Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Time and Frequency from A to Z, E


A group of clocks or oscillators whose outputs are averaged to create a time scale. Typically, the relative value of each clock is weighted, so that the best clocks contribute the most to the average. The BIPM uses an ensemble of clocks located around the world to produce UTC, and NIST uses an ensemble of clocks located in Boulder, Colorado to produce UTC(NIST).

Ephemeris Time (ET)

An obsolete time scale based on the ephemeris second, which served as the SI second from 1956 to 1967. The ephemeris second was a fraction of the tropical year, or the interval between the annual vernal equinoxes, which occur on or about March 21. The tropical year was defined as 31,556,925.9747 ephemeris seconds. Determining the precise instant of the equinox is difficult, and this limited the uncertainty of Ephemeris Time (ET) to ±50 ms over a 9-year interval. ET was used mainly by astronomers, and was replaced by Terrestial Time (TT) in 1984.


The beginning of an era (or event) or the reference date for a system of measurements.

Created May 12, 2010, Updated March 1, 2023