"Nature Guide Journal"

11 May 2002

Very low tides next week and later in May will draw clammers and tidepoolers down to the water's ever-changing edge.

While some of the tides this month are especially low, we experience two high tides and two low tides every 24 hours and 50 minutes, all year long.

Looking down on the earth from high above a pole, the tides would look rather like a circle set in an oval, with the oval being the water pulled deeper on opposite sides of the planet. As we ride on the rotating earth we pass through the deep part of the oval ("high tide") then the shallow part of the oval ("low tide"). We then pass through the deep and shallow on the other side to make a whole trip (a day).

The water is bulged out on one side by the gravitational pull of our moon; this moon-caused bulge is balanced by the bulge on the opposite side of the earth, caused by centrifugal force. The paired bulges follow the moon as it slowly orbits the earth. The gravitational pull of the sun also tugs on the water, but the sun's much greater distance lessens the gravitational effect of its larger size, so the moon's role in tides is greater.

While minute on a global scale, in human terms the coming and going of a bulge is more significant: in our part of the world, the bulge appears as 6 – 10 foot change in the water level at land's edge.

The arrangement of the moon, sun, and earth affect the tides, as well as create the effect we see as the phases of the moon.

Twice a month, the moon and sun are lined up with the earth enough for their gravitational pulls to effectively combine. When that happens, the bulges of water we rotate through are a little deeper—and the spaces between the bulges are a little shallower—and we experience a greater difference between high and low tides. Such big-difference tides are "spring tides," named for the springing effect, not for the season. Spring tides occur twice each month, during and just after full moons and new moons.

"Neap tides" are the in-between tides, when the sun and moon aren't lined up with the earth and their forces tend to counteract each other, producing a smaller difference between high and low tides. Neap tides occur during and just after waxing and waning moons.

In our part of the world, the two high tides in a day may be very different levels, and the two low tides may be very different levels. They're different for essentially the same reason the moon doesn't rise over the same point on the horizon each night: the moon's orbital plane is not level with the earth's equator. As the moon orbits around us, it drifts north and south a bit, dragging the bulge of water with it. As we ride the earth around, our part of the world usually goes through the shoulders of the tide-bulges, resulting in the experience of high tides that are different heights—and low tides that are different heights.

This all ends with a set of two high tides (of different height) each day, and two low tides (of different height) each day: a higher high tide, a lower low tide, a lower high tide, and a higher low tide. The zero on tide tables is the average level of the lower low tides.

Why are our low tides more extreme in early summer than in, say, autumn? The extreme tilt of the earth around the summer and winter solstices augments the tidal causes, resulting in "bigger" tides.

Also playing roles in the precise time and level of local tides are friction, the placement and shape and orientation of the land masses, the water currents, and the atmospheric pressure.

While many forces continually work together to drive the tides, every month and each season, low tide opportunities for clam-diggers and tidepool-explorers are especially rich now.