The tide at C is due to the
attraction of the moon less the effect due to the excess of the
centripetal force over centrifugal force. These tides are also
known as "spring" tides. Fig. 3 [Footnote: Plate I] shows the
positions occupied when the moon is in the first quarter; the
position at the third quarter being similar, except that the
moon would then be on the side of the earth nearest to B, The
tide at A is compounded of high water of the solar tide
superimposed upon low water of the lunar tide, so that the sea
is at a higher level than in the case of the low water of
spring tides. The tide at D is due to the attraction of the
moon less the excess of centripetal force over centrifugal
force, and the tide at B is due to the excess of centrifugal
force over centripetal force. These are known as "neap" tides,
and, as the sun is acting in opposition to the moon, the height
of high water is considerably less than at the time of spring
tides. The tides are continually varying between these extremes
according to the alterations in the attracting forces, but the
joint high tide lies nearer to the crest of the lunar than of
the solar tide. It is obvious that, if the attracting force of
the sun and moon were equal, the height of spring tides would
be double that due to each body separately, and that there
would be no variation in the height of the sea at the time of
neap tides.
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