Isochrones

Isochrone, from the Greek isos meaning equal and chronos meaning time, describes a curve or line that connects points sharing the same time value under a specified context. The concept is widely used in different disciplines to represent equal-time boundaries or relationships.

In geography and transportation analysis, isochrones map areas that can be reached from a fixed origin within a given travel time using a defined transportation network. They are used to study accessibility, urban planning, emergency response, and real estate. Construction relies on network data such as road layouts and speed profiles, and may account for different modes of transport (walking, driving, transit) and varying conditions. Algorithms such as Dijkstra’s or A* on time-dependent networks compute the reachable region, which is then rendered as a polygon or a set of contours.

In astronomy, isochrones are lines on a color-magnitude diagram that connect stars of the same age, produced from theoretical models of stellar evolution at specified chemical compositions. By fitting isochrones to observed clusters or stellar populations, astronomers estimate ages and metallicities, test stellar physics, and study star formation histories.

In geology and geochronology, the standard term for a line indicating equal isotopic age is isochron rather than isochrone. The term isochrone may appear in some non-specialist texts, but isochron is the preferred term in that field. Across disciplines, isochrones share the unifying idea of a boundary defined by equal time.