Etymologically
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equatorial

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Pronunciation

  • /ˌɛkwəˈtɔːɹiəl/[1]

Etymology

It is formed from English equator + -ial,[2][3][4][5][6] on the model of Latin, the OED tracing it to Late Latin aequātor, and compares French équatorial.[3] Equator is from Medieval Latin aequātor (“equalizer (of day and night)”), an agent noun from aequō (“to make equal”), from aequus (“equal”).[2][3]

Earliest recorded meaning

Relating to the "equator" of a magnet or of any round body: 1664, in Henry Power's Experimental Philosophy.[3][5] Etymonline dates the adjective to the 1660s,[2] and Dictionary.com to 1655–65.[6]

Semantic evolution

The adjective was first used of the equator of any spherical body, then from 1713 of the earth's equator and the regions around it.[3][4] Johnson (1755) defined it as "pertaining to the equator; taken at the equator".[7] In astronomy an equatorial instrument or equatorial telescope (1791, in a letter of Thomas Jefferson's) was a telescope mounted on an axis parallel to the earth's;[3] from 1793 such an instrument was simply called an equatorial.[3] Later technical uses include the equator of a balloon (1838), the equatorial plate of a dividing cell (1887, translating French plaque équatoriale), and in fisheries the north–south migration of fish (1879).[3][4]

Timeline

  • 1664 – relating to the equator of a magnet or other round body.[3][5]
  • 1713 – of or on the earth's equator.[3]
  • 1791 – equatorial instrument, a telescope with an axis parallel to the earth's.[3]
  • 1793 – noun, such a telescope.[3]
  • 1838 – relating to the equator of a balloon.[3]
  • 1887 – equatorial plate, in biology.[3]

Meanings

Adjective

  1. Of, at or near the equator: equatorial rainforests.[8][5]
  2. Typical of the region around the equator, as of its climate: equatorial heat.[5][4]
  3. (astronomy) Relating to the celestial equator.[8]
  4. Of a telescope: mounted on two axes at right angles, one parallel to the earth's axis.[8][3]

Noun

  1. A telescope with such a mounting.[8][5][3]

Word family

  • equatorially
  • nonequatorial
  • subequatorial
  • transequatorial
  • equatorial plane
  • Equatorial Guinea
  • French Equatorial Africa

References

  1. ^ Wiktionary, the free dictionary, https://en.wiktionary.org
  2. ^ a b c Harper, Douglas. Online Etymology Dictionary, https://www.etymonline.com
  3. ^ a b c d e f g h i j k l m n o p Oxford English Dictionary Online (accessed March 2026), Oxford: Oxford University Press
  4. ^ a b c d Shorter Oxford English Dictionary on Historical Principles (2002), 5th ed., ed. William R. Trumble and Angus Stevenson, Oxford: Oxford University Press
  5. ^ a b c d e f Merriam-Webster Unabridged Dictionary (2024), online ed., Springfield, Mass.: Merriam-Webster
  6. ^ a b Dictionary.com Unabridged (2016), based on the Random House Unabridged Dictionary, New York: Random House
  7. ^ Johnson, Samuel (1755), A Dictionary of the English Language, London; 4th ed. 1773; Johnson's Dictionary Online, ed. Beth Rapp Young et al.
  8. ^ a b c d Collins English Dictionary (2023), Glasgow: HarperCollins