The knot is a unit of speed commonly used in maritime and aviation contexts. It represents one nautical mile per hour. A nautical mile, in turn, is defined as 1,852 meters or approximately 1.15078 statute miles. This standardization is based on the Earth’s circumference1 and the concept of Earth’s circumference2, with one nautical mile corresponding to one minute of latitude on the Earth’s surface.
Knots are the preferred unit of speed in maritime and aeronautical navigation because they directly correlate with the geographical coordinates used in navigation. For example, when a ship or aircraft moves at 10 knots, it travels 10 nautical miles in one hour. This alignment with navigational charts and latitude lines makes knots more practical
for calculating distances and travel times on a global scale compared to kilometers per hour or miles per hour. The term “knot” originated from a method of measuring a ship’s speed using a device called a log line. This tool consisted of a rope with knots tied at regular intervals and a piece of wood (the “log”) at the end. Sailors would throw the log into the water and allow it to drift while the rope unwound from a reel.
From Wikipedia
They could estimate the ship’s speed by counting the number of knots that passed through their hands in a set time interval (measured using a sandglass). This system was widely used in the Age of Sail3 and became the basis for modern nautical speed measurement. Today, knots are a standardized unit of speed recognized internationally. In aviation, knots are used in flight plans, airspeed indicators, and air traffic control communications.
In maritime navigation, they remain crucial for charting courses, managing ship speeds, and adhering to regulations. Knots are also used in meteorology to describe wind speeds, particularly in forecasts for sailors and pilots.
Using knots simplifies navigation calculations. Because a nautical mile is directly tied to the Earth’s geometry, the use of knots allows seamless integration with navigational charts, which are based on degrees of latitude and longitude4. This ensures compatibility and accuracy in global travel, whether by sea or air.
Footnotes
- The Earth’s circumference varies slightly depending on how it is measured. Along the equator, the Earth’s circumference is approximately 40,075 kilometers (24,901 miles), while the circumference measured through the poles (a meridional circumference) is about 40,008 kilometers (24,860 miles). This difference arises because the Earth is not a perfect sphere but an oblate spheroid, flattened at the poles and bulging at the equator due to its rotation. These measurements are derived from geodetic studies and satellite observations, which account for the Earth’s irregular shape. ↩︎
- Latitude is a geographic coordinate that specifies the north-south position of a point on the Earth’s surface. It is measured in degrees (°), ranging from 0° at the Equator to 90° at the poles—northward for the Northern Hemisphere and southward for the Southern Hemisphere. Lines of latitude, also called parallels, run horizontally and are equidistant from each other, forming circles that decrease in size as they approach the poles. Latitude is essential for navigation, mapping, and understanding Earth’s climate zones, as it influences temperature, daylight, and seasonal variations. The concept is based on the Earth’s rotation and the angle between a point, the equatorial plane, and the Earth’s axis. ↩︎
- The Age of Sail refers to a historical period, roughly spanning from the 16th to the mid-19th century, when sailing ships dominated global maritime trade, exploration, and naval warfare. It began with the European powers’ expansion during the Age of Discovery, marked by the exploration of new trade routes and continents, and ended with the advent of steam-powered vessels, which rendered sail-driven ships less efficient for commercial and military purposes. During this era, advancements in ship design, navigation, and maritime technology significantly expanded global trade networks and facilitated cultural exchanges and colonization. Iconic vessels of the Age of Sail include caravels, galleons, and clippers. This period also saw the rise of naval empires such as Spain, Portugal, Britain, and the Netherlands, which competed for dominance over the seas. ↩︎
- Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth’s surface. It is measured in degrees (°), ranging from 0° at the Prime Meridian—an arbitrary line that passes through Greenwich, England—to 180° east or west, meeting at the International Date Line in the Pacific Ocean. Unlike lines of latitude, lines of longitude, or meridians, converge at the poles and are widest at the equator. Longitude is crucial for navigation and timekeeping, as the Earth rotates 15° per hour, aligning longitude with time zones. Determining longitude accurately historically required precise time measurement, a challenge resolved in the 18th century with the invention of marine chronometers. ↩︎
Further Reading
Sources
- Wikipedia “Knot (unit)” https://en.wikipedia.org/wiki/Knot_(unit)
- Marine Insight “Why Nautical Mile and Knot Are The Units Used at Sea?” https://www.marineinsight.com/guidelines/nautical-mile-knot-units-used-sea/
- NOA “What is the difference between a nautical mile and a knot?” https://oceanservice.noaa.gov/facts/nautical-mile-knot.html
- Aerotime “Nautical mile vs. knot: understanding the differences and their importance in aviation” https://www.aerotime.aero/articles/nautical-mile-vs-knot
