My father was a Chief Mate in the United States Merchant Marine. He left the sea in the 1950s but every few years he would pull out charts and books and a sextant. He would study over the weekend, take an exam and renew his license.
I was intrigued by the sextant and learned how to use it. A sextant is used to measure the angle of the sun with respect to the horizon. Latitude can be calculated from this measurement. This is celestial navigation where the sun, the moon and the stars are all used by a sailor to identify a ship’s latitude and longitude.
The sextant is not useful if the sky is overcast for days on end or if the ship is rocked by high seas from side to side with massive waves washing tons of water over the deck. My father had experienced such conditions in the North Atlantic and Arctic Oceans in Winter during WW II. Celestial navigation was unreliable for those reasons and unavailable before an eighteenth century inventor perfected the sextant, which was the successor to less reliable instruments.
When Columbus sailed, he used a navigation method called “dead reckoning.” To use dead reckoning, a sailor measured the ship’s direction, velocity and elapsed time at that velocity. The sailor then computed the distance covered (velocity multiplied by elapsed time) and marked the ship’s progress on a chart. Of course, Columbus’ chart was mostly empty, so he created his own charts of the islands and continents he discovered. Unfortunately for the native inhabitants that he “discovered,” the charts enabled Columbus and others to return again and again.
Columbus used a compass to determine the direction the ship was traveling. The Europeans had been using compasses for a few centuries before Columbus. The technology had arrived in Europe from China via trade routes that had opened up as a result of the Mongol conquest of Asia, the Middle East and Eastern Europe.
Velocity was measured by observing the water run past the ship as the ship moved forward. This primitive method was replaced in the next century by use of a line with knots tied in it at regular intervals. The line was secured to the ship at one end and the other end was tossed overboard. The end of the line thrown overboard sped to the rear of the ship, pulling the rest of the line behind it. Knots were counted as they sped overboard and elapsed time was measured. The speed in knots (nautical miles per hour) was then calculated based on the knots counted and time measured. But Columbus’ eyeball estimates were accurate, resulting in an average and consistent nine percent overestimate of distance traveled.
Elapsed time was measured in half hour increments with an ampolleta, a sandglass, which was attended to 24/7. A good navigator at that time was expected to make landfall within thirty nautical miles of goal on voyages of three thousand nautical miles. Columbus was a very good navigator.
Dead reckoning may seem primitive today but Samuel Eliot Morison* wrote, in 1942, that “Dead reckoning is still the foundation of navigation. On the navigating table of every ocean liner or battleship today you will find the ship’s ‘D.R.’ laid down on a chart, and the most modern systems of celestial navigation are worked out by assuming your D.R. position and correcting it by celestial observation.” Morison should know as he was the official historian of and an officer in the United States Navy in World War II, serving at sea during the war and retiring as a Rear Admiral. A statue of Morison, sitting on a rock and gazing towards the sea, can be found in a pretty tree lined park along Commonwealth Avenue in Boston. D.R. is still used today in some situations.
The here!apps family of apps implements dead reckoning. Smart phones have sensors that provide the phone’s direction of travel, velocity and elapsed time. These are the compass, accelerometer and clock, respectively. A gyroscope, found in most phones, provides the orientation of the phone such that the compass and accelerometer data make more sense. Most importantly, phones have powerful processors that can make the complex dead reckoning calculations required, more quickly than a sailor.
GPS is analogous to celestial navigation in that overcast skies and other situations can block the signal, similar to how they block use of a sextant. More importantly, GPS requires power . . . too much power to be used continuously. Dead reckoning can provide an important supplement to GPS, allowing GPS to be shut off while dead reckoning provides approximate location information.
Smart phone sensors and processors are improving with each new generation of phones. Dead reckoning will become more accurate as here!apps continuously integrates the new sensor and processor capabilities and improves the here!apps algorithms. But here!apps will never eliminate the reliance on GPS. After all, even Columbus needed to know his starting position in order to dead reckon.
* Admiral of the Ocean Sea by Samuel Eliot Morison