is usually very See also:

• LONG, GEORGE (1800-1879)
• LONG, JOHN DAVIS (1838– )

long, being sometimes measured by centuries, but generally by thousands of years . It is conceivable that a comet might revolve in a hyperbolic orbit . Although there are several of these bodies observations on which indicate such an orbit, the deviation from the parabolic See also:

• FORM (Lat. forma)

form has not in any case been so well marked as, to be fully established . Circumstances See also:

• LEAD
• LEAD (pronounced iced)

lead to the classification of newly appearing comets as expected and unexpected . An expected comet is a periodic one of which the return is looked for at a determinate See also:

• TIME (0. Eng. Lima, cf. Icel. timi, Swed. timme, hour, Dan. time; from the root also seen in " tide," properly the time of between the flow and ebb of the sea, cf. O. Eng. getidan, to happen, " even-tide," &c.; it is not directly related to Lat. tempus)
• TIME, MEASUREMENT OF
• TIME, STANDARD

time and in a certain region of the heavens . When this is not the case the comet is an unexpected one . See also:

• PHYSICAL

Physical Constitution of Comets.—The subject of the physical constitution of these bodies is one as to the details of which much uncertainty still exists . The considerations on which conclusions in this See also:

• FIELD (a word common to many West German languages, cf. Ger. Feld, Dutch veld, possibly cognate with O.E. f olde, the earth, and ultimately with root of the Gr. irAaror, broad)
• FIELD, CYRUS WEST (1819-1892)
• FIELD, DAVID DUDLEY (18o5-1894)
• FIELD, EUGENE (1850-1895)
• FIELD, FREDERICK (18o1—1885)
• FIELD, HENRY MARTYN (1822-1907)
• FIELD, JOHN (1782—1837)
• FIELD, MARSHALL (183 1906)
• FIELD, NATHAN (1587—1633)
• FIELD, STEPHEN JOHNSON (1816-1899)
• FIELD, WILLIAM VENTRIS FIELD, BARON (1813-1907)

field See also:

• REST (O. Eng. rest, reste, bed, cognate with other Teutonic forms, e.g. Ger. Rast, Riiste, rest, and probably Gothic Rasta, league, i.e. resting or stopping place)

rest are very various, and can best be set forth by beginning with what we may consider to be the best established facts . We must regard it as well established that comets are not, like See also:

• PLANETS, MINOR

planets and satellites, permanent in See also:

• MASS (O.E. maesse; Fr. messe; Ger. Messe; Ital. messa; from eccl. Lat. missa)
• MASS, IN

mass, but are continuously losing See also:

• MINUTE
• MINUTE (Lat. minutes, small; minuere, to make less)

minute portions of the See also:

• MATTER

matter which belongs to them, through a progressive dissipation—at least when they are in the neighbourhood of the sun .

When near perihelion the matter of a comet is seen to be undergoing a See also:

• PROCESS

process in the nature of evaporation, successive envelopes of vapour rising from the nucleus to form the coma, and then gradually repelled from the sun to form the tail . If this process went on indefinitely every comet would, in the course of ages, be entirely dissipated . This result has actually happened in the case of some known comets, the best established example of which is that of Biela, in which the process of disintegration was clearly followed . As the amount of matter lost by a comet at any one return cannotbe estimated, and may be very small, it is impossible to set any limit to the period during which its See also:

• LIFE

life may continue . It is still an unsettled question whether, in every case, the evaporation will ultimately cease, leaving a residuum as permanent as any other mass of matter . The next question in logical See also:

• ORDER
• ORDER (through Fr. ordre, for earlier ordene, from Lat. ordo, ordinis, rank, service, arrangement; the ultimate source is generally taken to be the root seen in Lat. oriri, rise, arise, begin; cf. " origin ")
• ORDER, HOLY

order is one of great difficulty . It is whether the nucleus of a comet is an opaque solid See also:

• BODY

body, a cluster of such bodies, or a mass of particles of extreme tenuity . Some light is thrown on this and other questions by the spectroscope . This See also:

• INSTRUMENT (Lat. instrumentum, from insincere, to build up, furnish, arrange, prepare)

instrument shows in the spectrum of nearly every comet three bright bands, recognized as those of See also:

• HYDROCARBONS

hydrocarbons . The obvious conclusion is that the light forming these bands is not reflected sunlight, but light radiated by the gaseous hydro-carbons . Since a See also:

• GAS

gas at so great a distance from the sun cannot be heated to incandescence, the question arises how incandescence is excited . The generalizations of See also:

• RECENT

recent years growing out of the phenomena of See also:

• RADIOACTIVITY

radioactivity make it highly probable that the source is to be found in some form of See also:

• ELECTRICAL
• ELECTRICAL (or ELECTROSTATIC) MACHINE

electrical excitation, produced by electrons or other corpuscles thrown out by the sun .

The resemblance of the cometary spectrum to the spectrum of hydrocarbons in the See also:

• GEISSLER, HEINRICH (1814-1879)

Geissler See also:

• TUBE (Lat. tuba)

tube lends great plausibility to this view . It is remarkable that the great comet of 1882 also showed the bright lines of See also:

• SODIUM [symbol Na, from Lat. natrium; atomic weight 23.00 (0=16)]

sodium with such intensity that they were observed in daylight by R . See also:

• COPELAND, HENRY

Copeland and W . O . Lohse . In addition to these gaseous spectra, all but the fainter comets show a continuous spectrum, crossed by the See also:

• FRAUNHOFER, JOSEPH VON (1787-1826)

Fraunhofer lines, which is doubtless due to reflected sunlight . It happens that, since the spectroscope has been perfected, no comet of great brilliancy has been favourably situated for observation . Until the opportunity is offered, the conclusions to be derived from spectroscopic observation cannot be further extended . In the telescope the nucleus of a bright comet appears as an opaque mass, one or more seconds in See also:

• DIAMETER (from the Cr. &a, through, µErpov, measure)

diameter, the See also:

• ABSOLUTE (Lat. absolvere, to loose, set free)

absolute dimensions comparing with those of the satellites of the planets, sometimes, indeed, equal to our See also:

• MOON (a common Teutonic word, cf. Ger. Mond, Du. maan, Dan. maane, &c., and cognate with such Indo-Germanic forms as Gr. µlip, Sans. ma's, Irish mi, &c.; Lat. uses luna, i.e. lucna, the shining one, lucere, to shine, for the moon, but preserves the word i
• MOON, SIR RICHARD, 1ST BARONET (1814-1899)

moon . But the actual results of micrometric See also:

• MEASURES

measures are found to differ very widely . In the case of See also:

• DONATI, GIOVANNI BATTISTA (1826-1873 )

Donati's comet of 1858 the nucleus seemed to grow smaller as perihelion was approached . This is evidently due to the fact that the coma immediately around the nucleus was so bright as apparently to form a See also:

• PART

part of it at considerable distances from the sun .

G . P . See also:

• BOND
• BOND, SIR EDWARD AUGUSTUS (1815-1898)

Bond estimated the diameter of the actual nucleus at 500 M . That the nucleus is a body of appreciable mass seems to be made probable by the fact that, except for the central attraction of such a body, a comet would speedily be dissipated by the different attractions of the sun on -different parts of the mass, which would result in each particle pursuing an orbit of its own . It follows that there must be a mass sufficient to hold the parts of the comet, if not absolutely together, at least in each other's immediate neighbourhood . How great a central mass may be required for this is a subject not yet investigated . It might be supposed that the amount of matter must be sufficient to make the nucleus quite opaque . But two considerations based on observations militate against this view . One is that an opaque body, reflecting much sunlight, would show a brighter continuous spectrum than has yet been found in any comet . Another and yet more remarkable observation is on See also:

• RECORD (Lat. recordari, to recall to mind, from cor, heart or mind)

record which goes far to prove not only the tenuity, but the transparency of a cometary nucleus . The great comet of 1882 made a transit over the sun on the 17th of See also:

• SEPTEMBER (Lat. septem, seven)

September, an occurrence unique in the See also:

• HISTORY

history of astronomy . But the fact of the transit escaped See also:

• ATTENTION (from Lat. ad-tendo, await, expect; the condition of being " stretched " or " tense ")

attention except at the See also:

• OBSERVATORY

observatory of the Cape of See also:

• GOOD, JOHN MASON (1764-1827)

Good See also:

• HOPE, ANTHONY
• HOPE, THOMAS (c. 1770-1831)

Hope .

Here the comet was watched by W . H . See also:

• FINLAY, GEORGE (1799-1875)

Finlay and by W . L . Elkin as it approached the sun, and was kept in sight until it came almost or quite in contact with the sun's disk, when it disappeared . It should, if opaque, have appeared a few minutes later, projected on the sun's disk; but not a trace of it could be seen . The sun was approaching Table See also:

• MOUNTAIN (0. Fr. montaigne; popular Lat. montanea, an adjectival form from the classical mons, montis, whence Eng. " mount," a form usually used along with the name of an individual mountain, e.g. Mt Everest)
• MOUNTAIN, THE (La Montagne)

Mountain at the See also:

• CRITICAL (in alphabetical order of authors)

critical moment, and its See also:

• LIMB

limb was undulating badly, making the detection of a minute point difficult . The possibility of a very small opaque nucleus is therefore still See also:

• LEFT

left open; yet the remarkable conclusion still holds, that, immediately around a possible central nucleus, the matter of the head of the comet was so rare as not to intercept any appreciable fraction of the sun's light . This result seems also to show that, with the possible exception of a very small central mass, what seems to telescopic vision as a nucleus is really only the central portion of the coma, which, as the distance from the centre increases, becomes less and less dense by imperceptible gradations . Another fact tending towards this same conclusion is that after this comet passed perihelion it showed several nuclei following each other . Evidently the powerful attraction of the sun had separated the parts of the apparent nucleus, which were following each other in nearly the same orbit . As they could not have been completely brought together again, we may suppose that in such cases the smaller nuclei were permanently separated from the See also:

• MAIN (from the Aryan root which appears in " may " and " might," and Lat. magnus, great)
• MAIN (Lat. Moenus)

main body .

In addition to this, the remarkable similarity of the orbit of this comet to that of several others indicates a See also:

• GROUP

group of bodies moving in nearly the same orbit . The other members of the group were the great comets of 1843, 188o and 1887 . The latter, though so bright as to be conspicuous to the naked eye, showed no nucleus whatever . The closely related orbits of the four bodies are also remarkable for approaching nearer the sun at perihelion than does the orbit of any other known body . All of these comets pass through the matter of the sun's See also:

• CORONA (Lat. for " crown ")

corona with a velocity of more than See also:

• LOO (formerly called " Lanterloo," Fr. lanturlu, the refrain of a popular 17th-century song)

loo m per second without suffering any retardation . As it is beyond all reasonable See also:

• PROBABILITY (Lat. probabilis, probable or credible)

probability that several See also:

• INDEPENDENT

independent bodies should have moved in orbits so nearly the same, the conclusion is that the comets were originally portions of one mass, which gradually separated in the course of ages by the powerful attraction of the sun as the collection successively passed the perihelion . It may be remarked that observations on the comet of 1843 seemed to show a slight See also:

• ELLIPTICITY

ellipticity of the orbit, corresponding to a period of several centuries; but the deviation of all the orbits from a parabola is too slight to be established by observations . The periods of the comets are therefore unknown except that they must be counted by centuries and possibly by thousands of years . Another fact which increases the complexity of the question is the well-established connexion of comets with meteoric showers . The shower of See also:

• NOVEMBER (Lat. novem, nine)

November 13-15, now known as the Leonids, which recurred for several centuries at intervals of about one-third of a century, are undoubtedly due to a stream of particles left behind by a comet observed in 1866 . The same is true of Biela's comet, the disintegrated particles of which give rise to the Andromedids, and probably true also of the Perseids, or See also:

• AUGUST (originally Sextilis)

August meteors, the orbits of which have a great similarity to a comet seen in 1862 . The See also:

• GENERAL
• GENERAL (Lat. generalis, of or relating to a genus, kind or class)

general and well-established conclusion seems to be that, in addition to the visible features of a comet, every such body is followed in its orbit by a swarm of meteoric particles which must have been gradually detached and separated from it .

(See See also:

• METEOR (Gr. µerEwpa, literally " things in the air,” from µera, beyond, and aeip6Lv, to lift up)

METEOR.) The source of the repulsive force by which the matter forming the tail of a comet is driven away from the sun is another question that has not yet been decisively answered . Two causes have been suggested, of which one has only recently been brought to light . This is the repulsion of the sun's rays, a form of See also:

• ACTION

action the probability of which was shown by J . Clerk See also:

• MAXWELL
• MAXWELL, JAMES CLERK (1831–1879)

Maxwell in 1870, and which was experimentally established about See also:

• THIRTY

thirty years later . The intensity of this action on a particle is proportional to the See also:

• SURFACE

surface presented by the particle to the rays, and therefore to the square of its diameter, while its mass, and therefore its See also:

• GRAVITATION (from Lat. gravis, heavy)

gravitation to the sun, are proportional to the See also:

• CUBE (Gr. K46os, a cube)

cube of the diameter . It follows that if the See also:

• SIZE

size and mass of a particle in space are below a certain limit, the repulsion of the rays will exceed the attraction of the sun, and the particle will be driven off into space . But, in order that this repulsive force may See also:

• ACT (Lat. actus, actum)

act, the particles, however minute they may be, must be opaque . Moreover, theory shows that there is a See also:

• LOWER

lower as well as an upper limit to their magnitude, and that it is only between certain definable limits of magnitude that the force acts . Conceiving the particle to be of the See also:

• DENSITY (Lat. densus, thick)

density of See also:

• WATER

water, and considering its diameter as a diminishing variable, theory shows that the repulsion will See also:

• BALANCE (derived through the Fr. from the Late Lat. bilantia, an apparatus for weighing, from bi, two, and lanx, a dish or scale)

balance gravity when the diameter has reached o•ooi5 of a millimetre . As the diameter is reduced below this limit' the ratio of the repulsive to the attractive force increases, but soon reaches a maximum, after which it diminishes down to a diameter of 0.00007 mm., when the two actions are again balanced . Below this limit the light speedily ceases to act . It follows that a purely gaseous body, such as would emit a characteristic bright See also:

• LINE

line spectrum, would not be subject to the repulsion .

We must therefore conclude that both the solid and gaseous forms of matter are here at See also:

• PLAY

play, and this view is consonant with the fact that the comet leaves behind it particles of meteoric matter . Another possible cause is electrical repulsion . The probability of this cause is suggested by recent discoveries in radioactivity and by the fact that the sun undoubtedly sends forth electrical emanations which may ionize the gaseous molecules rising from the nucleus, and lead to their repulsion from the sun, thus resulting in the phenomena of the tail . But well-established See also:

• LAWS

laws are not yet sufficiently See also:

• DEVELOPED

developed to lead to definite conclusions on this point, and the question whether both causes are combined, and, if not, to which one the phenomena in question are mainly due, must be left to the future . A curious circumstance, which may be explained by a duplex See also:

• CHARACTER (Gr. xapareri7p, from xap&crew, to scratch)

character of the matter forming a cometary tail, is the great difference between the visual and photographic aspect of these bodies . The soft, delicate, feathery-like form which the comet with its tail presents to the eye is wanting in a photograph, which shows principally a See also:

• ROUND (O. Fr. rond, Lat. rotundus, the Fr. is the source also of Du. rond; Ger., Swed., Dan. and Nor. rond)

round head with an irregularly formed tail much like the knotted stalk of a plant . It follows that the light emitted by the central See also:

• AXIS (Lat. for " axle ")

axis of the tail greatly exceeds in actinic See also:

• POWER [WILLIAM GRATTAN] TYRONE (1797-1841)

power the diffuse light around it . A careful comparison of the form and intensity of the photographic and visual tails may throw much light on the question of the constitution of these bodies, but no good opportunity of making the comparison has been afforded since the See also:

• ART

art of See also:

• CELESTIAL, OR STELLAR

celestial See also:

• PHOTOGRAPHY (Gr. bias, light, and ypa w, to write)
• PHOTOGRAPHY, CELESTIAL

photography has been brought to its See also:

• PRESENT

present See also:

• STATE
• STATE, GREAT OFFICERS OF

state of perfection . The main conclusion to which the preceding facts and considerations point is that the matter of a comet is partly solid and partly gaseous . The gaseous form is shown conclusively by the spectroscope, but in view of the extreme delicacy of the indications with this instrument no quantitative estimate of the gas can be made . As there is no central mass sufficient to hold together a continuous See also:

• ATMOSPHERE (Gr. fir µ6r, vapour; orkaipa, a sphere)

atmosphere of elastic gas of any sort, it seems probable that the gaseous molecules are only those rising from the coma, possibly by See also:

• ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

ordinary evaporation, but more probably by the action of the ultra-See also:

• VIOLET

violet and other rays of the sun giving rise to an ionization of disconnected gaseous molecules . The matter cannot be wholly gaseous because in this case there could be no central force sufficient to keep the parts of the comet together .

The facts also point to the conclusion that the solid matter of a comet is formed of a swarm or See also:

• CLOUD (from the same root, if not the same word, as " clod," a word common in various forms to Teutonic languages for a mass or lump; it is first applied in the usual sense in the late 13th century; the Anglo-Saxon chid is only used in the sense of " a ma

cloud of small disconnected masses, probably having much resemblance to the meteoric masses which are known to be flying through the solar system and possibly of the same general See also:

• KIND (O. E. ge-cynde, from the same root as is seen in " kin," supra)

kind as these . The question whether there is any central solid of considerable mass is still undecided; it can only be said that if so, it is probably small relative to See also:

• COSMIC (from Gr. Kovµos, order or universe)

cosmic masses in general—more likely less than greater than loo m. in diameter . The light of the comet therefore proceeds from two See also:

• SOURCES

sources: one the incandescence of gases, the other the sunlight reflected from the solid parts . No estimate can be formed of the ratio between these two kinds of light until a bright comet shall be spectroscopically observed during an entire apparition . Origin and Orbits of Comets.—The great difference which we have pointed out between comets and the permanent bodies of the solar system naturally suggested the See also:

• IDEA (Gr. Ibia, connected with i&eiv, to see; cf. Lat. species from specere, to look at)

idea that these bodies do not belong to that system at all, but are nebulous masses, scattered through the stellar spaces, and brought one by one into the See also:

• SPHERE (Gr. vclsa?pa, a ball or globe)

sphere of the sun's attraction . The results of this view are easily shown to be incompatible with the observed facts . The sun, carrying the whole solar system with it, is moving through space with a See also:

• SPEED, JOHN (1552–1629)

speed of about ro in. per second . If it approached a comet nearly at rest the result would be a relative motion of this amount which, as the comet came nearer, would be constantly increased, and would result in the comet describing relative to the sun a markedly hyperbolic orbit, deviating too widely from a parabola to leave any doubt, even in the most extreme cases . Moreover, a large majority of comets would then have their aphelia in the direction of the sun's motion, and therefore their perihelia in the opposite direction . Neither of these results corresponds to the fact . The conclusion is that if we regard a comet as a body not belonging to the solar system, it is at least a body which before its approach to the sun had the same motion through the stellar spaces that the sun has . As this unity of motion must have been maintained from the beginning, we may regard comets as belonging to the solar system in the sense of not being visitors from distant regions of space .

The See also:

• ACCEPTANCE (Lat. acceptare, frequentative form of accipere, to receive)

acceptance of this seemingly inevitable conclusion leads to another: that no comet yet known moves in a really hyperbolic orbit, but that the limit of eccentricity must be regarded as 1, or that of the parabola . It is true that seeming evidence of hyperbolic eccentricity is sometimes afforded by observations and regarded by some astronomers as sufficient . The objections to the reality of the hyperbolic orbit are two . (1) A comet moving in a decidedly hyperbolic orbit must have come from so great a distance within a finite time, say a few millions of years; as to have no relation to the sun, and must after its approach to the sun return into space, never again to visit our system . In this case the motion of the sun through space renders it almost infinitely improbable that the orbit would have been so nearly a parabola as all such orbits are actually found to be . (2) The apparent deviation from a very elongated See also:

• ELLIPSE (adapted from Gr. EXkei 'tc, a deficiency, iXXeirely, to fall behind)

ellipse has never been in any case greater than might have been the result of errors of observation on bodies of this class . This being granted, a luminous view of the causes which lead to the observed orbits of comets is readily gained by imagining these bodies to be formed of nebulous masses, which originally accompanied the sun in its See also:

• JOURNEY (through O. Fr. jornee or journee, mod. Fr. journee, from med. Lat. diurnata, Lat. diurnus, of or belonging to dies, day)

journey through space, but at distances, in most cases, vastly greater than that of the farthest See also:

• PLANET (Gr. ssXavirrns, a wanderer)

planet . Such a mass, when See also:

• DRAWN

drawn towards the sun, would move round it in a nearly parabolic orbit, similar to the actual orbits of the great majority of comets . The period might be measured by thousands, tens of thousands, or hundreds of thousands of years, according to the distances of the comet in the beginning; but instead of bodies extraneous to the system, we should have bodies properly belonging to the system and making revolutions around the sun . Were it not for the effect of planetary attraction long periods like these would be the general See also:

• RULE

rule, though not necessarily universal . But at every return to perihelion the motion of a comet will be to some extent either accelerated or retarded by the action of See also:

• JUPITER

Jupiter or any other planet in the neighbourhood of which it may pass . Commonly the action will be so slight as to have little influence on the orbit and the time of revolution .

But should the comet See also:

• CHANCE (through the O. Fr. cheance, from the Late Lat. cadentia, things happening, from cadere, to fall out, happen; cf. " case ")

chance to pass the orbit of Jupiter just in front of the planet, its motion would be retarded and the orbit would be changed into one of shorter period . Should it pass behind the planet, its motion would be accelerated and its period lengthened . In such cases the orbit might be changed to a hyperbola, and then the comet would never return . It follows that there is a tendency towards a See also:

• GRADUAL (Med. Lat. gradualis, of or belonging to steps or degrees; gradus, step)

gradual but See also:

• CONSTANT, BENJAMIN (1845-1902)

constant diminution in the See also:

• TOTAL

total number of comets . If we See also:

• CALL (from Anglo-Saxon ceallian, a common Teutonic word, cf. Dutch kallen, to talk or chatter)

call Ae the amount by which the eccentricity of a cometary orbit is less than unity, .e will be an extremely minute fraction in the case of the See also:

• ORIGINAL

original orbits . If we call 5 the See also:

• CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change which the eccentricity 1— Ae undergoes by the action of the planets during the passage of the comet through our system, it will leave the system with the eccentricity 1— Ae 5 . The possibilities are even whether 3 shall be See also:

• POSITIVE (or PORTABLE) ORGAN

positive or negative . If negative, the eccentricity will be diminished and the period shortened . If positive, and greater than De, the eccentricity 1— Ae + 5 will be greater than 1, and then the comet will be thrown into a hyperbolic orbit and become for ever a wanderer through the stellar spaces . The nearer a comet passes to a planet, especially to Jupiter,the greatest planet, the greater 3 may be . If 3 is a considerable negative fraction, the eccentricity will be so redaced that the comet will after the approach be one of See also:

• SHORT, FRANCIS JOB (1857– )

short period . It follows that, however long the period of a comet may be, there is a possibility of its becoming one of short period if it approaches Jupiter .

There have been several cases of this during the past two centuries, the most recent being that of See also:

• BROOKS, CHARLES WILLIAM SHIRLEY (1816-1874)
• BROOKS, PHILLIPS (1835-1893)

Brooks's comet, 1889, V . Soon after its See also:

• DISCOVERY

discovery this body was found to have a period of only about seven years . The question why it had not been observed at previous returns was settled after the orbit had been determined by computing its motion in the past . It was thus found that in See also:

• OCTOBER

October 1886 the comet had passed in the immediate neighbourhood of Jupiter, the action of which had been such as to change its orbit from one of long period to the short observed period . A similar case was that of Lexel's comet, seen in 1770 . Originally moving in an unknown orbit, it encountered the planet Jupiter, made two revolutions round the sun, in the second of which it was observed, then again encountered the planet, to be thrown out of its orbit into one which did not admit of determination . The comet was never again found . A general conclusion which seems to follow from these conditions, and is justified by observations, so far as the latter go, is that comets are not to be regarded as permanent bodies like the planets, but that the conglomerations of matter which compose them are undergoing a process of gradual dissipation in space . This process is especially rapid in the case of the fainter periodic comets . It was first strikingly brought out in the case of Biela's comet . This object was discovered in 1772, was observed to be periodic after several revolutions had been made, and was observed with a See also:

• FAIR

fair degree of regularity at different returns until 1852 . At the previous apparition it was found to have separated into two masses, and in 1852 these masses were so widely separated that they might be considered as forming two comets .

Notwithstanding careful See also:

• SEARCH, or VISIT AND SEARCH

search at times and places when the comet was due, no trace of it has since been seen . An examination of the table of periodic comets given at the end of this See also:

• ARTICLE (from Lat. articulus, a joint)

article will show that the same thing is probably true of several other comets, especially Brorsen's and Tempel's, which have each made several revolutions since last observed, and have been sought for in vain . In view of the seemingly inevitable dissipation of comets in the course of ages, and of the actually observed changes of their orbits by the attraction of Jupiter, the question arises whether the orbits of all comets of short period may not have been determined by the attraction of the planets, especially of Jupiter . In this case the orbit would, for a period of several centuries, have continued to nearly intersect that of the planet . We find, as a matter of fact, that several periodic comets either pass near Jupiter or have their aphelia in the neighbourhood of the orbit of Jupiter . The approach, however, is not sufficiently See also:

• CLOSE
• CLOSE (from Lat. clausum, shut)
• CLOSE, MAXWELL HENRY (1822-1903)

close to have led to the change unless in former times the proximity of the orbits was much greater than it is now . As the orbits of all the bodies of the solar system are subject to a slow See also:

• SECULAR (Lat. saecularis, of or belonging to an age or generation, saeculum)

secular change of their form and position, this may only show that it must have been thousands of years since the comet became one of short period . The two cases of most difficulty are those of See also:

• HALLEY, EDMUND (1656–1742)

Halley's and See also:

• ENCKE, JOHANN FRANZ (1791–1865)

Encke's comets . The orbit of the former is so elongated and so inclined to the general See also:

• PLANE

plane of the planetary orbits that its secular variation must be very slow indeed . But it does not pass near the orbit of any planet except See also:

• VENUS

Venus; and even here the proximity is far from being sufficient to have produced an appreciable change in the period . The orbit of Encke's comet is entirely within the orbit of Jupiter, and it also cannot have passed near enough to a planet for thousands of years to' have had its orbit changed by the action in question . It therefore seems difficult to regard these two comets as other than permanent members of the solar system .

See also:

• SPECIAL

Special Periodic Comets.—One of the most remarkable periodic comets with which we are acquainted is that known to astronomers as Halley's . Having perceived that the elements of the comet of 1682 were nearly the same as those of two comets which had respectively appeared in 1531 and 1607, See also:

• EDMUND
• EDMUND, or EADMUND (c. 980-1016)
• EDMUND, SAINT [EDMUND RICE] (d. 1240)

Edmund Halley concluded that all the three orbits belonged to the same comet, of which the periodic time was about 76 years . After a rough estimate of the perturbations it must sustain from the attraction of the planets, he predicted its return for 1757,-a bold prediction at that time, but justified by the event, for the comet again made its appearance as was expected, though it did not pass through its perihelion till the See also:

• MONTH
• MONTH (a common Teutonic word, cf. Ger. Mond, Du. maand, Dan. maaned, &c., and cognate with Lat. mensis, Gr. µi7v, &c., in other branches of the Indo-Germanic family; all ultimately from the root seen in the word for the moon in nearly all those languages

month of See also:

• MARCH
• MARCH (1) (from Fr. marcher, to walk; the earliest sense in French appears to be " to trample," and the origin has usually been found in the Lat. marcus, hammer; Low Lat. marcare, to hammer; hence to beat the road with the regular tread of a soldier: cf.
• MARCH, AUZIAS (c. "1395-1458)
• MARCH, EARLS OF
• MARCH, FRANCIS ANDREW (1825– )

March 1759, the attraction of Jupiter and See also:

• SATURN
• SATURN [SATURNUS]

Saturn having caused, as was computed by See also:

• CLAIRAULT (or CLAIRAUT), ALEXIS CLAUDE (1713-1765)

Clairault previously to its return, a retardation of 618 days . This comet had been observed in 1o66, and the accounts which have been preserved represent it as having then appeared to be four times the size of Venus, and to have shone with a light equal to a See also:

• FOURTH

fourth of that of the moon . History is silent respecting it from that time till the year 1456, when it passed very near to the See also:

• EARTH
• EARTH (a word common to Teutonic languages, cf. Ger. Erde, Dutch aarde, Swed. and Dan. jord; outside Teutonic it appears only in the Gr. 'pq'e, on the ground; it has been connected by some etymologists with the Aryan root ar-, to plough, which is seen in
• EARTH, FIGURE OF
• EARTH, FIGURE OF THE

earth: its tail then extended over 6o° of the heavens, and had the form of a sabre . It returned to its perihelion in 1835, and was well observed in almost every observatory . But its brightness was far from comparing with the glorious accounts of its former See also:

• APPARITIONS

apparitions . That this should have been due to the process of dissipation does not seem possible in so short a period; we must therefore consider either that the earlier accounts are greatly exaggerated, or that the brightness of the comet is subject to changes from some unknown cause . Previous appearances of Halley's comet have been calculated by J . R . See also:

• HIND

Hind, and more recently by P . H .

See also:

• COWELL, JOHN (1554—1611)

Cowell and A . C . D . Crommelin of See also:

• GREENWICH

Greenwich, the latter having carried the comet back to 87 B.C. with certainty, and to 240 B.C. with fair probability . It was detected by Max See also:

• WOLF
• WOLF (Canis lupus)
• WOLF, FRIEDRICH AUGUST (1759-1824)
• WOLF, HUGO (1860-1903)
• WOLF, JOSEPH (1820-1899)

Wolf at See also:

• HEIDELBERG

Heidelberg on plates exposed on See also:

• SEPT

Sept . 11, 1909, and subsequently on a Greenwich See also:

• PLATE

plate of Sept . 9 . The known comet of shortest period bears the name of J . F . Encke, the astronomer who first investigated its orbit and showed its periodicity . It was originally discovered in 1789, but its periodicity was not recognized until 1818, after it had been observed at several returns . This comet has given rise to a longer See also:

• SERIES (a Latin word from serere, to join)

series of investigations than any other, owing to Encke's result that the orbit was becoming smaller, and the revolutions therefore accelerated, by some unknown cause, of which the most plausible was a resisting See also:

• MEDIUM

medium surrounding the sun .

As this comet is almost the only one that passes within the orbit of See also:

• MERCURY
• MERCURY (MERCU1uus)
• MERCURY (symbol Hg, atomic weight = 2oo)

Mercury, it is quite possible that it alone would show the effect of such a medium . Recent investigations of this subject have been made at the Pulkova Observatory, first by F . E. von Asten and later by J . O . Backlund who, in 1909, was awarded the See also:

• GOLD
• GOLD [symbol Au, atomic weight 195.7(H = 1),197.2(0 =16)]

Gold See also:

• MEDAL (Fr. medaille, from Lat. metallum)

Medal of the Royal Astronomical Society for his researches in this field . During some revolutions there was evidence of a slight See also:

• ACCELERATION (from Lat. accelerare, to hasten, celer, quick)

acceleration of the return, and during others there was not . The following is a See also:

• LIST (O.E. lisle, a Teutonic word, cf. Dut. lust, Ger. Leiste, adapted in Ital. lista and Fr. lisle)
• LIST, FRIEDRICH (1789-1846)

list (compiled in 1909) of comets which are well established as periodic, through having been observed at List of Periodic Comets observed at more than one Return . one or more returns . In addition to what has already been said of several comets in this list the following remarks may be made . Tuttle's comet was first seen by P . F . A .

Mechain in 1790, but was not recognized as periodic until found by Tuttle in 1858, when the resemblance of the two orbits led to the conclusion of the identity of the bodies, the period of which was soon made evident by continued observations . The comets of Pons and See also:

• OLBERS, HEINRICH WILHELM MATTHIAS (1758-1840)

Olbers are remarkable for having an almost equal period . But their orbits are otherwise totally different, so that there does not seem to be any connexion between them . Brorsen's comet seems also to be completely dissipated, not having been seen since 1879 . There are also a number of cases in which a comet has been observed through one apparition, and found to be apparently periodic, but which was not seen to return at the end of its supposed period . In some of these cases it seems likely that the comet passed near the planet Jupiter and thus had its orbit entirely changed . It is possible that in other cases the apparent periodicity is due to the unavoidable errors of observation to which, owing to their diffused outline, the nuclei of comets are liable . (S . N.) COMET-SEEKER, a small telescope (q.v.) adapted especially to searching for comets: commonly of short See also:

• FOCAL

focal length and large See also:

• APERTURE (from Lat. aperire, to open)

aperture, in order to secure the greatest brilliancy of light .