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The sun and the moon both exert tidal forces on the Earth.
Sometimes the solar and lunar tidal forces pull in the same direction giving us very high or very low tides, and sometimes they pull in opposite directions giving us intermediate tides.
The result is a trend like that shown here for Nanaimo.
Although the tides do not flow all of the time—for example at low tides or high tides—it is easy to accurately predict the tides and tidal currents at any location, so we can know, well in advance, how much energy can be produced from a tidal installation.