Tides and currents

There is a tide in the affairs of men.

Which, taken at the flood, leads on to fortune;

Omitted, all the voyage of their life

Is bound in shallows and in miseries.

On such a full sea are we now afloat,

And we must take the current when it serves,

Or lose our ventures.

--William Shakespeare

Julius Ceasar, Act 4, scene 3

A tide is the rise and fall of water (and to a much lesser degree, land as well) due to changes in the gravitational attraction among the Earth, Moon, and Sun. A tide is technically the vertical part of this movement; tides, it is said, rise and fall. The horizontal movement is called current, or tidal current, and it is said to ebb or flood.

Causes. The Moon is the main force behind tides and currents on Earth. The Moon's gravity pulls at both land and water on Earth, but because it is liquid the water moves more. The point where this bulge of water is highest is said to be at high water, and is at the point on Earth closest to the Moon or at the point on Earth directly opposite (see diagram above). The point where the bulge is stretched thinnest is at low water. This bulge of water follows the Moon as it orbits the Earth, which in turn rotates within the bulge. This relative movement of land and sea causes tides and currents.

The Sun adds its attraction to the mix as well, although its pull is only 46-percent that of the Moon. When the Sun, Moon, and Earth are lined up -- at the time of the full moon or new moon -- the tidal range is the greatest and tides are called spring tides. When the pulls of the Sun and Moon are at right angles to each other -- at the time of the first and third quarters, or "half moons" -- the tidal range is lowest and is called a neap tide.

Other planets have a negligible effect on tides and, even if they're all lined up as Mercury, Venus, Mars, Jupiter and Saturn were in May 2000, they exert less than 0.00006 times the effect of the Moon.

Patterns. Tides vary throughout the world depending on specific local characteristics. This in turns determines what type of tide an area experiences. The east coast of the United States, for instance, had a semidiurnal tide, in which there are two tides a day (a cycle of two highs and two lows), with relatively little difference between the highs and lows. The Gulf of Mexico experiences diurnal tides, in which a single high and low occur each day. The US west coast, however, experiences a mixed tide cycle, in which usually has two tides a day, but sometimes becomes diurnal. The differences between the two tides in a day can vary significantly.

Predicting tides. Predicting tides today is a lot like predicting the weather. To create a tide table for a given station, the forecaster will first look at historical data of tide heights and times called a time series. Then a model is created using several harmonic components including distance between the Earth and the Moon and Sun and the phase of the Moon. Even then, most publishers of tide tables are careful to point out that these are only predictions and actual tide heights and durations may be affected by barometric pressure, flooding in coastal rivers, and other factors. Tide tables and current tables are printed separately. Tide tables show heights of the tide at various times. Current tables show speed and direction of the current at various times.

Tidal extremes. The greatest tidal extreme is in Nova Scotia's Bay of Fundy, with ranges of 57 feet between high and low tides. Canada also boasts the fastest tidal current at British Columbia's Nokwakto Rapids, where currents reach speeds more than 18 miles per hour.