Where the tides turn

How high does the high tide go? In the Bay of Fundy, Canada, the difference between high and low tides is more than 16 metres. But at several points in the world’s oceans, called the tidal nodes, the sea level doesn’t change at all.

Amphidromic points
R. Ray, NASA Goddard Space Flight Center, Jet Propulsion Laboratory, Scientific Visualization Studio / Public domain

The planet’s oceans are pulled around by the sun and moon all the time – that’s where we get tides. But the water has to come from somewhere, and it has to go somewhere, and there have to be points of equilibrium between the high tide on one side and the low on the other.

I imagine this a little like a seesaw. The seats on the ends of the seesaw represent the height of the tides: as one side goes up (high tide) the other side has to go down (low tide). As the tides change, the high tide drops and the low tide rises until they have swapped heights. But! Somewhere between the two sides there has to be a pivot: a point between the two extremes that does not change height. The tidal nodes, also known as the amphidromic points, are those pivots.

The difference between the average low tide and the average high tide is called the tidal range. At the tidal nodes, this range is close to zero. The further away from the nodes you get, the higher the tidal range. The Bay of Fundy between New Brunswick and Nova Scotia in Canada has the highest recorded tidal range, 16.3 metres. Out in the open ocean, it’s more often a scant 0.6 metres.

The tides are a little more complicated than a seesaw, of course: the tides not only go up and down but they also go around and around. The high tides move up or down the coastlines depending on whether the tidal node rotates the tides clockwise or anti-clockwise. (About half of the world’s tidal notes rotate clockwise and the other half the other way around.) So it’s really closer to a seesaw combined with a merry-go-round, or a teeter-totter combined with a carousel. Some kind of amusement park ride, anyway. The tidal nodes are still the points in the ocean in which the sea level doesn’t go up or down much at all, and these are the points around which the world’s tides turn.


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