We Got The Beat

 Time: it's a funny thing.

We can't measure it directly. The best we can do is construct mechanisms that have some kind of periodic behavior and then count the "beats" (as watchmakers call it) that they produce.

There have been all kinds of sources of periodicity used during human history. The heart beat for short periods of time. The movement of the Sun across the sky for the day. The phases of the Moon for periods of a "moonth". The seasons for the year.

There were many attempts to make time keeping devices - candles with marks drawn on them, water clocks that counted drips, sundials, hourglasses. But none of these were accurate enough to measure longitude, the angular east-west distance across the Earth. (Latitude can be determined by the height of the Sun above the horizon, taking the season and the hemisphere into account. There are almanacs still published today with the numbers you need to do this.)

Navigators going all the way back at least to the ancient Greeks and Polynesians had known that timekeeping could be used to determine longitude, by comparing local time (e.g. local noon, determined by the sun) with a clock set to the time at the port from which you departed. But it wasn't until the mid-to-late 1700s that there was a "chronometer" design accurate, precise, stable, and reliable enough to carry on board ship that navigators were able to use it to determine their longitude.

All "modern" clocks, from those initial chronometers until today, consist of an oscillator - a source of stable precise beats - and a counter - the watch face. And all oscillators are made of three basic components: a resonator (a source of periodicity derived from nature), a power source (a falling weight, a spring, a battery, the mains), and a feedback loop (known as an "escapement" in a mechanical clock).

Many things have been used as resonators over the centuries (and all of these are still used today): a pendulum, a balance wheel, a quartz crystal, an atom of cesium, rubidium, aluminum, ytterbium. But no matter how sophisticated clocks become, they still have the three components that can be classified as a resonator, a power source, and a feedback loop.

The clock below - on display at NIST in Boulder Colorado and whose photograph I took in 2018 - was sold by IBM in 1956. It is an electric pendulum "Type 37" clock that set itself from the NIST WWV/WWVH telegraphic time code using a vacuum tube radio receiver. It was typically used in factories as the master clock from which all other clocks were set.