Ephemeris TimeEdit

Ephemeris Time (ET) is a historical dynamical time scale that was once the standard for computing celestial ephemerides and for describing the motion of the planets and the Moon. It emerged from mid-20th-century efforts to base time on the dynamics of the solar system rather than on the irregular rotation of the Earth itself. In practice, ET provided a uniform, predictable time base for astronomical calculations, navigation, and spaceflight planning. With the rise of atomic timekeeping and the adoption of Terrestrial Time (TT) as its successor, Ephemeris Time has become largely a historical reference, but its influence shaped how modern time scales are defined and interrelated. For historians of science and navigators alike, ET marks a pivotal move from a rotation-based notion of time toward a celestial-mechanics–based standard that underpins much of modern astronomy and space science. See also Timekeeping, Celestial mechanics, and Ephemeris.

History and definitions

Origins and purpose

In the mid-20th century, astronomers and metrologists sought a time standard that would reflect the true dynamical motion of the Earth–Sun–Moon system. The goal was to remove the irregularities of Earth's rotation as a clock-maker, replacing them with a smooth, calculable basis tied to celestial mechanics. This led to the concept of Ephemeris Time as the time scale used to generate and read off planetary ephemerides. The effort was coordinated by international bodies such as the International Astronomical Union (IAU) and the precursor committees of the Bureau International de l'Heure (BIH).

Units and epoch

Ephemeris Time was defined with a specific unit for the second, the ephemeris second, which was set so that a year in ET would correspond to a tropical-year-based interval. Concretely, one ephemeris second was defined as 1/31,556,925.9747 of the tropical year of 1900, giving ET a distinct but closely related length to the solar-day–based second used in civil timekeeping. This linkage to a tropical year anchored ET to the motions of the Sun and Earth in a way that made planetary ephemerides internally consistent over decades. See Tropical year and Ephemeris second for related terms.

Adoption and use

ET served as the de facto dynamical timescale for nautical and astronomical ephemerides, including the planetary series that underlie modern navigation and mission planning. Ephemerides computed in ET underpinned early space programs, long-range celestial forecasts, and the formal definitions used by astronomical almanacs of the era. See also JPL Development Ephemerides and Astronomical Almanac for related avenues where ET figures in historical practice.

Transition to modern standards

From ET to Terrestrial Dynamical Time and Terrestrial Time

As atomic timekeeping matured, the need for a time standard tied to celestial dynamics in the daily practice of civil life and technology diminished. The transition culminated in the adoption of Terrestrial Dynamical Time (TDT) and its successor Terrestrial Time (TT), formalized by the International Astronomical Union in the latter part of the 20th century. TT was designed to be a close, stable realization of the dynamical time scale on the Earth's surface, but defined in terms of the atomic timebase so that long-term continuity with historical ephemerides would be maintained. In practical terms, TT is realized by adding a fixed offset to International Atomic Time (TAI): TT = TAI + 32.184 s, with TT continuing smoothly through leap seconds and other adjustments that govern civil timekeeping. See Terrestrial Time and International Atomic Time for the modern framework.

The SI second and atomic clocks

The modern second is defined by a physical constant of nature—the cesium-133 atom. This atomic definition enables extremely stable, reproducible timekeeping, which is essential for precision navigation, communications, and science. The shift from a celestial, solar-year–based second to an atomic second allowed the time scale used in technology and science to outrun the drift inherent in human-scale Earth rotation and in historical celestial models. For the contemporary base of time, see SI second and TAI.

Current status and usage

Ephemeris Time, as a formal label, is now regarded as historical. Its practical role lives on in the way ephemerides are constructed and interpreted, and in the vocabulary of astronomical history. The modern backbone of precise time is TT (and its ties to TAI and UTC), while celestial ephemerides continue to be computed in a form that harmonizes with these scales. See Timekeeping and Ephemeris Time (historical context).

Relevance and uses

Ephemerides and navigation: The ET framework helped standardize how planetary positions were tabulated and used in celestial navigation, missions, and spaceflight planning. See Development Ephemeris and Planetary ephemeris for more on how such data are produced and applied.
Astronomy and almanacs: Historical almanacs and astronomical tables relied on ET as the reference framework for predicting the positions of planets, the Moon, and the Sun. See Astronomical Almanac for a primary source in this tradition.
Time standards and intercomparison: The shift from ET to TT and the embedding of TT in the SI time framework illustrate how astronomical and civil time standards interact in practice. See Timekeeping and the pages on UTC and UT1 for broader context.

Controversies and debates

Civil time versus celestial time: A long-standing policy question concerns whether civil time should be tied to the irregular rotation of the Earth (requiring leap seconds to stay aligned with solar time) or anchored to a continuous atomic timescale. Proponents of keeping leap seconds emphasize compatibility with the day-night cycle and traditional civil time. Critics argue that leap seconds introduce complexity for computing, communications, and automated systems. The debate has practical consequences for technology, finance, and international coordination, not merely for theory. See Leap second and UT1.
Decoupling from Earth's rotation: Some technologists advocate a future where civil time is fully decoupled from Earth's rotation, embracing a continuous, uniform scale based on atomic time. Others warn that such decoupling would create a more abrupt disconnect between timekeeping and human experience of day and night. The discussion reflects broader tensions between scientific precision, technological convenience, and cultural norms. See TAI and UTC for related standards.
Respecting historical baselines: From a scholarly perspective, Ephemeris Time remains a crucial historical stage in the story of timekeeping. Debates about how to interpret ET-era data or how to translate ET-based ephemerides into modern TT-based analyses involve careful historical and technical work, ensuring that long-running datasets remain usable across generations. See Ephemeris Time and Development Ephemerides.