How Logging Sized the World

Before big data, there was the humble logbook

Counting knots on the log-line

Operational metrics should be designed to capture more than just errors and anomalies. Software developers should give internal measurement the attention it deserves.

Clarissa, the founder of Tangled Web Services, opened the Tech Tuesday gathering, “I’ve invited Capt. Cayman to tell us about the Magellan expedition of 1521, and more to the point, about how the world’s first circumnavigation was recorded.

“I know this may seem random, but I think you’ll find it relevant to our own work in technology. There’s more than one lesson in this story.”

Capt. Cayman projected an image onto the front wall of the conference room, and began with, “This is the best tribute I know of, to honor one of mankind’s greatest exploratory achievements.”

“Derrotero del viaje de Fernando Magallanes en demanda del estrecho desde el paraje del cabo de San Agustín [AMN Ms.0141]”

As he paused for effect, the others in the room looked up to see lines of terse abbreviations with unfamiliar words and numbers and dates.

“This is a page from Francisco Alvo’s logbook of the fleet’s progress across the South Seas in 1521.

For more than three months, the crew saw nothing but the sun, the stars, and the open waters of the Pacific.

Remember, at that time, Earth’s size had been greatly underestimated. What they thought would be just a short passage from South America to the Spice Islands, instead became a grueling test of endurance.

Each day of that passage was recorded in the derrotero, the logbook, with entries consisting of the date, the ship’s direction, its latitude, and the distance traveled. Because of these entries, the ship’s course could be plotted across the vast uncharted expanse, and the size of the Earth determined from empirical data.

Incidentally, not all of this data was easy to obtain.

The date was of course no problem — as long as the record keeper was diligent.

The ship’s direction could be obtained by averaging the compass readings taken throughout the day.

And the latitude could be obtained from the sun’s noon-time altitude, adjusted for the Earth’s current tilt (that is, the day of the year).

But obtaining the distance traveled — and by further calculation, the ship’s current longitude — was not easy. It required careful dead reckoning. That was accomplished by throwing a log overboard, paying out the rope that was attached to it, waiting for the sand in the sand-glass to reach its end, hauling in the rope, and counting the number of evenly-spaced knots in the rope that had been payed out. This measurement was taken multiple times a day.

The ship’s pilot averaged these measurements, adjusted for what he estimated the water’s current speed to be, and recorded the ship’s overall speed (in knots) and distance traveled, in the official logbook.

All of these efforts might have seemed superfluous to the operation of the fleet, because in the moment, they provided no relief from monotony, no protection from storms, no estimate of when (if ever) they would reach the end of the ocean. But in the end, it was those very measurements that allowed others to follow in their wake.

Magellan’s voyage across the Pacific revealed a major discrepancy in the Earth’s size. His careful logging established that the Earth’s circumference was actually 5250 kilometers greater than previously thought!