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HISTORY OF PHYSICAL ASTRONOMY.

through which the solar rays pass, is so loaded with vapour in some partsas in a great degree to absorb the rays, while in other parts it allows themto be transmitted with facility to the moon’s surface. Hence during thetotality of a lunar eclipse, the moon’s surface will appear quite obscure insome parts, and in other parts will exhibit a high degree of illumination.Thus Kepler states that during the total eclipse of the moon, which hap-pened on the 16th of August, 1598, while one-half of her disk was seenonly with great difficulty, the other half was discernible by a deep redlight of such brilliancy, that at first he was doubtful whether it was im-mersed in the shadow at all*. If dense clouds be interspersed throughoutthe whole of the atmospheric zone through which the rays pass, there willmanifestly ensue an irregular distribution of light and darkness over thewhole of the moon’s surface. Such was the appearance, which, according toSir John Herschel , the moon presented during the total eclipse of October13, 1837 f.

The phenomena presented by the transits of the inferior planets overthe sun’s disk are among the most interesting that arise from the relativemovements of the various bodies composing the solar system. The im-portance of the transits of Venus , in determining the solar parallax, hasbeen already referred to. The physical features of such phenomena tendalso to attract the attention of the enquirer. The transits of Mercury areof less utility than those of Venus in ascertaining the absolute dimensionsof the solar system; but, as in the case of that planet, the phenomenaaccompanying such occurrences are calculated to throw some light on thephysics of the celestial regions.

Long before the invention of the telescope it was asserted that Mercury had been seen upon the sun’s disk. Even as early as the ninth century,it was stated by the author of the “ Life of Charlemagne ” that the planethad been visible upon the sun for eight days. As it was impossible, fromthe rapidity of his orbitual motion, that Mercury could have remained solong visible upon the sun, Kepler suggested that the expression octo dies,or eight days, might have been erroneously substituted for the barbaricLatin octoties, or eight times. It is now well known, however, that theangular magnitude of the planet is by far too small to allow its being seenwith the naked eye on such an occasion, so that in all probability thephenomenon referred to was no other than a solar spot of more than ordi-nary magnitude. Coming down to a more recent age, Averroes , a famousphysician of Cordova, in Spain , who wrote a commentary on the Alma-gest, positively asserted that he had seen Mercury on the sun at a timewhen, from the motion of the planet, it ought to have been passingthrough its inferior conjunction J. This statement, however, must berejected as unworthy of belief, for the same reason as that alluded to inthe case just cited. Kepler himself was induced to believe on one occasionthat he had seen the planet on the sun, but the invention of the telescopea few years afterwards having speedily led to the discovery of spots on thesun’s disk, he retracted his opinion, and candidly acknowledged that thephenomenon seen by him was, in all probability, no other than one of suchmacula.

says Mr. Walkey, “and during that period I have several times beheld an eclipse ofthe moon, but never before did my eyes behold the moon positively give good light fromher disk during a total eclipse.”

* Ad Vitellionem Paralipomena, p. 276. f Outlines of Astronomy, p. 256.

J Copern., Revol., lib. x. The real name of this individual, who was of Moorishorigin, was Ebn Roschd.