734
ASTRONOMY
what rapid rotation. In the case of Neptune, although no well-marked ellipticity has been observed, it has been established in a satisfactory though intricate way. The plane of the orbit of the satellite is found to be in continuous motion, and the only cause to which this can be attributed is the ellipticity of the planet itself. The result of this cause would be that the axis of the orbital plane would perform a slow revolution around the axis of the planet. The observed motion is undoubtedly a revolution of this kind, but up to the present time the arc described has been so small that neither its radius nor its period can be determined. Were the planets surrounded by comparatively dense atmospheres, especially atmospheres markedly different from that of the earth, that fact would be most eas of the™ ily made known by the spectroscope, since planets. unknown substances in such an atmosphere would show absorption lines differing from those of the solar spectrum. A very dense atmosphere of the same general character as that of the earth would be indicated by a strengthening of the telluric lines. We remark in this connexion that the lines observed in the solar spectrum are of two classes—those which arise from the absorption of the gases surrounding the sun and those produced by our own atmosphere. The latter seem to be principally due to the aqueous vapour in the air. As the light which, reaches us from a planet necessarily comes from the sun and passes through our atmosphere, we must expect to see all the lines of the solar spectrum in the spectrum of any planet, and the important question is whether any new lines or any changes in the strength of the known ones are also found. Studies with this end in view have been made by Huggins and Vogel. In the case of the planets Mercury, Venus, Mars, and Jupiter, the evidence of any well-marked modification of the solar spectrum does not seem conclusive. If, as appears from what has just been said, the inferior and denser regions of the atmospheres of these bodies are mostly filled with clouds and vapour, we should not expect to gain much evidence in this way. In the case of Mars and Jupiter, Vogel thought he detected an increased absorption in the red region. The general conclusion from the studies of these two investigators seems to be that, at least in the case of the three inner planets, there is no evidence of an atmosphere differing materially from that of the earth. In the spectrum of Jupiter, however, a line was found in the red of wave length 6178 which does not belong either to the sun’s or the earth’s atmosphere. Vogel remarks that it may be doubtful whether this line arises from some new and unknown substance in the atmosphere of Jupiter or simply from some combination with which we are not familiar. The case with Uranus and Neptune is different. The extreme faintness of the light from these planets renders it difficult to distinguish the Fraunhofer lines in their spectra, but a number of dark bands were found by Huggins, Keeler, and Vogel in the easily visible portion of the spectrum of Uranus. The following is a list of the bands on which the observers are substantially in agreement :— Wave Length. 618. A broad band fading off towards the red. 596. A narrow faint band. 575. Darkest part of a broad band, extending from 578 to 565. 543. Middle of the darkest band. 486. A group of fine lines. Vogel found a few faint bands above 486, and by photographing the spectrum from F into the ultra-violet Huggins was able to distinguish the stronger Fraunhofer lines. The spectrum of Neptune seems to be of the same general character as that of Uranus; but the bands are more difficult to see, owing to the extreme faintness of
the light. The inference to be drawn from these studies is that these two planets are surrounded by very deep, dense atmospheres, probably materially different in constitution from our own; but until we learn what combinations of known substances, if any, might be adequate to produce such bands as those just described, it is impossible to reach any conclusion as to the nature of those atmospheres. While the photometry of the fixed stars has, in recent years, been placed on a definite scientific basis, the same can scarcely be said of that of the bodies of the solar system. This is owing largely to the difficulty, if not the impossibility, of Photometry so,ar of establishing general laws as to the proportion of s stem light reflected from bodies at various angles of inci- y ’ deuce and reflection. We must, therefore, confine ourselves to a statement of the apparent magnitude of the principal bodies of the solar system under mean conditions. A fundamental datum of the subject is an expression for the quantity of light received from the sun as compared with that from a fixed star of given magnitude, a result which is best expressed in the form of a stellar magnitude of the sun. The results of attempts to fix this datum are so discordant that entire confidence cannot be felt in any of them. To express it as a stellar magnitude of the sun we remark that, on the photometric scale now, adopted, an increase of a hundredfold in the quantity of light corresponds to a drop of 5 units of stellar magnitude in the body emitting the light. In general, a change of n stellar magnitudes is equivalent to a multiplication or division of the amount of light by lO0-4'1, a number whose common logarithm is O'in. The best results for the stellar magnitude of the sun as thus defined seem to be - 26-6 (Wollaston), - 25-8 (Bond), and - 26'6 (Zbllner). Giving Zbllner’s result double weight, we have the stellar magnitude of sun= -26'4. The two best determinations of the ratio of sunlight to that of the full moon seem to be those of Bond and Zollner, which are 470980 and 619000 respectively. Muller (Photometric der Gestirne) estimates the best mean result to be 569500. For the stellar magnitudes of the planets and satellites, we have room only to state what seem to be the best mean results. In the case of Mercury and Venus, the variations are so great that definite results cannot be given. Mag. Mars : at mean opposition ; = -1-75 Jupiter ,, ,, =- 2 '23 Saturn without rings ,0-88 Uranus : at mean opposition 5Neptune ,, ,, 7Satellites of Mars : Phobos 12-8 ,, Deimos 13T Satellites of Jupiter : First 6,, ,, Second 6T ,, ,, Third 5,, ,, Fourth 6Satellites of Saturn Mimas 11 T9 Enceladus 11*4 Tethys 10-5 Dione 10-6 Rhea 9-9 Titan 8Hyperion 12-8 Japetus— from 10to. 11Satellites of Uranus; Titania 14-2 Oberon 14-5 )> )) Satellite of Neptune 13-3 For details respecting the new comets, especially periodic ones, which have been discovered since 1880, reference may be made to the article on that subject; what we are here concerned with is the general question of the origin and constitution of these bodies. A striking confirmation of the view that the comets of short period from time to time become members of our system through the action of one of the larger planets, nearly always Jupiter, is afforded by the case of Comet V. of 1889, discovered by Brooks on 6th July of that year. It was soon found to 'be moving in an orbit with a period of about eight years, and when its motions were traced back it was ascertained to have passed very near the planet Jupiter in June and July 1886. Attempts to compute its orbit previous to this approach were made by Chandler and Poor. The
