equatorial

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

Related words

• equator
• equal
• equate
• equinox

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