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body moving around the sun. Taken in its widest sense it applies to the satellites which are sometimes termed secondary planets. Each of these moves around a planet larger than itself, which it accompanies in its revolution See also:

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

round the sun. A planet not revolving round another is termed a See also:

PRIMARY

primary planet. The primary planets are classified as See also:

major and See also:

minor. The former are eight in number and, with the sun, See also:

FORM (Lat. forma)

form the See also:

PRINCIPAL

principal members of the See also:

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

solar See also:

SYSTEM

system, under which See also:

head their arrangement is described. The See also:

earth on which we live is the third in the See also:

order of the major planets from the sun. With respect to the positions of their orbits relative to the earth, the other planets are distinguished as inferior and See also:

SUPERIOR

superior. The former, only two in number, comprise Mercury and Venus, which revolve between the earth and the sun. The superior planets are those whose orbits are outside that of the earth.

The synodic revolution of an inferior planet is the See also:

time in which it performs a revolution relative to the See also:

LINE

line joining the earth and the sun. This is greater than its actual time of revolution. The phases or appearances presented by such a planet depend upon its configuration with respect to the earth and sun, and therefore go through their See also:

COMPLETE

complete periods in a synodic revolution. At superior See also:

CONJUNCTION (from Lat. conjungere, to join together)

conjunction the illuminated hemisphere of the planet is presented to the earth so that it presents the form of a full moon. As it moves towards inferior conjunction, the lines from the planet to the sun and to the earth, or the See also:

ANGLE (from the Lat. angulus, a corner, a diminutive, of which the primitive form, angus, does not occur in Latin; cognate are the Lat. angere, to compress into a bend or to strangle, and the Gr. ayKOS, a bend; both connected with the Aryan root ank-, to

angle sun-earth as seen from the planet, on which the phase depends, continually make a greater angle. At the time of greatest See also:

ELONGATION

elongation this angle is 900, and the planet appears one See also:

HALF

half illuminated, like the moon at first or last See also:

QUARTER (through Fr. from Lat. quartarius, fourth part)

quarter. Then, as it approaches inferior conjunction, the visible portion of the disk assumes the See also:

CRESCENT (Lat. crescens, growing)

crescent form, and while the circle bounding the disk continually increases owing to the approach of the planet to the earth, the crescent becomes thinner and thinner until, near inferior conjunction, the planet is no longer visible. After conjunction the phases occur in the See also:

REVERSE

reverse order. The brilliancy of the planet, as measured by the See also:

TOTAL

total amount of See also:

LIGHT

light we receive from it, goes through a similar See also:

cycle of 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. The point of greatest brilliancy is between inferior conjunction and greatest elongation. In the See also:

case of Venus this phase occurs about three or four See also:

WEEKS, EDWIN LORD (1849-1903)

weeks before and after inferior conjunction. See also:

TIRE

Tire dots round the orbits See also:

elm. the poeitipn of the planets at intervals of ten days The symbols) t indloate the greatest distance of an See also:

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

orbit See also:

north and See also:

south of the prone of the See also:

ECLIPTIC

Ecliptic The dots round the orbits show the position of the planets at intervals of a thousand days The arrow-head on rash orbit shows r the direction of reuolution,also the ' See also:

PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place of each planet an See also:

Jan. ist.10l0 at See also:

NOON

noon Ly Q e 6 .9 rP Mars The Earth The Moon Mercury Clef o'.

Ere tf afO:.k n See also:

Scale ten thousand times that of Orbits Seale about one See also:

HUNDRED

hundred times that of Oitfte (The Earth,Mars,and the See also:

SUE, EUGENE [JosEPH MARIE] (1804—1857)

Sue, are shown as seen from the dirsetiw of the See also:

pole of the ecliptic in their true axial poeitioer) Moos 0 Marna Venus See also:

Barth Mars • See also:

Uranus See also:

Neptune' See also:

Boa/e of planets 20,000 times that of Orbits /Jupiter and Saturn are shown in their true axial position, Uranus and Neptune in the axial positions inferred from the motions of their Satellites/. Scale about 4000 times that of the Orbits Saturn's I- :I- System of Sateilltes System IO Uranus's Satellites Jupiter's of SatellitesNeptune's See also:

SATELLITE (from the Lat. satelles, an attendant)

Satellite In the figures given above are shown the relative orbits of the planets, the orbits of Mars, the Earth, Venus and Mercury (fig. i) being See also:

DRAWN

drawn to a scale twenty times that of the See also:

OUTER

outer ones—Neptune, Uranus, Saturn, Jupiter (fig. 2). The positions of the planets at ten-See also:

day intervals; their actual position on the 1st of See also:

JANUARY

January 1910 at noon, of their nodes and nearer apses, and the points when they are farthest distant north and south of the ecliptic, are also given. The relative sizes of the planets are also given, orientated in their true axial position with regard to the ecliptic. The nearer planets (and also the Moon) are separately compared (fig. 3) ; and then shown (on a smaller scale) in comparison with the more distant ones (fig. 4). Finally scale diagrams of the distances of the orbits of the satellite-systems of Saturn, Uranus, Jupiter and Neptune are given (fig. 5). The phases of a superior planet are less strongly marked, because the lines from the planet to the earth and sun never increase to a right angle. The result is that although the apparent disk of Mars is sometimes gibbous in a very marked degree, it is always more than half illuminated.

In the case of the other superior planets, from Jupiter outward, no variation in phase is perceptible even to telescopic See also:

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

vision. The entire disk always seems fully illumina ted. The most favourable time for viewing an inferior planet is near that of greatest brilliancy. As it recedes further from the earth, although a continually increasing proportion of its disk is illuminated by the sun, this See also:

ADVANTAGE

advantage is neutralized by the diminution in its See also:

SIZE

size produced by the increasing distance. When a superior planet is in opposition to the sun it rises at sunset and is visible all See also:

NIGHT

night. This is also the time when nearest the earth, and therefore when the circumstances are most favourable for observation. The greater the distance of a planet from the sun the less is the See also:

SPEED, JOHN (1552–1629)

speed with which it moves in its orbit. The orbit being larger, the time of its revolution is greater in a yet larger degree. An approximation to the See also:

general See also:

LAWS

laws of speed in different planets is that the linear speed is inversely proportional to the square See also:

root of the mean distance. From this follows See also:

KEPLER, JOHANN (1571-1630)

Kepler's third See also:

law, that the squares of the times of revolution are proportional to the cubes of the mean distances. Notes on the See also:

PLATE

Plate showing Planetary Spectra. Only those lines and bands are mentioned which are See also:

PECULIAR

peculiar to the planets; the See also:

FRAUNHOFER, JOSEPH VON (1787-1826)

Fraunhofer lines are therefore omitted.

See also:

WAVE

Wave Remarks. length. 4600 Neptune. 4800 F See also:

HYDROGEN [symbol H, atomic weight x-oo8 (0=16)]

hydrogen, H$ strong. Neptune, Uranus, Saturn (?) 5090 Neptune, Uranus. 5190* Broad. Neptune, Uranus. 5370 Neptune, Uranus. 5430 Broad, unsymmetrical, Neptune, Uranus, Saturn, 5570 * strong. Jupiter. Neptune, Uranus (?). 5700* Broad, unsymmetrical, Neptune, Uranus, Saturn (?) 5980 strong.

Jupiter (?). Strong. Neptune, Uranus. 6090 Neptune, Uranus. 6190 Very strong. Neptune, Uranus, Saturn, 6400 Broad (?). Jupiter. Neptune, Uranus. 6500* Neptune, Uranus, Jupiter, 656o C hydrogen, Ha. Saturn (?). Neptune, Uranus. 6670* Broad See also:

BAND

band.

[678o See also:

Bright region due to ab- Neptune, Uranus, Saturn, upiter. Neptune, Uranus. 682o sence of selective ab- Neptune, Uranus, Saturn, sorption which is strong both above and below. Strong, narrow, near 7020 above B. Jupiter. Strong, broad. Neptune, Uranus, Saturn, [7140 Bright, unabsorbed region Jupiter. Neptune, Uranus. 7260 similar to that at 6780. Saturn, Jupiter. Strongest band See also:

PRESENT

present. 7500 Band (?).

Saturn. It was once supposed that the planets were surrounded by comparatively dense atmospheres. The question whether such spectra and is the case, and, if so, what is the See also:

PHYSICAL

physical constitu-Atmo- tion of the atmospheres, is a difficult one, on which See also:

SPHERES, MUSIC OF THE

spheres of little light is thrown except by the spectroscope. the Planets. If any of these bodies is surrounded by a transparent See also:

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

atmosphere like that of the earth, the light which reaches us from it will have passed twice through this atmosphere. If the latter were materially different in its constitution from that of the earth, that fact would be made known by the spectrum showing absorption lines or bands different from those found in the solar spectrum as we observe it. If, how-ever, the planetary atmosphere had the same See also:

composition as ours we should see only an intensification of the atmospheric lines, which might be imperceptible were the atmosphere rare. Actual observation has thus far shown no well marked deviation in the spectra of any of the inner See also:

GROUP

group of planets, Mercury, Venus and Mars, from the solar spectrum as we see it. It follows that any atmospheres these planets may have must, if transparent, be rare. The See also:

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

evidence in the cases of Venus and Mars is given in the articles on these planets. Taking the outer group of planets, it is found that the spectrum of Jupiter shows one or more very faint shaded bands not found in that of the sun. In Saturn these bands become more marked, and in Uranus and Neptune many more are seen. The spectra in question have been observed both optically and photographically by several observers, among whom See also:

HUGGINS, SIR WILLIAM (1824-1910)

Huggins, See also:

Vogel and See also:

Lowell have been most successful.

It may be said, in a general way, that seven or eight well marked dark bands, as well as some fainter ones are observable in the spectra of the two outer planets. The general conclusion from this is that these planets are surrounded by deep and dense atmospheres, semi-transparent, of a constitution which is probably very different from that of the earth's atmosphere. But it has not, up to the present time, been found practicable to determine the chemical constitution of these appendages, except that hydrogen seems to be an important constituent. (See Plate.) Intimately associated with this subject is the question of the conditions necessary to the permanence of an atmosphere round a planet. Dr See also:

JOHNSTONE

Johnstone Stoney investigated these stability of conditions, taking as the basis of his See also:

WORK

work the Planetary kinetic theory of gases (Trans. See also:

ROY, WILLIAM (1726-1790)

Roy. Dubl. See also:

Soc. vi. Atmo-305). On this theory every See also:

MOLECULE (from mod. Lat. molecula, the diminutive of moles, a mass)

molecule of a gaseous spheres. See also:

mass is completely disconnected from every other and is in rapid motion, its velocity, which may amount to one or more thousand feet per second, depending on the temperature and on the atomic See also:

WEIGHT

weight of the See also:

gas. At any temperature the velocities of individual molecules may now and then increase without any well-defined limit. If at the boundary of an atmosphere the velocity should exceed a certain limit fixed by the mass and force of gravity of the planet, molecules might See also:

FLY (formed on the root of the supposed original Tent. fleugan, to fly)

fly away through space as See also:

INDEPENDENT

independent bodies.

The See also:

ABSENCE (Lat. absentia)

absence of hydrogen from the atmosphere of the earth, and of an atmosphere from the moon, may be thus explained. If the fundamental hypo-theses of Dr Stoney's investigations are correct and complete, it would follow that neither the satellites and minor planets of the solar system nor Mercury can have any atmosphere. If the See also:

SEPARATE

separate molecules thus flying away moved according to the laws which would govern an See also:

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

ordinary body, they would, after leaving their respective planets, move round the sun in independent orbits. The possibility is thus suggested that the See also:

MATTER

matter producing the zodiacal light may be ail agglomeration of gaseous molecules moving round the sun; but several questions respecting the intimate constitution of matter will have to be settled before any definite conclusions on this point can be reached. It is not to be assumed that a molecule would move through the See also:

ETHER, (C2H5)2O

ether without resistance as the minutest known body does, and there is probably a See also:

RADICAL (Lat. radix, a root)

radical difference between the minutest particle of meteoric matter and the molecule of a gas. The relations of identity or difference between such finely-divided matter as See also:

SMOKE (from O. Eng. smeocan, to smoke, reek, cf. Dutch meek, Ger. Schmauch, probably allied to Gr. vol9caiv)

smoke and atmospheric haze and a true gas have yet to be fully established, and until this is done a definite and satisfactory theory of the subject does not seem possible. Since the See also:

RADIATION, THEORY OF

radiation of See also:

HEAT (0. E. haktu, which like " hot," Old Eng. hat, is from the Teutonic type haita, hit, to be hot; cf. Ger. hitze, heiss; Dutch, hitte, heet, &c.)

heat by a planet is, with our present See also:

INSTRUMENTS

instruments, scarcely capable of detection and measurement, the temperature of these bodies can be estimated Temperature only from general physical laws. The laws govern- of the See also:

ing the radiation of heat have been so See also:

DEVELOPED

developed Planets. during See also:

RECENT

recent years that it is now possible to See also:

state at least the general principle on which a conclusion as to the temperature of a planet may be reached. At the same time our knowledge of the conditions which prevail on other planets is so limited, especially as regards their atmospheres, that only more or less probable estimates of the temperature of their surfaces can even now be made. Summarily stated, some of the physical principles are these: r. A neutrally coloured body—understanding by that term one which absorbs the same fraction of the thermal radiation falling upon it whatever the wave length of this radiation-exposed to the sun's radiation in void space tends to assume a definite temperature, called the normal temperature, the degree of which depends upon the distance of the body from the sun. This is a result of See also:

Kirchhoff's laws of radiation.

2. An atmosphere surrounding such a body, if at 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, will tend to assume a state of thermal See also:

EQUILIBRIUM (from the Lat. aequus, equal, and libra, a balance)

equilibrium, in which the temperature will be the same at all heights. 3. If the atmosphere is kept in See also:

CONSTANT, BENJAMIN (1845-1902)

constant motion by an inter-change between its higher and See also:

LOWER

lower portions, the tendency is towards adiabatic equilibrium, in which the temperature diminishes at a constant See also:

RATE

rate with the height, until it may approach the See also:

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

absolute zero. The rate of diminution depends upon the intensity of gravity and the physical constants of the gases composing the atmosphere. 4. In the actual case of a planet surrounded by an atmosphere and exposed to the sun's radiation, the actual rate of diminution of temperature with height above the See also:

SURFACE

surface of the planet lies between the extreme limits just defined, the rate varying widely with the conditions. The general tendency will be towards a See also:

CONDITION (Lat. condicio, from condicere, to agree upon, arrange; not connected with conditio, from condere, conditum, to put together)

condition in which the temperature at the See also:

BASE

base of the atmosphere is higher than the normal, while in the upper regions it is lower. The temperature of the surface of the planet on which the atmosphere rests is determined partly by the sun's radiation and partly by the temperature of the See also:

air. What we should generally expect in the absence of any selective absorption by the air is that the temperature of the lower air would be higher every See also:

PART

part of it is equally exposed to the suns rays by a rapid than that of the material surface on which it rests. But this ; revolution. As has already been intimated, the actual temperature condition might be reversed by the effect of such absorption may also depend upon the interior heat of the planet, which is an in either the air or the material of the planet. unknown quantity.

(S. N.) Planet. Mean Distance from Sun. Eccentricity g Lonitude See also:

LONGITUDE (from Lat. longitudo, " length ")

Longitude Inclina- See also:

Period of Mean Daily MeanLon g-j of Orbit. of See also:

PEN (Lat. penna, a feather, pen)

Pen- of See also:

NODE (Lat. nodus, a loop)

Node. tion. Revolution. Motion. itude 1910, Astronomical Thousands See also:

Units. of See also:

MILES, NELSON APPLETON (1839— )

Miles. helion. Jan. o. Mercury . 0.3870987 36,000 0.205614 75° 54' 47° 9' 7° 0' Days. 4°'0927 3° 32' 87.969256 ' Venus 0.7233315 i 67,269 0.006821 130° 10' 75° 47' 3° 24' 224.700798 10.6021 73° 53' Earth . . 1.0000000 92,998 0.016751 101 ° 13' - - 365.256360 0°.9856 99° 17' Mars .

. . 1.523688 141,701 0.093309 334° 13' 48° 47' 1° 51' 686.979702 00.52403 47° 39' Jupiter . . 5.202804 483,853 0.048254 12° 36' 99° 37' I° 19' 4332'5879 0°•083091 181° 43' Saturn 9.538844 887,098 0.056061 90° 49' 113°3' 2° 30' 10759.2010 0°.033460 28° 56' Uranus . . 19.19096 1,784,732 0.047044 169° 3' 73° 29' o° 46' 30586.29 00.011770 286° 42' Neptune 30.07067 2,796,528 0.008533 43° 45' 130° 41' 1° 47' 60187.65 0°'006020 107° 1' Planet. Angular Semidiameter. At See also:

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

Diameter Reciprocal of See also:

DENSITY (Lat. densus, thick)

Density. Gravity at Orbital Normal Mass. Surface. Velocity. Temperature. See also:

EQUATORIAL

Equatorial. Polar.

Dist. in Miles. (Q's mass =1) (See also:

WATER

water =1) (®- ,) (®=1) Milespersec. Centigrade. Mercury 3.30" 3.30" I 2,976 10,000,000 3.5 .633 0.24 29.76 195° Venus . . 8.46" 8.46" I 7,629 408,000 5.05 •913 o•88o 21.77 70° Earth . . 8.79" 8.76" I 7,917 333,430 5.53 I.000 1.00 18.52 19° Mars 4'80" 4'76" I 4,316 3,093,500 3.68 •666 0.363 15.00 - 36° Jupiter 18.75" 17.65" 5.203 86,259 1,047.35 1.363 '247 2.68 8.12 -144° Saturn 8.75" 7.88" 9'539 72,772 3,500 0.678 •123 I.13 6•oo - 177° Uranus 1.90" 1'90" 19.19 32,879 22,869 1.13 .204 0.85 4.24 -205° ~ LNeptune 1.1o" I.10" 30'07 29,827 19,314 1.79 •322 1.22 3.40 -218° ment. Something of this sort has been suspected in the case of Jupiter, which has several points of resemblance to the sun. The planets Uranus and Neptune which, but for their atmospheres, would approximate to the absolute zero in temperature, may be prevented from doing so by the dense atmosphere which the spectroscope shows around them. A very elaborate investigation of the probable mean temperatures of the surfaces of the several planets has been made by J. H. Poynting, Phil. Trans.

(vol. 202A, 1904). Tables of Planetary Elements and Constants. Table I. gives the elements determining the motions of each major planet, and Table II. certain See also:

numbers pertaining to its physical condition. For explanation of terms used see ORBIT. The elements are given for the See also:

EPOCH (Gr. E7roxij, holding in suspense, a pause, from E1rxav, to hold up, to stop)

epoch 1900, Jan. o, See also:

GREENWICH

Greenwich mean time, except the mean longitudes, which are for 1910, Jan. o. In interpreting or using the numbers it must be remembered that only the mean distances and mean daily motions can be regarded as well determined and invariable quantities. The other elements are subject to a See also:

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

secular variation, and all vary more or less from the See also:

ACTION

action of the planets. In Table II. the reciprocal of the mass is given, the mass of the sun being unity. Some of these and other quantities are extremely uncertain. This is especially the case with the mass of Mercury, which the astronomical tables put at 1/6,000,000 that of the sun, while G. W.

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