What does ephemeris mean?
An ephemeris (plural: ephemerides) is a table or almanac that gives the calculated positions of celestial bodies — the sun, moon, planets, and sometimes stars — at regular intervals, typically daily. Ephemerides are indispensable tools in astronomy, celestial navigation, and astrology: before satellite positioning, sailors used a nautical ephemeris together with a sextant to determine a ship's latitude and longitude from observations of the heavens. Today most ephemerides are produced by computer models of the solar system's gravitational dynamics rather than printed tables. The word derives from the Greek 'ephēmeros', meaning 'daily', which also gives us 'ephemeral' — a reminder that the positions it records change from one day to the next. While largely confined to specialist use, the term retains a certain romance, evoking centuries of stargazers and seafarers who trusted printed tables of the sky to find their way.
nounA table or almanac giving the calculated positions of celestial bodies such as the sun, moon, and planets at regular intervals, used in astronomy and navigation.
- A table or almanac listing the computed positions of celestial bodies at regular intervals, used chiefly in astronomy and navigation.
"Before GPS, sailors plotted their course using an ephemeris to determine their position from the stars."
"The navigator consulted the nautical ephemeris each evening to fix the ship's position by the stars."
"Modern ephemerides are generated by supercomputers modelling the gravitational interactions of the solar system."
The plural follows the Greek pattern (like 'thesis' → 'theses'), pronounced roughly /ˌɛfəˈmɛrɪdiːz/. In technical writing 'ephemerides' is preferred over 'ephemerises'.
"The Jet Propulsion Laboratory publishes high-precision ephemerides for all the major bodies of the solar system."
The plural of ephemeris isn't 'ephemerises' — it's 'ephemerides', and it's been guiding sailors across oceans since the age of sail.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of ephemeris
Ephemeris comes from the Greek 'ephēmeros', meaning 'lasting only a day' or 'daily', formed from 'epi-' ('on') and 'hēmera' ('day'). It entered English in the early modern period via Latin, when printed almanacs of planetary positions became widespread in Renaissance Europe. It shares its Greek root with 'ephemeral', which originally described insects or fevers lasting a single day before acquiring its broader sense of transience.
Related word forms
How ephemeris is actually used
A technical term used mainly by astronomers, navigators, astrologers, and historians of science. The plural 'ephemerides' (pronounced /ˌɛfəˈmɛrɪdiːz/) is standard in technical contexts. Often appears with a modifier specifying its domain, as in 'nautical ephemeris' or 'planetary ephemeris'.
Easily confused with ephemeris
An ephemeris is a table of astronomical positions, while ephemeral describes something short-lived; both share the Greek root for 'daily' but differ entirely in meaning.