isochrone

An isochrone (from Greek isos meaning equal and chronos meaning time) is a curve or line that represents equal time or equal age, depending on the context. The term is used in astronomy and geography to denote different, but related, concepts involving a constant quantity across a set of locations or objects.

In stellar astrophysics, an isochrone is a theoretical line on a color-magnitude diagram or Hertzsprung–Russell diagram that connects stars of the same age but different masses. By comparing observed photometry of star clusters to model isochrones computed for various ages and chemical compositions (metallicity), astronomers estimate cluster ages, distances, and reddening. Isochrone fitting adjusts distance, extinction, and metallicity to match the observed distribution of stars.

In cartography and geographic information science, an isochrone is a line on a map that connects locations reachable within the same travel time from a given origin, using a specified mode of transport and network data. Isochrones are used to study accessibility, urban planning, evacuation planning, and service delivery, and can incorporate schedules, speeds, constraints, and network changes.

Although the two uses arise in different domains, they share the underlying idea of a constant time. In astronomy, the constant is age; in geography, the constant is travel time or time distance.