Tides 43 
each particle of water. If the earth were isolated by itself in 
space, its unvarying gravitational force would produce a 
smooth globular surface, and there would be no such things 
as tides. But the universe in which we live is inhabited by 
other celestial bodies besides the earth, and these other celes- 
tial bodies, the moon and sun in particular, also exert gravi- | 
tational forces upon the ocean waters. However, since they 
are so much further away from the ocean than the earth it- 
self is, their influence is far less than that of earthly gravity. 
Their attractions are sufficient, however, to cause a heaping 
up of water toward them and also on the side of the earth op- 
posite them. It is this heaping up of water due to the attrac- 
tions of the sun and moon that is called the tides. It is worth 
while to note that the moon or sun produces a high tide not 
only directly beneath it on the earth, but also at the antipodes 
of that point (on the other side of the world). 
Both the moon and sun produce tides of their own, and 
since the sun and moon move around the earth at different 
speeds, their two tides do not always coincide. At new and 
full moon however, the earth, sun, and moon are all in a line 
and hence both tides coincide, and as a result we have extra 
large tides called spring tides. At first and last quarter phases 
of the moon, the sun and moon act against one another and 
cause rather small tides called neap tides. Spring tides are 
about 2.8 times the size of neap tides. 
HOW ARE TIDES PREDICTED? 
Experience shows us that high tides do not follow the mo- 
tions of the celestial bodies as faithfully as the above ideal- 
ized picture would suggest. The contradiction between the 
simple theory outlined, and the actual facts is largely due to 
the obstacles in the ocean, the continental land masses, which 
