76

HISTORY OF PHYSICAL ASTRONOMY.

earth. Laplace investigated this interesting question, and arrived at theconclusion that such a condition was inconsistent with the theory of gra-vitation. He found, in fact, that the terrestrial attraction would alwaysdraw the moon’s axis into coincidence with the line joining the earthand moon, so that the rotatory motion will participate in the secular acce-leration of the motion in longitude, and consequently the lunar hemi-sphere, which is turned away from us, will remain for ever concealed fromview, with the exception of the small portion disclosed by the periodicinequalities.

Laplace has considered the circumstances which .determine the sta-bility of the singular mechanism with which the planet Saturn isfurnished. lie considers that the rotatory motion of the rings may heaccounted for by supposing the particles composing them to be homoge-neous, and to move freely among each other like the particles of afluid. Under such conditions, he shews that they would be maintained inequilibrium by the action of the planet and the centrifugal force generatedby their own rotatory motion, the exterior surfaces assumed by both ringsbeing such, that all sections perpendicular to them, and passing throughthe centre of the planet, would be ellipses, whose major axes when producedwould pass through that point. Laplace hence concluded that the period ofthe rotation of the rings is equal to that of a satellite revolving at thedistance of the centre of the generating ellipse. This period he found tohe equal to lOh. Sd'.SG". It is remarkable that Herschel inferred, fromcertain periodic changes in the appearance of the rings, that they accom-plished a revolution round the planet in lOh. 32'.15"*.

If the rings were uniform and circular, and were not exposed to theaction of any extraneous force, it would still be possible for them to re-volve constantly round the planet; but it is clear that the least disturb-ance, as the action of a satellite or comet, would affect their stability, andultimately precipitate them upon the. body of the planet. In order, there-fore, to assure the permanence of the rings, Laplace conceived that it wasnecessary to suppose their figures to he irregular, so that any disturbanceeither of them might suffer would he rapidly checked in course of rota-tion by the unequal distribution of the mass.

CHAPTER VII.

Jupiter ’s Satellites.—Galileo.—Simon Marius.—Hodierna.—Borelli.—- Cassini.—His firstTables.—He is invited to France. —He publishes his Second Tables.—His Rejection ofthe Equation of Light. — Researches of Maraldi I.—He discovers that the Inclinationof the second Satellite is variable.— Bradley’s Discoveries.—Maraldi II.—His Dis-coveries relative to the third and fourth Satellites. —He adopts the Equation of Light.—Wargentin.—He discovers the Inequalities in Longitude of the first and secondSatellites . — He remarks that the third Satellite has two Equations of the Centre.—■Motion of the Nodes of the fourth Satellite.—Inclination of the third Satellite.—Libratory Motion of the Nodes.—Inclination of the fourth Satellite.

The .discovery of Jupiter 's satellites is one of the most interesting eventsin the history of astronomy. Even in any age it would have been deemed

* Phil. Trans. 1790.