Definition

1. Noun:
   • A table or data set giving the calculated positions of a celestial object at regular intervals throughout a period: An ephemeris is a publication, typically annual, containing astronomical tables that provide the predicted positions of celestial bodies (such as the sun, moon, planets, and stars) for each day of a given year.
   • (By extension) Any table of numerical data predicted for regular intervals: While primarily astronomical, the term can be used more broadly for any similar tabulation of predicted values over time.

Usage Examples

• Noun:
  • The astronomer consulted the ephemeris to determine the exact time of the solar eclipse.
  • Before modern software, navigators relied on a printed nautical almanac and ephemeris for celestial navigation.
  • The satellite's orbital ephemeris is updated daily by ground control.

Advanced Usage

• "Ephemeris data": The specific numerical information within an ephemeris, often used in fields like satellite tracking and GPS technology.
  • The accuracy of your GPS device depends on receiving current ephemeris data from the satellites.
• "Ephemeris time": A former standard of astronomical time based on the orbital motion of the Earth, now superseded by Terrestrial Time (TT).
  • Ephemeris Time was defined to be independent of the Earth's variable rotation.

Variants and Related Words

• Ephemerides (plural noun): The standard plural form of ephemeris.
  • The library's collection includes ephemerides from the 18th century.
• Ephemeral (adjective): Lasting for a very short time. This is a related word sharing the Greek root ( meaning "lasting a day") but is not a variant of the noun "ephemeris."
  • The beauty of cherry blossoms is ephemeral.

Synonyms

• Almanac: A book published annually containing information, often including astronomical data. (Note: An almanac is broader; an ephemeris is specifically a table of positions.)
• Astronomical table: A general term for tabulated astronomical data.

Notes on Meaning

• The primary and most specific meaning of "ephemeris" is an astronomical position table. The reference to a mayfly ("con phù du") in some bilingual contexts comes from the shared etymological root with the adjective "ephemeral," not from a direct meaning of the noun "ephemeris" itself. The noun "ephemeris" does not mean "mayfly."

Noun

1. an annual publication containing astronomical tables that give the positions of the celestial bodies throughout the year
   • today computers calculate the ephemerides