raction
for the earth as well as the moon. But from the fact that the sun is
something like 400 times further from the earth than the moon is, and
also the fact that the attraction of one body for another varies
inversely as the square of the distance, the moon has an immense
advantage over the sun, although so much smaller. If the power of the
moon were entirely suspended, or if the moon were blotted out of
existence, there would still be a tide. The fluctuation between high and
low tide would not be nearly so great as it is at present, but it would
occur at the same time each day, because it would be wholly a product of
the sun.

It will be easily seen that these two forces acting upon the water at
the same time will cause a complicated condition in the movement of the
waters of the ocean. There will come a time once in twenty-eight days
when the sun and the moon will act conjointly, and both will pull in the
same direction at the same time upon the water. This joint action of the
sun and moon produces the highest tide, which is called the "spring"
tide. From this point, however, the tides will grow less each day,
because the relation of the sun and moon is constantly changing, owing
to the fact that it requires 365 days for the sun to complete his
apparent revolution around the earth, while the moon does her actual
course in twenty-eight days. When the sun and moon have changed their
relative positions so that they are at right angles to each other with
reference to the earth--at a quarter-circle apart--the sun and moon will
be pulling against each other; at least this is the point where the moon
is at the greatest disadvantage with reference to its ability to attract
the water.

Because one-quarter around the earth the sun is creating his own tide,
which to that extent counteracts the effect produced by the moon, the
tide under the moon at this point is at its lowest point and is called
the "neap" tide. When the moon has passed on around the earth to a point
where it is opposite to that of the sun--at a half-circle apart--there
will be another spring tide, and then another neap tide when it is on
the last quarter, and from that point the tide will increase daily until
it reaches the point where the sun and moon are in exact line with
reference to the earth's center, when another spring tide occurs. From
this it will be seen that there are two spring tides and two neap tides
in each twenty-eight days. This is the f