Etymologically
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equator

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Pronunciation

  • UK: /ɪˈkweɪ.tə/[1]
  • US: /ɪˈkweɪ.tɚ/, /iˈkweɪ.tɚ/[1]

Etymology

It is inherited from Middle English equator,[2] borrowed from Medieval Latin aequātor (“one who makes equal, equalizer”),[3][4][5][6] short for circulus aequator diei et noctis (“the circle that makes day and night equal”),[3][7][8][9] an agent noun from Latin aequō (“to make equal”),[4][5][6][9] from aequus (“equal”).[3] Hoad and the Shorter Oxford allow that it may have come through Old French équateur.[7][8] The name refers to the fact that when the sun stands on the celestial equator, twice a year, day and night are of equal length.[4][6][3] It is related to equate and equal.[4][10]

Earliest recorded meaning

The great circle of the celestial sphere whose plane is at right angles to the earth's axis: 1391, in Chaucer's Treatise on the Astrolabe,[6][3] which says that the equinoctial circle "is clepid equator, or the weyere of the day".[2] Chaucer used the word once.[11]

Semantic evolution

The word began as a term of astronomy, and the celestial sense is the oldest.[4][3][7] It was carried over to the corresponding circle on the earth, midway between the poles, in the early 17th century: the OED's first record is in Brerewood's Enquiries (before 1613), Barnhart dates it 1612 and Etymonline the 1610s.[3][6][4] Johnson (1755) still defined it for both the earth and the heavens, as the circle that divides the globe into a northern and a southern hemisphere.[12] Later uses transferred it to any similar circle: on a heavenly body (1746), the line on which a compass needle has no dip, the magnetic equator (1832), the widest horizontal line of a balloon (1784), and in biology the middle plane of a dividing cell (1888).[3]

Timeline

  • 1391 – the celestial equator, in Chaucer.[6][3][2]
  • before 1613 – the earth's equator, midway between the poles.[3][6][4]
  • 1625 – the equator of the magnet.[3]
  • 1746 – a similar circle on another heavenly body or sphere.[3]
  • 1784 – in aeronautics, the line of the widest horizontal section of a balloon.[3]
  • 1832 – magnetic equator, the line on which the needle has no dip.[3]
  • 1888 – in biology, the plane of division of a cell or nucleus.[3]

Meanings

Noun

  1. The imaginary great circle around the earth at latitude 0°, equally distant from the two poles, which divides the northern and southern hemispheres: The ship crossed the equator on the third day.[10][13][3]
  2. (astronomy) The celestial equator, the great circle of the sky in the plane of the earth's equator.[13][10][3]
  3. A similar great circle midway between the poles of rotation of a planet, star or other body.[13][3]
  4. A circle that divides a sphere or other surface into two equal, symmetrical halves.[10][3]
  5. The magnetic equator.[10][3]

Word family

  • equatorward
  • equatorial
  • celestial equator
  • Equatorial Guinea
  • equatorial telescope
  • magnetic equator

References

  1. ^ a b Wiktionary, the free dictionary, https://en.wiktionary.org
  2. ^ a b c Kurath, Hans; Kuhn, Sherman M.; Lewis, Robert E., eds. (1952–2001), Middle English Dictionary, Ann Arbor: University of Michigan Press; online edition ed. Frances McSparran et al. (2000–2018)
  3. ^ a b c d e f g h i j k l m n o p q r s t Oxford English Dictionary Online (accessed March 2026), Oxford: Oxford University Press
  4. ^ a b c d e f g Harper, Douglas. Online Etymology Dictionary, https://www.etymonline.com
  5. ^ a b The American Heritage Dictionary of the English Language (online 2024), 5th ed., Boston: Houghton Mifflin Harcourt
  6. ^ a b c d e f g Barnhart, Robert K. (1988), The Barnhart Dictionary of Etymology
  7. ^ a b c Hoad, T. F., ed. (1986), The Concise Oxford Dictionary of English Etymology, Oxford: Clarendon Press
  8. ^ a b Shorter Oxford English Dictionary on Historical Principles (2002), 5th ed., ed. William R. Trumble and Angus Stevenson, Oxford: Oxford University Press
  9. ^ a b Stevenson, Angus, ed. (2010), Oxford Dictionary of English, 3rd ed., Oxford: Oxford University Press
  10. ^ a b c d e Collins English Dictionary (2023), Glasgow: HarperCollins
  11. ^ Benson, Larry D., Glossarial Concordance to Middle English (Chaucer, Gower), Johns Hopkins University, github.com/jhu-digital-manuscripts/gcme
  12. ^ Johnson, Samuel (1755), A Dictionary of the English Language, London; 4th ed. 1773; Johnson's Dictionary Online, ed. Beth Rapp Young et al.
  13. ^ a b c Merriam-Webster Unabridged Dictionary (2024), online ed., Springfield, Mass.: Merriam-Webster