![]() It is however a sinusoidal modulation over the half-month period between successive springs or neaps. ![]() NOTE: The range of the combined solar and lunar tides does not change suddenly at springs and neaps. The resultant small range neap tides occur approximately a week after springs. The result is a smaller than average range at neap tides. Thus neap tides occur on the day that the sun’s and moon’s high waters most closely coincide with the others low waters. With the moon at its first or last quarter, its tide-raising force is partially counteracted by that of the sun. They occur at approximately fortnightly intervals. ![]() Actual big range tides spring tides occur a couple of days afterwards due to the inertia of the water mass. Solar equilibrium tide has two bulges, one facing Sun, one on opposite side of Earth Lunar bulge about twice as large as solar bulge Spring tides occur when solar and lunar bulges are lined up, neap tides occur when the bulges work against each other Lunar tides delayed by 50 minutes every day. The result is a larger than average range of semi-diurnal tide at spring tide. The two low waters are also reinforced, but in the opposite sense, making them lower than average. During the spring tide, the high water’s occur near local noon and local midnight and are higher than average due to the reinforcement of the two. At new and full moon, the sun and moon act together producing a large tide-raising force. The sun’s tide producing forces sometimes acts with or against those of the moon. Nevertheless, many of the characteristics of the two are similar except for magnitude and time. It cannot be stressed enough that at no place on the Earth is the actual tide the same as the equilibrium tide at that place. Thus the greatest diurnal effects occur when the moon is full, at solstices, ie in June and December in the Southern Hemisphere. Thus the combined effect of the sun and the moon will increase the diurnal effect on the tides when their combined declinations are both large, either in the same or opposite sense. The solar declination varies seasonally from 23.5° in June to –23.5° in Dec. The maximum lunar monthly declination north and south of the equator varies from 18.3° to 28.6° over the 18.6 year nodal period. Both water levels are equal at the equator, but in high latitudes one high water disappears altogether. Fig 3 shows, in an exaggerated form, how the declination produces an asymmetry between the two high and two low water levels observed at the mid-latitude point P as it rotates under the two bulges. Diurnal tides are effected by the changes in solar and lunar declination. The origin of semi-diurnal tide has already been explained, but what about diurnal tides? We know that even semi-diurnal tides are often of unequal amplitude and in some places the tides are entirely diurnal.
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