Ocean tides are driven mainly by differences in the Moon’s gravitational pull across Earth, with the Sun also contributing. As Earth rotates relative to the resulting tidal pattern, many coasts experience repeated high and low water levels.
Why there are usually two high tides
The Moon produces tidal effects on both the near and far sides of Earth. Earth’s rotation carries many coastlines through two tidal bulges and two lower-water regions in a lunar day, although local coastal geography can change that pattern.
What the Sun adds
The Sun also exerts a tide-generating force. Around new and full moons, solar and lunar effects reinforce each other, often producing a larger tidal range called spring tides. Around quarter moons, the range is often smaller.
Why local tide times vary
Coastlines, ocean depth, bays and the movement of water can change the timing and size of tides. A high tide does not occur at exactly the moment the Moon is overhead everywhere.
Where to get a reliable prediction
For a beach trip, fishing or boating, use published tide predictions for your particular station rather than assuming a universal two-tides-per-day schedule. Weather and storm surge can also shift actual water levels.
Frequently asked question
Do waves cause tides?
No. Wind typically causes ordinary surface waves; tidal water-level changes are primarily gravitational.
Source and further reading
See the NASA reference for additional scientific background.
Leave a Reply