Measuring accurate time aboard The Aegre

And why it was important

Sailing along the coast is one thing; going out of sight of land is quite another.

For Julie and I, as our ambition for sailing The Aegre turned to crossing the Atlantic, we needed to master astro-navigation, the ability to plot our position at sea far out of sight of land based on the sun and stars. Today an inexpensive pocket GPS does all that, giving an instant readout of latitude and longitude, but back in 1973, such a thing was unimaginable.

Measuring the altitude of the sun using a sextant aboard The Aegre.

At that time, astro navigation was the conventional approach. You used a sextant to measure the angle (from the horizon) to the sun or a known star), and the (near exact) time of the measurement, using a very accurate clock showing the time at 0 degrees longitude (the line of longitude passing through Greenwich, England).

Template for calculating sun and star astro sights using Air Sight Reduction Tables

Then by a fairly easy calculation drawing on data for that day contained in a thick calendar book for the year called a Nautical Almanac and ‘Sight Reduction’ tables in another book, you could draw a line on a chart along which you were somewhere. A similar measurement of another star (or the sun perhaps 6 hours later) could give you another line, preferably at a large angle to the first, and where the two crossed, well, you might be somewhere near there. A third line could narrow it down.

The accuracy of such a position depended on your ability with the sextant and the accuracy of your clock. It’s challenging to measure the angle of the sun or a star to + or – a minute or two of arc (that’s 1/60 of a degree) on a boat that’s randomly rolling from side to side while simultaneously pitching up and down while traversing swells that would commonly be 2 – 4 metres, while standing less than 2 metres above sea level. Lots of practice helps.

Aboard The Aegre, we bought a half-size brass sextant long before we left. It was a little beauty. At the same time, I bought a little book, The Sextant Simplified by OM Watts and later Celestial Navigation for Yachtsmen by Mary Blewitt, and devoured them.

But equally important is the accuracy of the time measurement of the sight. In 1973 the most accurate clock for a boat was a marine chronometer, essentially a large watch designed and built to keep accurate time despite the constant movement of the vessel and changes in temperature. More difficult than you might think, and they were consequently pretty expensive. Too much for our slim budget. An alternative, we decided, was a regular water-resistant clockwork watch which we’d keep in a safe place. But we knew we couldn’t rely on it to keep accurate time. However, using a small transistor shortwave radio receiver, we thought that every day we could tune into WWV, a station that constantly broadcasts on the short wave a time signal (every minute) and compare this to the time showing on the watch, and thus calculate the (near) exact time of an astro sight.

In reality… WWV broadcasts on the shortwave from Colorado, and we could really only pick up the signal at night when it bounced off the ionosphere. Every night we would tune in at about the same time, to the reassuring deep beat every second, as if a heartbeat of the universe, then 7.5 seconds before the minute approached, the announcement of ‘Coordinated Universal Time’. (You can listen to a simulation of it here). We’d compare that to the watch to find the latter slow or fast, and always by varying amounts. And it might be 12 hours before we took an astro sight and actually needed the watch, by when the watch error could have gone any which way.

How we managed that I describe in the book.

But then, along the way, we discovered that one yacht owner had an innovative and relatively inexpensive solution, an electronic watch that used an oscillating tuning fork to keep consistent time. It didn’t tick; it hummed. He said it didn’t seem to be affected by the movement of the yacht or the temperature and kept consistent time, with a consistent day-to-day error of no more than 2 seconds. This was a Bulova Accutron watch, and we drooled over it.

Much later, as I tell in the book, we acquired one of our own, and accurate time came to The Aegre, matching my improved sun and star sights. The Accutron’s ruggedness and reliability would help to save our lives.

accurate time

I still have it today, humming along.

And then came quartz clocks and watches, then GPS systems. No one needs a Bulova Accutron today.

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