ἐφήμερος

The Greek ephḗmeros joins epí ('upon, for') and hēméra ('day') to mean 'upon a day' or 'lasting a day' — something that endures only as long as the light lasts. The adjective described mayflies (whose adult life spans a single day), daily journals, and anything of momentary duration. Hēméra is the same root that gives ephemeral its English meaning of 'short-lived.' The leap from 'daily' to 'astronomical table' happened through the practice of astronomical record-keeping: the positions of the sun, moon, planets, and stars change each day, and a table recording where each body will be on each successive day is, in the most literal sense, a record of daily (ephemeral) celestial positions. An ephemeris is a book of daily skies.

The earliest ephemerides (the proper plural) were Babylonian in origin — cuneiform tablets recording planetary positions and lunar phases stretching back to at least the 7th century BCE. Babylonian astronomers had tracked celestial cycles with such meticulous regularity that they could predict planetary positions years in advance using arithmetic schemes derived from observed periodicities, without any geometrical model of why planets moved as they did. Greek astronomers built on this tradition, and Ptolemy's Handy Tables (Procheiroi Kanones) were a working ephemeris compiled from his Almagest. The Greek term ephemeris was applied retrospectively to all such tables.

For over a millennium, ephemerides were the practical output of astronomical scholarship — the thing observatories actually produced that the world actually needed. Astrologers needed them to cast horoscopes. Navigators needed them to determine longitude at sea by measuring the angular distance of the moon from reference stars and comparing results with predicted positions in the ephemeris. Calendar-makers needed them to fix the dates of Easter and Ramadan. The Nautical Almanac, first published by the British Admiralty in 1767, was an ephemeris designed specifically for maritime navigation; it remained essential until GPS rendered celestial navigation optional rather than mandatory.

Modern ephemerides are produced by institutions including the U.S. Naval Observatory and the Jet Propulsion Laboratory, whose DE series (Development Ephemeris) is the most precise planetary position table ever computed. DE430 and subsequent editions account for relativistic effects, the masses of hundreds of asteroids, and decades of laser-ranging data from retroreflectors left on the Moon by Apollo astronauts. The precision required for deep-space navigation — pointing a probe at a moon of Jupiter and arriving on schedule — demands an ephemeris accurate to kilometers across billions of kilometers of space. The daily sky-table of ancient astronomers became a numerical integration of the solar system.