EISE OF ASTRONOMICAL SCIENCE. 3 



History teaches us that this first step was taken by the 

 Greeks, who, judging from the facts that a vessel when coming 

 from sea into port first shows the tops of her masts, and then 

 seems to rise higher and higher out of the water as she 

 approaches, that the sun rises later and later as we travel from 

 east to west, and that the shadow of the earth, which appears 

 during lunar eclipses on the surface of the moon, always shows a 

 circular form, proclaimed its spherical shape, and even made the 

 first attempts to measure its size. 



But as their geographical knowledge was very limited, and their 

 instruments of measurement imperfect, their calculations could 

 not but be extremely defective ; and thus it was reserved for 

 later times to ascertain that the earth is a globe flattened at 

 the pole, with an equatorial diameter of 6,864, and a polar 

 diameter of 6,852, geographical miles. 



The dimeDsions of the earth being thus known, it was now a 

 comparatively easy task to measure the distances of the various 

 planets belonging to our Solar System ; for the mathematician 

 requires but to know the length of a line, and the angles which 

 its extremities make with a third point, to obtain a full know- 

 ledge of the dimensions of the triangle thus formed, and 

 consequently of the comparative distances of all its parts. 

 With triangles he invades the celestial space and subjects them 

 to the dominion of science, as surely as by means of triangles he 

 measures the extent of his fields or the height of his mountains. 



By this means it has been calculated that the moon is about 

 208,000 geographical miles distant from the earth, while the sun 

 sends us his enlivening rays from a distance of 80,000,000 

 miles : and thus we know that while torrid Mercury, the planet 

 nearest to the sun, revolves at a distance of 32,000,000 miles 

 from his orb, frigid Neptune receives his scanty supply of 

 warmth and light from the amazing distance of 2,800,000,000 

 miles ! 



According to the delusive testimony of our eyesight, the sun 

 and all the planets move round our earth, as if it were the 

 centre of the universe. We see the sun and moon rise and set, 

 and the stellar canopy slowly revolving round the Polar Star. 

 But we seem to repose in majestic immobility, and thus it ap- 

 pears as if all those luminous worlds acknowledged the supre- 

 macy of our globe and paid homage to its superior power. 



B 2 



