Definition: The term "isochrons" is a mathematical concept in astronomy that refers to the principle of measuring distances by using the same unit of measurement for both the distance and time. This principle was first introduced by the astronomer Johannes Kepler, who used it to calculate the orbits of planets around the Sun. Isochrons are commonly used in navigation systems, such as GPS, where they allow users to determine the position of a spacecraft or object using different units of measurement for its distance. Isochrons