Sailors have used stars, horizon to navigate seas
"What is the unit knots measured by?" asked a student who attended Schaumburg District Township Library's graphic novel class.
The seas lured voyagers whose boats covered vast distances using stars and the horizon to navigate, while discovering new territories for settlements thousands of years ago.
Originally, navigation was charted by observing constellations as a fixed guide to identify location. As knowledge of seafaring advanced, boats became ships, and devices were invented to calculate and record distance and speed at any time of day and under any weather conditions.
Sailors used the curve of the earth to compute distances.
Dr. Joshua Smith, professor of humanities and museum director at the U.S. Merchant Marine Academy - one of the five U.S. military service academies - explains the measurement.
"Knots means nautical miles per hour. A nautical mile is 6,076.1 feet long, which correlates to one minute of arc of a great circle of the earth, for example, at the equator."
Charting new territories meant devising ways to measure distance and time. Using the circumference of the earth at the equator as a starting point, the globe is a circle and measures 360 degrees. Imaginary lines of latitude (horizontal) create a way to measure east to west and lines of longitude (vertical) measure north-south.
Each line is divided into even smaller units called minutes and seconds. The lines follow the earth's curve, with longitudinal lines meeting at the poles.
You can chart latitude and longitude using NASA's calculator at mynasadata.larc.nasa.gov/latitudelongitude-finder. Sailors used the sextant, which calculates distance from the Polaris star. Smith said these and other navigational devices date to the early 17th century when European mathematicians began calculating the circumference of the planet.
Speed was calculated using knots tied in a rope at equal measures. A log or piece of wood looped on a rope festooned with regularly spaced knots was tossed off the ship's stern.
A half minute hourglass filled with sand ticked off time while sailors counted the number of knots yanked into the water. The result was the ship's speed in knots per hour, or nautical miles per hour. One knot is 1.15 miles per hour.
Christopher Columbus was able to convince the Queen and King of Castile, Spain, to invest in his voyage to discover a new route to Asia and hopefully return loaded with spices and gold. He charted a course and set sail. About 10 weeks later, Columbus inscribed this in his ship's log: "Thursday, 11 October. Steered west-southwest; and encountered a heavier sea … After sunset steered their original course west and sailed twelve miles an hour till two hours after midnight, going ninety miles, which are twenty-two leagues and a half; and as the Pinta was the swiftest sailer, and kept ahead of the Admiral, she discovered land."
Today's gargantuan container ships travel from 12 to 22 knots per hour and can take more than two weeks to cross the Pacific Ocean from China. The system of knots is also used in air travel.
"Aircraft and ships both use great circle navigation, and aircraft navigation used to be very similar to that of ships - they even used sextants," Smith said.