XX.] THE AIOVEMENTS OF THE EARTH. 349 



the sun appears to move backwards among them. The stars 

 are so extremely far off that their apparent position is not 

 sensibly affected by our yearly march round the sun ; but the 

 sun is so much nearer, that its apparent position is materially 

 affected, and the sun, consequently, seems to lag behind every 

 day. As a complete revolution is effected in the course of a 

 year, -g^g^th part of the journey will be accomplished in a 

 day. But a circle is divided into 360 degrees : therefore 

 nearly one degree will be travelled over every day. Now the 

 360th part of 24 hours is 4 minutes ; hence the change of 

 jiosition, due to one day's apparent annual motion of the sun, 

 is equal to the change of position due to about four minutes 

 apparent daily motion. 



The position which the earth occupies in relation to the 

 sun, at different periods of its annual journey, will be under- 

 stood by reference to Fig. 112. This shows the earth at 

 four successive positions corresponding to the four seasons. 

 The path of the earth round the sun is called the ecliptic ;'^ 

 and if it be supposed that a flat surface passes through this 

 path, and through the centres of the earth and sun, that sur- 

 face will form the plane of the ecliptic, or the plane of the 

 earth's orbit. 



From what has been already said (p. 340), it might be 

 concluded, that the earth's axis is neither in this plane nor 

 perpendicular to it, but that it is inclined to the plane. It 

 is in fact sloped, as represented in the figure, at an angle of 

 66° 32'; and the extent of the slope remains the same in 

 every part of the earth's path ; in other words, the axis may 

 be said to remain parallel to itself, and to point to the same 

 spot of the heavens.^ Great as the diameter of the earth's 



^ Ecliptic, so called because eclipses only happen when the moon is 

 either on or very near to this curved path. 



^ It should be mentioned, however, that the earth's pole undergoes 



