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HISTORY OF PHYSICAL ASTRONOMY.

the phenomena may he explained in accordance with the above-mentionedprinciples. Thus, in the case wherein they have not been seen, whenthe image of the sun has been received upon a screen, it may justly besupposed that this circumstance arose from the feeble irradiation of thesolar image, whose brightness is so much inferior to that of the actual sur-face of the sun. In the great majority of cases, however, no explanationcan be assigned, which is capable of reconciling the conflicting state-ments of different observers.

Eclipses of the moon, although neither so impressive in their nature,nor so important in their results as those of the sun, attracted the at-tention of astronomers at a very early age, as in the absence of morerefined methods of observation, they served to fix the moon’s place inthe heavens with considerable precision. Hipparchus did not failto perceive that observations of lunar eclipses would furnish a solution ofthe great problem of the longitude. It was accordingly by means of suchphenomena, that the difference between the meridians of places on theearth’s surface was ascertained during a long succession of ages. Theresults so obtained were, however, very inaccurate, on account of the diffi-culty which is experienced in determining, by observation, the exact timeof the occurrence of a lunar eclipse. The method was therefore speedilyabandoned in modern times, upon the invention of others of a more trust-worthy nature. Ptolemy, in his great work the Syntaxes , has determinedthe elements of the lunar orbit by means of observation of eclipses of themoon, made by the Chaldeans, at Babylon, about 700 years before thecommencement of the Christian era. It has been already mentionedthat, by a comparison of the same eclipses with others observed in morerecent times, Halley was led to suspect the famous inequality of thesecular acceleration of the moon’s mean motion.

Observations of lunar eclipses are chiefly interesting in modern timeson account of the physical peculiarities which they occasionally exhibit.The ruddy hue, by means of which the moon is rendered visible, evenwhen she is totally immersed in the earth’s shadow, was a phenomenon,the cause of which was long involved in great obscurity. By some personsit was supposed to arise from a light naturally inherent in the moon’ssurface, but it seemed impossible to reconcile this explanation with theappearance of the moon in other parts of her orbit. The illustrious Keplerwas the first who shewed that the phenomenon was occasioned by therefraction of the earth’s atmosphere, which had the effect of turning thecourse of the solar rays passing through it so as to fall upon the mooneven when the earth rvas actually interposed between her and the sun *.

The deep red colour of the moon’s surface arises from the absorptionof the blue rays of light in passing through the terrestrial atmosphere, inthe same manner as the western sky is seen frequently to assume a ruddyhue when illuminated in the evening by the solar rays. On account of thevariable condition of the terrestrial atmosphere, the quantity of light whichis actually transmitted through it to the moon is liable to fluctuate con-siderably, and hence arises a corresponding variation in the appearancepresented by the moon’s surface during her immersion in the earth’sshadow. If the region of the atmosphere through which the solar rayspass, be everywhere deeply loaded with vapours, the red rays will be almosttotally absorbed as well as the blue, and the illumination of the moon will

Ad Vitellionem Paralipomena, p. 276.