THE EARTH. 97 



about five seconds before the more opaque part touched his 

 edge. When the elongation of Venus is about forty de- 

 grees, her lustre far exceeds that of the moon, at the same 

 apparent distance from the sun. For though the moon re- 

 flects more light to us than Venus does, yet this light is dull, 

 and has none of the briskness which attends the beams of 

 Venus, This difference is supposed to arise from the cir- 

 cumstance of Venus having an atmosphere far more dense 

 than that of the moon. 



Questions. — 1. What is the appearance of Mercury ? 2. What is 

 the length of his year ? — Distance from the sun ? 3. Why is it seldom 

 seen ? 4. What is its greatest elongation ? 5. What calculation 

 did Newton make with respect to the light and heat of Mercury ? 6. 

 What must be the consequence of such a degree of heat ? 7. What is 

 called a transit of Mercury ? 8. What is the distance of Venus from 

 the sun ? — Lene;th of her year ? — Day ? 9. When is" Venus evening 

 and when morning star ? — How long in each situation ? 10. To what 

 purpose have her transits been applied .'' 11. What is said of her at- 

 mosphere ? 12. When is the lustre of Venus greatest, and to what is 

 it attributed .'' 



LESSON 44. 



The Earth. 



Merid'ian, a great ci;rcle passing through the poles of the world, 

 <ind also through both zonith and nadir ; it crosses the equator 

 at right angles,'and divides the sphere into two hemispheres, 

 the eastern and the western ; it has its poles at the east and west 

 points of the horizon. 



The planet which we inhabit is called the earth. It re- 

 .^olves about the sun at the mean distance of ninety-five, or, 

 as some state, of ninety-three millions of miles, It com- 

 pletes this revolution in a year, and turns on its axis in a day, 

 or twenty-four hours. If the earth were seen from the sun, 

 it would appear to describe, while revolving in its orbit, a 

 circle among the stars. But to us on the earth, the sun ap- 

 pears to describe precisely the same circle, only beginning 

 at the opposite point. That imaginary great circle in the 

 heavens, which the sun appears to describe in the course of 

 the year, is called" the ediptic. The apparent diurnal, or 

 dailj/ motion of th- sun is very different from the path which 

 it appears to traverse in the course of a year. The former 

 9 



