PORTION OF THE COSMOS. THE SOLAR DOMAIN. 347 



( 553 )j we shall be surprised that Copernicus, who lived to 

 attain his 70th year, should have had to complain on his 

 death-bed that, much as he had tried, he had never seen 

 Mercury. Nevertheless, the Greeks designated this planet, 

 and justly so, "the strongly sparkling" (ori\/3wv) (554)^ on 

 account of its occasional intense light. Like Yenus, it 

 presents to us phases or varying forms in its illuminated 

 portion ; and appears to us sometimes as a morning, and 

 sometimes as an evening star. 



Mercury, at its mean solar distance, is little more than 8 

 millions of German, or 32 millions English, geographical 

 miles from the Sun, being exactly 0'3870938 parts of the 

 Earth's mean distance from the Sun. Prom the great ex- 

 centricity of its orbit (0-2056163), the distance of Mercury 

 from the Sun is, in perihelion, 6J, and in aphelion 10 mil- 

 lions of German geographical miles (25 and 40 millions 

 English). It completes its revolution round the Sun in 

 87 mean terrestrial days, 23 hours, 15 minutes, and 46 

 seconds. By the somewhat uncertain observations of the 

 shape of the southern horn of the sickle, and by noticing a 

 dark streak which was blackest towards the east, Schroter 

 and Harding estimated its time of rotation at 24 hours 5 

 minutes. 



According to BesseFs determinations made on the occa- 

 sion of the transit of Mercury on the 5th of May, 1832, 

 its true diameter is 671 German, or 2684 English geogra- 

 phical miles ( 555 ) i. e. 0'391 parts of the Earth's diameter. 



The mass of Mercury was assigned by Lagrange 

 from very hazardous assumptions respecting the recipro- 

 cities of ratios of densities and distances. Encke's comet 

 of short period first afforded a means of correcting this 



