24 FIRST YEAR SCIENCE 



In the journey around the sun the earth does not move 

 in a circle but in an ellipse, which is a figure something 



like a circle but having one 

 of its diameters longer than 

 the other. This figure can 

 be drawn by sticking two 

 pins into a piece of paper, a 

 little distance apart, and 

 tying to each pin one end of 

 a string, the length of which 

 is several times the distance 

 between the pins. Then put 



a pencil into the loop of the string and draw the curve 

 which will be formed on either side of the pins by the 

 pencil being moved over the paper in the extended loop. 



The points, where the pins pierce the paper are called the 

 foci, and, as will be seen, each of these points is nearer one 

 half of the curve than the other. If a body were placed at 

 one of these points and another body moved around it in 

 the line of the curve, the two bodies would be nearest each 

 other when passing one point in the line extending through 

 the foci, and farthest apart when passing the opposite point 

 in the same line. 



Now the sun is at one of the foci of the ellipse in which 

 the earth moves, so the distance between the sun and the 

 earth varies during the year. This variation is about 

 three millions of miles, the average distance of the earth 

 from the sun being about 93,000,000 miles. Strange as it 



94,500,OOOM I LES ^* 9I^OO,OOQMI LES 



SUMMER WINTER 



Fig. 9. 



may seem, we are nearest the sun in January and farthest 

 away in July (Fig. 9). We are not warmest in the north- 



