Aikaskaala

Aikaskaala, or time scale, is a system for defining, maintaining, and comparing time across clocks and instruments. It provides a reference for timestamps, synchronization, and calculations in science, technology, and daily life. Time scales can be based on Earth’s rotation (dynamical, or solar time) or on atomic processes (physical time).

Major time scales include UT1, the time based on Earth's rotation, which reflects variations in rotation rate and is used in astronomical calculations. UTC, the civil standard, combines International Atomic Time (TAI) with leap seconds to stay within about one second of UT1 for practical alignment with the day. TAI, the International Atomic Time, is a continuous atomic scale not adjusted for Earth's rotation. Terrestrial Time (TT) is used in ephemerides and equals TAI plus 32.184 seconds.

Other scales include GPS Time, used by the Global Positioning System; it started as TAI and does not include leap seconds, so its offset from UTC changes as leap seconds are added. UT2 is an older astronomical scale that was once used to approximate solar time. Relativistic scales such as Barycentric Dynamical Time (TDB) and Barycentric Coordinate Time (TCB) are used in celestial mechanics and account for relativistic effects in the solar system.

These scales are defined and maintained by international bodies such as the BIPM (Bureau International des Poids et Mesures), the IERS, and the IAU. The SI second, the unit of time, is defined by the radiation of cesium-133. Aikaskaala underpins precise science, navigation, and telecommunications by ensuring clocks across locations agree within defined tolerances.