ephemeris second: meaning, definition, pronunciation and examples
Very Low (C2+)Technical/Scientific (Astronomy, Metrology, History of Science)
Quick answer
What does “ephemeris second” mean?
The fundamental unit of time in astronomy, defined as a specific fraction of the tropical year at a given epoch (e.
Audio
Pronunciation
Definition
Meaning and Definition
The fundamental unit of time in astronomy, defined as a specific fraction of the tropical year at a given epoch (e.g., 1/31,556,925.9747 of the tropical year for 1900). It is a precise, constant time interval derived from astronomical observations.
A highly precise historical definition of the second based on the Earth's orbital period around the Sun, used as the standard from 1960 to 1967. It has since been replaced by the atomic second but remains important for understanding long-term astronomical calculations and the historical development of timekeeping standards.
Dialectal Variation
British vs American Usage
Differences
No significant differences in meaning or usage. Spelling follows standard national conventions for other words in a sentence (e.g., 'metre' vs. 'meter' in surrounding text).
Connotations
None. Purely technical term.
Frequency
Equally rare and specialised in both varieties.
Grammar
How to Use “ephemeris second” in a Sentence
The ephemeris second was defined as...Prior to 1967, the standard second was the ephemeris second.One ephemeris second equals 1/31,556,925.9747 of the tropical year for 1900.Vocabulary
Collocations
Examples
Examples of “ephemeris second” in a Sentence
adjective
British English
- The ephemeris-second definition was superseded.
American English
- The ephemeris-second definition was superseded.
Usage
Meaning in Context
Business
Never used.
Academic
Used in specialised papers on metrology, history of science, or precise historical astronomy.
Everyday
Virtually never used.
Technical
Primary context. Used in discussions of time standards, calibration of historical data, or celestial mechanics.
Vocabulary
Synonyms of “ephemeris second”
Neutral
Weak
Vocabulary
Antonyms of “ephemeris second”
Watch out
Common Mistakes When Using “ephemeris second”
- Using it to refer to the current SI second.
- Pronouncing 'ephemeris' as /ˈiːfəmərɪs/ (it's /ɪˈfɛmərɪs/).
- Thinking it is related to something short-lived.
FAQ
Frequently Asked Questions
No. It was replaced in 1967 by the atomic second, which is based on the vibration of caesium atoms and is far more stable and reproducible.
Because its precise value was determined from astronomical 'ephemerides'—tables predicting the positions of the Sun, Moon, and planets over time.
It was defined as 1/31,556,925.9747 of the tropical year for the epoch 1900 January 0 at 12 hours ephemeris time.
Primarily historians of science, astronomers working with very old data, metrologists (scientists of measurement), and advanced physics or engineering students studying the evolution of standards.
The fundamental unit of time in astronomy, defined as a specific fraction of the tropical year at a given epoch (e.
Ephemeris second is usually technical/scientific (astronomy, metrology, history of science) in register.
Ephemeris second: in British English it is pronounced /ɪˈfɛmərɪs ˈsɛkənd/, and in American English it is pronounced /ɪˈfɛmərɪs ˈsɛkənd/. Tap the audio buttons above to hear it.
Learning
Memory Aids
Mnemonic
Link 'ephemeris' to 'ephemeral' (lasting a short time) – ironic, as it defined a fundamental, constant unit of time based on the long Earth year, not atomic vibrations.
Conceptual Metaphor
TIME IS MEASURED MOTION (of celestial bodies).
Practice
Quiz
What replaced the ephemeris second as the primary standard in 1967?