The varying values of 
and 
for a given point
on the earth obviously lead to varying combinations of the
tide producing forces 
and 
. We
may illustrate the important short period variations associated
with the rotation of the earth by examining one of these
forces, say that due to the moon.
The declination of the moon relative to the equator varies from
about 28 
S to 28 N during a lunar
month (28 days). This may be compared with the declination
of the sun which varies from about 23.5 S to
23.5 N during a year. At that time when the declination
of the moon is zero then the distribution of the lunar
tide-producing force is symmetrical with respect to the equator.
In terms of of the potential tide 
, each point on the
earth experience two maximum and two minimum tides
(Fig. 2.04-8) with each culmination of the moon (24 lunar
hours or 24.84 solar hours). Consequently, the apparent
period of this tidal effect is 12.42 hours.
When the declination of the moon differs from zero, then the variation
of the tide potential with time at a given point on the earth
contains a diurnal period as well as the semidiurnal, except at
the equator. The situation existing for maximum declination of
the moon is illustrated in Fig. 2.04-9. The variation of
at latitude 20 clearly shows the diurnal
component induced by the asymmetry of the semidiurnal oscillations.
At the equator the diurnal component vanishes. At the latitude
, which equals (90 - 
) where
is the declination of the moon, the semidirnal component
vanishes. The minimum value of 
is 62 . Consequently,
at no time does the semidiurnal lunar component of the tide
vanish at latitudes between 62 N and 62 S, except
where its effect may tend to be offset by the solar semidiurnal tide at
those times during the lunar month when the moon and sun are
90 out of phase.
The period of the principal diurnal component of the tide associated with the moon is 25.82 hours.
The tidal potential associated with the sun varies in a similar
manner to that of the moon, but the periods of the diurnal and
semidiurnal components differ. Also, the lunear and solar tide
producing forces vary in phase relationship, thus leading
to an amplitude modulation effect. Spring tides, representing a maximum
range
of tide, and neap tides, representing a minimum range of tide, occur
twice each month when the sun and moon are in phase and
90 out of phase, respectively.
A list of the major tide components is given in The Oceans, Table 70, p. 550.
with longitude (for the complete 300
range of longitude) at the latitudes 
and
N at the time of the moon's maximum
declination above the
equator, i.e. 28.5 .
with
longitude for the same parallels of latitude as above.