90 PHYSIOGRAPHY. 



(114) THE MOON. 



The horizontal parallax of the moon is 57' 2*7" ; her greatest dis- 

 tance from the earth is 259,600 miles, and her least distance 

 221,000 miles. The eccentricity of her orbit is 0*055. 



The mean distance of the moon is about 60'3 radii of Hie earth, 

 or 240,000 miles. The inclination of the plane of the moon's orbit 

 to the ecliptic is about 5 8' 42". 



The moon is very nearly spherical, with a radius of 1,080 miles ; 

 her volume is 5 '2765 x 10 9 cubic miles, or about -^ of the volume 

 of the earth ; her mass is about -gV of the mass of the earth, hence 

 the acceleration of gravity at her surface would be about 5 '4 feet per 

 second per second. The density of the moon is about 3 '5, or rather 

 more than half that of the earth. 



(115) THE CALENDAR (//. C). 



The tropical year is 335 days 5 hours 43 minutes 46 seconds, or 

 365*2422 mean solar days. 



The solar cycle is 28 Julian years, after which period the same day 

 of the week falls on the same day of the solar month (1894 is 

 the 27th). 



The Sothic period was 1460 (more nearly 1500) years. 



The cycle of the Roman Indiction was 15 years (1894 is 

 the 7th. 



A sidereal month or complete circuit of the moon in the heavens 

 is 27'3217 days. 



A lunar month (lunation) is 29*5306 days. 



An anomalistic month (perigee to perigee) is 27*5446 days. 



A tropical month (vernal equinox to vernal equinox) is 27*3216 

 days. 



A nodical month (node to node of the same kind) is 27*2122 

 days. 



The Saros, or cycle of the conjunction of the sun and moon in 

 nearly the same place on the ecliptic (223 lunations), is 6585*3212 

 days, or 18 years and 10 or 11 days. 



