The speed of ships and aircraft is measured in knots instead of miles per hour (MPH) because the knot is based on a unit of distance directly tied to the Earth’s geography, which is essential for navigation. A knot is defined as one nautical mile per hour, while MPH uses the statute mile, a unit designed for land travel. This difference makes the knot the standard for maritime and aviation use, where precise geographical positioning is paramount.
The Fundamental Difference: Nautical vs. Statute Miles
The core distinction between knots and miles per hour lies in the distance unit each uses. A statute mile, the basis for MPH, is a fixed length of 5,280 feet, originally derived from the Roman mile and used primarily for measuring distances on land. This unit has no inherent relationship to the Earth’s coordinate system of latitude and longitude.
The nautical mile is a geographical unit of distance defined by the Earth’s circumference. Specifically, one nautical mile equals the length of one minute of arc along any line of longitude, or meridian. Since there are 60 minutes in a degree, and 360 degrees around the Earth, this definition directly links the distance unit to the planet’s spherical geometry.
Because one minute of latitude always equals one nautical mile, a navigator can use the latitude scale on a chart to measure distance directly. The international nautical mile is standardized at 1,852 meters, or approximately 6,076 feet. This makes it about 15% longer than the 5,280-foot statute mile. Since the nautical mile is integrated into the global coordinate system, it is the natural choice for calculating travel over the Earth’s curved surface.
The Origin and Definition of the Knot
The knot is simply a measure of one nautical mile traveled per hour. The term has a historical origin dating back to the 17th century, when sailors needed a practical way to measure a ship’s speed through the water. They used a device called a chip log, which consisted of a wooden panel attached to a rope, or log line, with uniformly spaced knots tied into it.
The wooden log was tossed overboard and remained relatively stationary while the ship moved away, causing the knotted line to pay out. A sailor would count the number of knots that passed through their hands during a specific time interval, typically measured with a 28-second sandglass. The number of knots counted roughly corresponded to the ship’s speed in nautical miles per hour, giving rise to the term “knot”.
One knot is equal to approximately 1.15 statute miles per hour. For example, a vessel traveling at 20 knots is moving at about 23 miles per hour. This highlights that while the knot is a speed unit, its foundation is the geographically defined nautical mile, not the land-based statute mile.
The Practical Advantage in Navigation
The continued use of knots in modern navigation stems from the practical utility it offers for plotting courses and calculating travel time. Navigational charts, used by both mariners and aviators, are based on the latitude and longitude grid. Since the nautical mile is defined as one minute of latitude, a navigator can easily use the latitude scale on the chart to measure distance.
Using knots simplifies the calculation of speed, distance, and time without requiring constant conversion between statute miles and geographical coordinates. If a ship travels at 15 knots for two hours, the distance covered is simply 30 nautical miles. This distance can be measured directly on the chart’s latitude scale. This direct relationship streamlines dead reckoning and course plotting. The knot is also the standard unit for measuring wind speed and ocean currents, ensuring all speed-related data is consistent and applicable to navigational charts.