When the gravity forces of sun & moon act together we have a neap / spring tide,
with highest / lowest high tides, and highest / lowest low tides.
7½ days later
When the gravity forces of sun & moon act at right angles to each other we have a neap / spring tide,
with highest / lowest high tides, and highest / lowest low tides.
15 days later
When the gravity forces of sun & moon act against each other we have a neap / spring tide,
with highest / lowest high tides, and highest / lowest low tides.
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