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From Chapter Four: Dead Reckoning
If you were on a desert island, how could you measure distances and communicate these to others? Readily available measures are on our body and in the environment. In ancient Egypt short measures were based on finger, hand, and arm lengths. The ancient Egyptian cubit is the length of the forearm. Longer units of distance were based on travel. The word mile comes from the Latin phrase mille pacem, which means “a thousand paces”. Roman legionnaires kept track of the distance traveled by counting paces. A pace is the distance you cover when the same foot (left or right) hits the ground. Two steps – left then right - equal one pace.
This practice of counting paces isn’t limited to humans. At least one species of ant navigates by counting paces. Harald Wulf studied ant habits by gluing tiny stilts onto their legs or cutting off sections of their legs to change the length of their paces. In Wulf’s study the distance the desert ants (Cataglyphis) traveled was directly related to how many paces they took. Ants with stilts would walk past a target and ants with shortened legs would come up short in a way that was consistent with pace counting.
Another natural measure is based on travel time. If you were giving directions to a friend driving a car, you might say something like “You stay on the interstate for two hours and take exit twenty-seven.” This assumes that everyone travels at the same speed, but it is usually clear from the context how fast your friend moves. Distance traveled (miles) is speed (miles per hour), times time, (hours).
The hour was and is one of the most widely used units of time. It has its origins in the ancient Egyptian use of rising stars to reckon the time of night. They used 36 bright stars in all, which would rise just before the sun in turn at different times of the year. The passage of one night was associated with the passage of 12 of these bright stars giving rise to the night being divided into 12 hours. The day was likewise divided into 12 units. This scheme, dating from roughly the fifth century BC, became widely adopted, but it was far from universal. Medieval Saxons reckoned the length of day in tides, with eight tides in one day.
Three miles per hour was, and is, a good reference for human-powered travel. This is how fast most people walk on level ground, how fast a person can row a boat, and how fast an old sailboat can move on water. It is far from universal. You could be running or limping, but it is a reliable standard shared among travelers and widely understood. If you convert this speed with an hour into a distance, it is three miles. In ancient Persian and Arab cultures, a parasang or farsakh was a unit based on this same combination of speed and time. The length of a league is close to the farsakh and is also based on the distance covered by a person in an hour. Longer distances can be reckoned in how many days or lunar months elapse over the course of a journey. Native Americans used these lengths of time as standards. A marhalah is a day’s journey in medieval Islamic reckoning and is equal to roughly eight farsakhs (24 miles), or eight hours of travel.
These are still viable measures. If GPS devices and odometers vanished overnight we could fall back on human measures instantly. When I tested a group of 30 students, they took 980 paces per mile, which is very close to the one thousand originating from soldiers in ancient Rome! They also averaged a walking speed of 3 miles per hour, so the concepts of a mile, farsakh, and league are true to their origins.
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