by Robert Gordon
A recent article on a rehabilitated tide-power installation describes it as a “newly revived link to lunar energy.” It isn’t. Tide power comes from the earth, not the moon. Think of a toy gyroscope. Spin the wheel and it gradually slows because of friction in its bearings and the air drag on its disc. The friction of tidal currents flowing over the sea floor is doing this for the earth. How do we know it is happening? Because the speed of the earth’s rotation about its axis is slowing. That means our day is getting longer, and the number of days in a month is decreasing. Not by much, but enough to be seen in the time kept by modern precise atomic clocks. But we already knew this before those precise clocks were invented, from the sex life of clams. Here’s how.
Some species of bivalves add to their shells day-by-day. Section a shell and you can see growth rings, like those on the end of a log or tree stump. Clams have to reproduce themselves, fairly frequently since they don’t have long lives. For some species the sex act is triggered by the full moon. Reproduction takes a lot of energy. This leaves less for growing a shell. Every month the spacing of the growth rings in the clam’s shell gets smaller for a few days. Count the number of large rings in the shell between the bands of narrow ones and you have the number of days in the month, about 30 in a modern shell.
Next visit the invertebrate paleontology collection at a natural history museum, as I did once with Cope McClintock, the collection manager at the Peabody Museum of Natural History in New Haven, Conn. He was sectioning clam shells millions of years old, and counting rings. He found there were 35 large rings between the small ones. When that clam was growing the month had 35 days; the earth was turning faster than it is today. There has to be friction somewhere slowing it down. It is in the flow of tide currents over the earth’s surface.
We can see how this works with the simplest model of the tide (known as the ‘equilibrium’ model; it is for an earth without continents). The gravitational attraction of the moon raises the level of the sea beneath it. As the earth rotates under the full moon the height of the sea rises and then falls, making the tide. It is high tide under the moon. That is why you can make pretty good tide predictions with just an almanac. (You may ask why there are two, not one, high tides a day. That is a story for another time.) The rise and fall of the sea as it passes under the moon sets tidal currents in motion.
These can be very large and they dissipate a lot of energy. You can experience this by sailing a boat without an engine through Woods Hole on Cape Cod, or the Race or Hell Gate at the ends of Long Island Sound, or as Captain Slocum did in his solo voyage around the world, though the Straits of Magellan. These tidal currents are strong and turbulent, and a bit scary to encounter. The turbulence of tidal flow creates friction and uses up energy. Adding a turbine increases the energy loss. This energy is drawn from the earth’s rotation, thereby slowing the rotation down just as the toy gyroscope is slowed by friction in its bearings. Our day is getting longer, and our months have fewer days.
So where did this energy first come from? It was part of the process of planet formation after the ‘Big Bang.’ It is not going to be renewed.
About the author:
Robert Gordon, D. Eng., is Professor Emeritus, Department of Geology and Geophysics, at Yale University. He has been an enthusiastic supporter of the Tide Mill Institute since 2013 and is a frequent presenter at TMI’s tide mill conferences.