ASTRONOMY. 



353 



passes to the north side of the earth's orbit being called its ascending 

 node. The apsides gradually advance, but the nodes retrograde; and 

 thus the direction in which these points appear, is changed, as seen 

 from the sun, and referred to the fixed stars or celestial sphere. 



The earth, is an oblate sphere, 7899 miles in polar, and 7925 

 miles in equatorial diameter : its mean distance from the sun being 

 95,000,000 miles. The principal circles of the earth, we have 

 already defined, in the introduction to Geography ; (p. 162) ; and 

 we there mentioned that the earth's diurnal rotation causes the alter- 

 nation of days and nights ; while the parallelism of its axis, which 

 makes an angle of 665 with the plane of its orbit, but points con- 

 tinually towards the north star in the heavens, and hence sometimes 

 inclines the north pole towards the sun, and sometimes from it, varies 

 the length of the days and nights, and causes the changes of the 

 seasons. The exact length of the tropical or equinoctial year, is 

 365 days, 5 hours, 48 minutes, and 48 seconds. The moon, is a 

 satellite, or secondary planet, 2160 miles in diameter, and 237,500 

 miles from the earth ; around which it revolves, while carried, with the 

 earth, around the sun. It performs a lunation, or synodic revolu- 

 tion, that is from new moon to new moon again, in 29 days, 12 

 hours, 44 minutes, and 3 seconds. When near either of its nodes, 

 at new moon, its shadow falls upon the earth, and causes a solar 

 eclipse, or eclipse of the sun ; and in the like case at full moon, it 

 passes into the earth's shadow, and causes a lunar eclipse, or eclipse 

 of the moon. 



Mercury and Venus are called inferior planets, because they are 

 nearer to the sun than the earth is ; but the other planets are called 

 superior. Their principal data are given in the following table. 



The planets Vesta, Juno, Ceres, and Pallas, sometimes called as- 

 teroids, may more properly be termed intercalary planets ; from their 

 filling a supposed chasm in the series. Their size is somewhat un- 

 certain. Jupiter has four satellites; Saturn, seven; and Uranus, 

 formerly called Herschel, has six. The extreme diameter of Sa- 

 turn's ring is 176,000 miles. The brightest planets, are Venus, and 

 Jupiter ; and next to these are Mars and Saturn ; Mars being recog- 

 nizable by its red color. Of the Comets, usually distinguished by 

 their bright train, Halley's returns once in about 75 years ; Encke's, 

 once in about 3| ; and Biela's, once in about 6| years. 

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