equator

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

Related words

• Ecuador
• equal
• equi-
• equate
• equinox
• equation

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