EARTH 



2763 



EARTH 



Earth. Three views of the earth showing the three mountain ridges which meet in the plateau oi Antarctica. These 

 ridges indicate the shape which is being assumed by the earth's crust as the earth itself cools and contracts 



as its satellite. Many solar systems 

 form the universe. 



The earth's rotation, or spin, 

 upon its axis through the poles is 

 the cause of the rhythmic succession 

 of day and night and the steady 



* \ ^l 



Saturn 



the path of the pendulum and was regions are tilted now away and 

 not underneath it again until a day later towards the light. The angle 



had passed. The floor tunied round 

 the pendulum in a day because the 

 earth rotated. 



The axis of rotation through the 



Earth. In th.s diagram the distances, in millions of miles, of the earth and the 

 other planets from the sun are indicated along the bottom line 



of tilt 23 determines the arctic 

 and antarctic circles (90 23 = 

 66J), which are the edges of the 

 areas which have no sunrise at their 

 midwinter and no sunset at their 

 midsummer. 



At midsummer, in England and 

 similar latitudes, the sun is 47 

 (twice 23|) higher in the sky than 

 at midwinter. At the equinoxes 

 the sun rises due east and sets due 

 west, and day lasts for 12 hours; in 

 England in the summer the sun 

 rises north of east and sets north of 

 west, and day varies from 12 to 18 

 hours ; in winter the sun rises south 



pulsation of the oceanic tides. The 

 direction of rotation from west to 

 east causes the sun to rise in the 

 east, and the cyclonic planetary 

 winds to swirl in different direc- 

 tions, anti-clockwise in the north- 

 ern and clockwise in the southern 

 hemisphere. 



The fact that the earth rotates is 

 demonstrated by Foucault's pen- 

 dulum. Foucault in 1851 sus- 

 pended a pendulum from the dome 

 of the Pantheon in Paris and 

 started it swinging above a mark on 

 the floor. A pendulum always 

 swings in the same path. The mark 

 along the floor moved away from 



Mercury Venus Earth Uars 



O Q 



Earth. Diagram showing the relative sizes ol the earth and the other seven 

 planets. The earth is one of the four smaller planets. Note the relatively 

 immense sizes and distances of the four major planets 



tilts through an angle of 



poles tilts through an angle 

 23J from the vertical towards the 

 line joining the centres of the earth 

 and the sun. Sunlight reaches the 

 earth in rays parallel to the line of 

 centres ; consequently the polar 



LAND HEMISPHERE WATER HEMISPHERE 



Earth. The world in hemispheres. The land hemisphere has its centre 

 approximately at London. The Arctic basin is antipodal to the Antarctic 

 continent ; Africa is antipodal to the gr*at basin of the Pacific. The water 

 hemisphere includes Argentina, Australasia, the East Indies, and the south- 

 east corner of Asia 



of east, sets south of west, and 

 day varies from 6 to 12 hours. The 

 tilt of the earth's axis is therefore 

 responsible for the seasons, and for 

 the fact that variation in mean 

 monthly temperature through the 

 year follows the same rhythmic 

 curve for all places on the earth, 

 differing only in amplitude from 

 place to place. Like the other 

 planets the earth has a spherical 

 shape, rotates upon an axis in- 

 clined to the plane through which 

 it revolves round the sun in an 

 elliptical orbit, and receives light 

 and heat from the sun. 



Owing to its rotation the earth 

 is not a perfect sphere, but has a 

 bulging belt round the equator and 

 a flattening at the poles, so that it 

 is an oblate spheroid. This bulge 

 is a reminder of the way in which 

 at an early stage in its career the 

 earth, then much larger in diame- 

 ter, had a greater bulge which even- 

 tually broke away from the earth in 

 fragments, which later coalesced to 



