92* GALILEO. 



are called secondary planets, moons, or satellites. The 

 number of comets is unknown. The sun is the centre of 

 the system, and the eleven primary planets, at different dis- 

 tances, and in different times, move round him, from west to 

 east, in the following order, Mercury, Venus, the Earth, 

 Mars, Vesta, Juno, Pallas, Ceres, Jupiter, Saturn, and Her- 

 schel or Uranus. The Earth has one moon, Jupiter four, 

 Saturn seven, and Uranus six. Venus and Mercury being 

 nearer to the sun than our earth, are called inferior planets, 

 and all the rest, which are without the earth's orbit, are 

 (failed 5Mpenor planets; some astronomers distinguish them by 

 the terms interior and exterior, which seem preferable. The 

 planets are retained in their orbits by the united operation of 

 the centripetal force, by which a body is attracted to the 

 centre of gravity, and the centrifugal force, by which it en- 

 deavours to persevere in a straight line. These two powers, 

 mutually balancing each other, compel them to make their 

 respective revolutions. The time of performing their revo- 

 lutions round the sun is called their year, and the time of 

 performing their revolution on their axis, their day. The 

 axis of a planet is an imaginary line conceived to be drawn 

 through its centre, about which it revolves, and the extremi- 

 ties of this line, terminating on opposite points of the planet's 

 surface, are called \is poles. 



The first material step in improving the science of astro- 

 nomy was the establishment of the present arrangement of 

 the sun and planets by Copernicus, who died in the year 

 1543. This doctrine, it is true, was held by Pythag'oras, 

 but it was now presented in a new and stronger light, with 

 its leading features more fully and distinctly unfolded. It 

 is remarkable, that in so many instances, it should have ex- 

 posed its authors and defenders to persecution. Pythagoras, 

 we are told, made it known only to a select few ; but one of 

 his disciples, who had the courage to teach it publicly,, was 

 obliged to flee in order to escape the odium it excited. 

 Copernicus meditated upon the subject for many years, be- 

 fore he undertook to give his thoughts to the world, and 

 scarcely surviving the publication of his work, he left to 

 others to receive the shock that awaited those who espoused 

 it. Galileo could not resist the accumulated evidence, that 

 presented itself to his enlarged and philosophic mind, in 

 iavour of this refined scheme, and was accordingly destined 



