Taylor Made: Step in time
In the days of astronomical navigation, chronometers were a critical tool. The idea for a chronometer first started floating around in the 1670s and evolved throughout the first half of the 18th century. While often eclipsed by digital technology, these highly specialized mechanical devices are still used today in specific applications.
Chronometers were developed specifically for keeping proper time on board ships. Common clocks of the era typically kept time using a pendulum, the motion of which would be upset by the motion of the ship. Chronometers were built such that their escapement, which controls the transformation of energy from potential, in the power source, to kinetic, in the time piece, is set in gimbals that keep it horizontal no matter what the lean of the ship. Their design also accounted for changes in temperature and pressure, which could affect tolerances governed by the mechanical properties of some components (e.g. spring elasticity). These design considerations assured that the chronometer’s time would not become skewed while at sea. Some chronometers had pieces made from precious metals and bearings of gemstone—built to last more than a lifetime with very little fatigue from long-term cyclic loading.
Astronomical navigation encompassed many different methods, but many involved the use of a sextant. Somewhat resembling a protractor, the sextant was used to take the angle between the horizon and a body visible in the sky (this was sometimes called sighting). Seasonal hemisphere charts showed determined positions of commonly known celestial bodies, like the sun and prominent stars, and these could be referenced to figure out a latitude location based on the measured angle. The observer could then draw a latitudinal line of position on the chart, knowing they were at some point along that line.
The chronometer came into play for measuring longitude, which would tell where exactly the ship was along the estimated line of position. It was set to keep the local time at the prime meridian, commonly known as universal coordinated time (or Zulu time in aviation). The prime meridian is the earth’s designated zero of longitude, so any other longitudinal point can be thought of as a time difference from meridian time. Again using celestial bodies, charts, and angles, the observer could derive an experimental time (think 24 hours = 360 degrees), and then compare it to Zulu time.
In similar analog fashion, speed of the ship was measured using a rope with knots in it. A weighted piece of rope (typically with a piece of wood at the end, called a chip) was tossed off the stern with uniform knots along the length. The rope could unroll freely for some designated amount of time that was usually measured using an hourglass. The number of knots that had been let out were tabulated, and the data were used to calculate a rate that could be equivalated to nautical miles per hour. A nautical mile was historically defined as one minute, or 160th of a degree, along any longitudinal line; the convention now is 6,076 ft. In comparison, our statute mile of 5,280 ft represents a flat distance.