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.
Continue reading “Where Does Tide Power Come From?”
