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October 26, 2001

Tidal Highs

Tidal changes are influenced by many factors other than lunar position.

Q: I fish the incoming tide for drum and cobia on channels and flats around the Chesapeake Bay. The actual maximum height that the high tide reaches over the flats seems to vary from day to day, rather than monthly as it should if the tide is controlled only by the moon. Is there some other influence on the tides? Why don't they just cycle higher and lower according to the phase of the moon?
- Cam Ressler, Virginia Beach, Virginia

A: Because the height of the tide is also affected by the sun and other factors. Tides are a bit complicated. A tide is a standing wave caused primarily by the gravitational pull of the moon and sun on the oceans. The world's oceans always have two standing waves, or high tides, on opposite sides of the earth. These two bulges move around the surface of the planet as it rotates, generally causing two high tides and two low tides per day.
The height of each of the two bulges or tides depends on the relative positions of the sun and moon in relation to the earth. When the sun, moon and earth are all in line - which occurs twice a lunar month (28 days) at the full moon and at the new moon - the tides are the strongest. That's when spring tides occur, when high tides are the highest of the year and low tides are the lowest. During the full moon, the three bodies line up with the earth in the middle - sun/earth/moon - and the tidal bulges on each side of the earth are about equal. During the new moon, the three bodies line up sun/moon/earth, with both the sun and the moon on the same side of the world. That causes the bulge or tide on the sunward side to be higher than the one on the opposite side of the planet, resulting in one higher high tide and one lower high tide in the same 24 hours.
The sun and moon are at right angles relative to the earth twice each lunar month during the half-moon phases. The moon is the primary motivating factor of the tides, but the weaker pull of the sun acts against it to partially cancel out the moon's effect, resulting in the lowest high tides and the highest low tides - called neap tides.
Many other factors, including the shape of the land and water basins, rivers, wind, currents, seismic activity, latitude and the movements of other heavenly bodies, all work to affect the height and rhythm of the tides, often making reliable, day-to-day tidal height and timing predictions quite uncertain.