HISTORY OF PHYSICAL ASTRONOMY.
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nature. A certain corona, resembling the Iris, surrounded the orb of thesun, and obscured his light.”
Plutarch , also, about the same time, alludes in one of his works to thephenomenon of the luminous ring. Speaking of a total eclipse of thesun which had recently happened, he endeavours to show why the dark-ness arising from occurrences of this nature is not so profound as thatwhich usually prevails when the sun is below the horizon. He beginsby assuming, as the basis of his reasoning, that the earth greatly exceedsthe moon in dimensions. After citing various authorities in support ofthis assertion, he then proceeds thus :—“ Whence it happens that theearth, on account of its magnitude, entirely conceals the sun from our
sight. But even although the moon should at any time hide
the whole of the sun, still the eclipse is deficient in duration as well asamplitude, for a peculiar effulgence is seen around the circumference,which does not allow a deep and very intense shadow.”* It cannotadmit of any doubt that the phenomenon alluded to in the above pas-sage is the luminous ring that has been invariably observed aroundthe moon in modern times on the occasion of a total eclipse of thesunf.
It would seem that Clavius observed the luminous ring at Borne duringthe eclipse which happened on April 9, 1607, although he was not con-scious of its real nature. It has been already mentioned, that when theobscuration was greatest, he perceived a narrow ring of light around themoon, which he supposed to be the margin of the solar disk. Kepler,however, maintained, that the luminous circle seen by Clavius, could notreally be a portion of the sun. He found, in fact, by calculation, thatduring the eclipse, the moon was at her mean distance from the earth, inwhich position, he remarked, her apparent diameter exceeds that of thesun even when he is in perigee; while at the same time the sun wasapproaching towards apogee, where bis apparent diameter is the leastpossible. He therefore came to the conclusion that the sun must havebeen totally covered by the moon during the eclipse, and, consequently,that the appearance observed by Clavius could not have been producedby the direct transmission of the solar rays. The explanation which that
* It will be seen that the passage in the text differs materially from that cited byM. Arago, in the Comptes Rendus, tome xiv., p. 848, which is to the following effect: —“ La Lune laisse deborder autour d’elle, dans les eclipses, une partie du Soleil, ce quidiminue I'obscurite.” It is merely on the strength of the last part of this sentence, andin direct contradiction to the first part, that M. Arago ventures to suggest that thephenomenon of the luminous ring is probably that to which Plutarch alludes. It ismanifest, however, that the sentence may be supposed, with greater plausibility, to applyto annular eclipses. With respect to the passage cited in the text, there cannot exist ashadow of a doubt that it refers to the luminous ring that is visible around the moonduring a total eclipse of the sun. The following are the express terms in which Plutarch alludes to the phenomenon :— « Si trtXwvi xav oXov Tori xgv^n rov %Xiov, six s %tt %govov,flvSe tXo.to;, 91 ixXir^i;, aXXa T:Oi<panarai rl; atryx Tioi rnv truv, ovx lu/ra /3 ahTav ytvitrSat
•rijv axiav xa) ux^am. (Plut.’, Opera Mor. et Phil., vol. ix., p. 682, Edit. Lips., 1778).M. Arago does not mention the part of Plutarch ’s works in which the original of thetranslation given by him is to be found. It is not improbable, therefore, that he mayrefer to some less unequivocal passage of the Greek author than that above cited.
+ This eclipse is sometimes referred to the year 98, A.D. ; Kepler, however, is dis-posed to believe that it happened in the year 113, A.D. ( Tahul. Rudolp. Prcecept.,p. 104).