THE MOVING WATERS 



the sun and moon attracts the earth's waters which He 

 closest to them. This pull, however, is so small compared 

 with that of the earth's pull on those same waters that 

 they are not pulled vertically away from the earth's 

 surface. The further we move away from this nearest 

 point, however, the more horizontal does the joint pull 

 of the sun and moon become. Obviously it takes far less 

 force to make the water move horizontally over the 

 earth's surface than to draw it up vertically, and so the 

 water is drawn from all directions to this nearest point 

 until it is piled up. This bulge is high tide. It is not so 

 easy to see why the water on the opposite side should bulge 

 outward, away from the sun or moon. Realize that the 

 earth itself is pulled away from those far-side waters and 

 the matter becomes clearer. It should now be obvious 

 that the highest tides will occur when the sun and moon 

 are on the same side of the earth, pulling together, and 

 when they are directly opposite each other with the earth 

 between. 



Such conditions occur only at intervals, between 

 which the sun and moon are pulling at constantly vary- 

 ing angles. The movements of the tides are also com- 

 plicated by the fact that (as seen from our point of view 

 on this earth) the sun and moon do not go round the 

 earth in unison but at different speeds. The lunar day is 

 one of 24.84 hours, compared with the solar day's 24. 

 This apparent ''lag" of the moon compared with the sun 

 causes the difference in the daily times of high tides. 



When you spend your holiday at the seaside and get 

 up before breakfast for a swim, you find the sea further 

 and further out as mornings pass. As a rule the high 

 tides around Great Britain are about fifty minutes later 

 each day. 



That there are two tides each day, and not one is due 

 to the rotation of the earth so that each place experiences 

 high water when it is nearest and furthest from the moon ; 

 and so, through all its phases, the tides are duplicated, 



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