20 



INTRODUCTION. 



Constellations, 



6. Equinox. Solstice. A circle of the celestial sphere, whose plane passes through the 

 earth's centre, and is perpendicular to its axis, is called the celestial equator ; this circle is cut 

 by the ecliptic in two opposite points, called the equinoctial points. The first point of Aries 

 coincides with the point of the vernal equinox, and the first point of Libra with that of the 

 autumnal equinox. As the sun on leaving the point of the vernal equinox advances in the 

 ecliptic, his meridian altitude above our horizon daily increases, and hence arises a gradual 

 increase in the length of the day, until the sun reaches his greatest altitude, and the day acquires 

 its greatest length ; the point of the ecliptic at which the maximum takes place is called the 

 Summer Solstice. Having reached this point the sun now turns back towards the equator 

 which he crosses at the point of the autumnal equinox ; his meridian altitude and the length of 

 the days are now gradually diminishing, and the point at which they become least is called the 

 Winter Solstice. The vernal equinox falls about the 2Ist of March, the summer solstice on 

 the 21st of June, .the autumnal equinox about the 23d of September, and the winter solstice 

 the 22d of December. 



7 Principal Elements of the Solar System. 





Sidereal 



Mean distance 



Rota- 



Diam- 





Sidereal 



Mean distance ] Rota- 



Diam 



Names. 



Period 



from Sun. 



tion. 



eter. 



Names. 



Period. 



trom Sun. 



tion. 



eter 





Days. 



Miles. 



Days. 



Miles. 





Days. 



Miles. 



Days. 



Miles 



Sun, 







25,05 



8.82,000 



Ceres, 



1681 -40 



262.0011,000 



? 





Mercury, 



87-97 



36,000,01)0 



J, 01 



3,200 



Pallas, 



1686-54 



263,000,000 



p 





Venus, 



224-70 



68,000,000 



-97 



7.800 



Jupiter, 



4332-58 



484,000,000 



•41 



87",000 



Earth, 



365-26 



95.000,000 



I 



7,920 



Saturn, 



10,7.59-22 



900,000,000 



•43 



79,000 



Mars, 



686-98 



143,000,000 



1-03 



4,100 



Herschel or Uranus, 



30,686-82 



1820,000.000 



? 



35,000 



Vesta, 



1325-74 



224,000,000 



? 



? 



Moon, 



27-32 



27-32 



2,160 



Juno, 



1592-66 



253,000,000 



? 



> 













The following illustration of the relative magnitudes and distances of the parts of our system, 

 as stated in the above table, is given by the celebrated astronomer Herschel. " Choose," 

 says he, " any well-levelled field or bowling-green. On it place a globe, two feet in diameter ; 

 this will represent the sun ; Mercury will be represented by a grain of mustard-seed, on the 

 circumference of a circle 164 feet in diameter for its orbit ; Venus a pea, on a circle 284 feet 

 in diameter ; the Earth also a pea, on a circle of 430 feet ; Mars a rather large pin"s head, on 



