Astronomical Almanac — Annual Ephemerides, Time Standards, and Celestial Navigation Data Astronomical Almanac Annual reference from the US Naval Observatory and HM Nautical Almanac Office supplying precise ephemerides, time-scale conversions, eclipse predictions and navigational coordinate…
Atomic clock — ultra-precise timekeeping using atomic transitions Atomic clock A clock that uses the natural frequency of atomic energy transitions as its timekeeping reference; the basis of modern time standards, navigation, telecom and precision science.
