Complex Numbers

Analog Log

This one's been hanging out in my queue for a long time, but it's not exactly time-sensitive. As they say, time and tide wait for no one.

    Lord Kelvin and His Analog Computer  
This tide-predicting machine was one of many advances he made to maritime tech

The source is a publication of the IEEE, the electrical engineering professional organization. But fear not; the article isn't very technical.

Civilizations recognized a relationship between the tides and the moon early on, but it wasn’t until 1687 that Isaac Newton explained how the gravitational forces of the sun and the moon caused them. Nine decades later, the French astronomer and mathematician Pierre-Simon Laplace suggested that the tides could be represented as harmonic oscillations. And a century after that, [William] Thomson used that concept to design the first machine for predicting them.

Thompson was Lord Kelvin and, yes, he's the one the temperature scale is named after. One wonders what it would have been called had Thompson not become a noble, because Thompson is a boring name for a unit of measure.

Thomson’s tide-predicting machine calculated the tide pattern for a given location based on 10 cyclic constituents associated with the periodic motions of the Earth, sun, and moon. (There are actually hundreds of periodic motions associated with these objects, but modern tidal analysis uses only the 37 of them that have the most significant effects.)

Translation: it's complicated.

The most notable one is the lunar semidiurnal, observable in areas that have two high tides and two low tides each day, due to the effects of the moon.

Which is what most of us think of when we think of tides, but it's not as simple as "it's high tide when the moon is directly overhead." There's a lag, and there are local conditions that affect the timing of tides (such as sea floor depth).

On Thomson’s tide-predicting machine, each of 10 components was associated with a specific tidal constituent and had its own gearing to set the amplitude.

Basically, it's a very complicated clock. Sure, the article calls it an analog computer, and I'm not going to argue with professionals (especially ones not in my field) but I think that's a categorization issue. At some point of increasing complexity, a clock stops being a clock and starts being an analog computer. But in my view, if it involves the timing of natural phenomena like the movement of solar system bodies, it's a clock.

The device marked each hour with a small horizontal mark, making a deeper notch each day at noon. Turning the wheel rapidly allowed the user to run a year’s worth of tide readings in about 4 hours.

But this bit, an output device, is probably what pushes it into the computer category. It also was designed for prediction, not for reading what the tide is right now.

As with many inventions, the tide predictor was simultaneously and independently developed elsewhere and continued to be improved by others, as did the science of tide prediction.

One thing I'm still unclear on when it comes to tidal prediction: the wind plays a role. And wind is way, way harder to predict than the future relative position of the sun and moon. I'd ask my dad, the sailor, but I seem to have misplaced my Ouija board.

In any case, mostly, I just liked the article and the history lesson, and I wanted to muse about the differences between clocks and analog computers.