HISTORY OF PHYSICAL ASTRONOMY.

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CHAPTER XIV.

Researches on the Solar Parallax.—Modem Determinations of this Element_Discovery

of the Solar Spots Consequences deduced from this Discovery.—Period of the Sun’s

Rotation.—Theories of the Solar Spots..—Wilson.—Herschel.—Researches on the

Lunar Parallax.—Ellipticity of Mercury_Researches on the Rotation of Venus.—

Discovery of the Ultra Zodiacal Planets.—Micrometrical measures of Jupiter ’sSatellites.—Micrometrical measures of Saturn , and of his Ring.—Discovery of theeighth Satellite of Saturn.—Researches on the Satellites of Uranus.—Lassel’s Dis-covery of the Satellite of Neptune.—Researches on Comets.—Halley’s Comet.—Comet of 1843.

The determination of the distance from the sun to any of the planetsrevolving round him, is one of the most important problems of astronomicalscience. When this object is effected in any individual instance, it is thenpossible by means of Kepler ’s third law to ascertain the distances of all theplanets from the sun, and hence, also, their distances from the earth cor-responding to any assigned instant. An accurate knowledge of the latter isindispensable in reducing the apparent positions of the planets to the truepositions which they would occupy if seen from the centre of the earth.These results may obviously be all derived from the solar parallax, which isexpressed by the reciprocal of the sun’s distance from the earth. The realvalue of this element has, in all ages, formed an interesting subject ofenquiry. Aristarchus of Samos, by observing the angular distance, betweenthe sun and moon, when the latter was dichotomized, inferred that thesun is nineteen times more distant from the earth than the moon is.Ptolemy assumed this result to be true, and, combining it with the valueof the lunar parallax as determined by his own observations (or ratherperhaps those of Hipparchus ), he obtained 3' for the amount of the solarparallax. This value was adopted by all his successors down to Tycho Brahe inclusive. Kepler , while engaged in his celebrated researches onthe motions of Mars , availed himself of the accurate observations of Tycho Brahe to institute a searching scrutiny into the value of the solar parallax.The conclusion he came to was, that it did not exceed 1', and in all pro-bability fell very short of that quantity. The researches of Cassini reducedthe superior limit considerably below this value. He attacked the problemnot by direct investigation, but by means of researches on the parallax ofMars . When this planet is in opposition, it is much nearer to the earththan the sun is, and consequently its parallax is then a much moreappreciable quantity than that of the sun. But if the parallax of anyplanet is once determined, the parallax of the sun or that of any otherplanet, becomes known by means of Kepler ’s third law. The method pro-posed by Cassini for determining the parallax of Mars , was to make simul-taneous observations of the planet when in opposition at two places of theearth considerably distant from each other, and then, by a comparison ofthe results, to ascertain the amount of displacement arising from thedifference of position. For this purpose, Richer was sent by the Academy of Sciences to Cayenne, in Africa , while Cassini, Roemer, and Picardobserved the planet at different places in France . A comparison of the

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