466 ASTRONOMY. 



up in a box, so as to move freely in every direction, 

 constitutes the mariner's compass. 



ABSTRACT OF ASTRONOMY. 



1. The fixed stars are probably suns at immense 

 distances : and our sun is only a fixed star much 

 nearer to us, forming the centre of our system. 



2. There are nine primary, and eighteen secon- 

 dary planets moving round the sun. 



3. The moon is a secondary planet moving round 

 the earth. 



4. All these planets move in orbits which are 

 nearly circular, but in reality are eliptical, having 

 the sun in one focus. 



5. They are preserved in their orbits by the 

 power of attraction and the centrifugal 'force, which 

 exactly balance each other. 



6. The comets are probably a species of planets, 

 moving in very eccentric orbits. Some of them, 

 come very near the sun; and they must go to im- 

 mense distances from him, as some of their periods 

 are very long, and but few are known to return. 

 With their nature we are not acquainted. 



7. The planets revolve round an imaginary axis, 

 in various periods of time, which constitute the 

 day and night of each planet. 



8. The time of the revolution of each planet 

 round the sun forms its year. 



9. Their axes are inclined to the planes of their 

 orbits, which occasions the diversity of seasons. 



10. Eclipses of the sun are occasioned by the 

 moon coming between the earth and the sun, and 

 thus covering his disc. 



