6 



THE SKY. 



shorter diameter, two centres called "foci," and the cir- 

 cumference or boundary so placed that the sum of any two 

 lines, drawn one from each centre, and meeting at any part 

 of the circumference, shall be equal to any two other lines 

 so drawn and meeting at any other point of the circumference, 

 and to the major axis or long diameter of the ellipse. In 

 fig. 4, each pair of lines drawn from A A and meeting res- 



pectively at B are each equal. An ellipse may be very 

 much elongated, or almost circular, but still have these 

 properties which are essential to it, and in the case of the 

 orbits of the planets, they are so nearly circular, that if 

 drawn a few inches wide, would hardly be detected to differ 

 from the circle. The orbit of the earth is about one thirtieth 

 part longer than broad. 



The rate at which the planets revolve round the sun is 

 not equable, that is, their progress is not through equal 

 distances at equal times, but a line drawn from the planet 

 to the sun would always pass over equal spaces or " areas " 

 in equal times, for example, in fig. 5, if the area of the angle 

 included in A B c be equal to that included in A D E, then a 

 planet would pass from B to c and from D to E in equal times. 



Mercury is the smallest of the greater planets, and the 

 nearest to the sun. Its diameter is about 3000 miles, and 

 it is about one-fifteenth part the volume of the earth ; its 



