equinox (English)

Where does "equinox" come from?

The English word 'equinox' descends from Middle English[1]. This traveled through Old French 'equinoce' or Medieval Latin 'ēquinoxium'[1], originating from the Latin 'aequinoctium'[1]. The Latin term combines 'aequus' meaning 'equal' and 'nox' meaning 'night'[1], a root ultimately tracing back to the Proto-Indo-European form '*nókʷts'[1]. This borrowing eventually replaced the earlier Old English term 'efnniht'[1].

Open questions

Scholars debate whether the ProtoIndoEuropean form represents ablaut variants within a single paradigm or distinct root nouns derived from a root meaning bare or naked. [2]
The ultimate origin of the Latin word aequus remains unknown. [5]

Full etymology tree

equinox (English), 'One of two times in the year (one in March and the other in September) when the length of the day and the night are equal, which occurs when the sun is directly overhead at the equator; this marks the beginning of spring in one hemisphere and autumn in the other.'
- borrowed equinox (Middle English)
  - borrowed equinoce (Old French)
    - inherited ēquinoctium (Medieval Latin)
      - inherited aequinoctium (Latin), 'equinox'
        compound aequus (Latin), 'equal'
        inherited aiquos (Sabellic)
        inherited *aikʷos (Proto-Italic)
        compound nox (Latin), 'night (period of time)'
        inherited *nokts (Proto-Italic), 'night'
        inherited *nókʷts (Proto-Indo-European), 'night (or possibly 'evening')'
        suffix -ium (Latin), 'Suffix used to form abstract nouns, sometimes denoting offices and groups. May no longer be productive.'
  - borrowed ēquinoctium (Medieval Latin)
    - inherited aequinoctium (Latin), 'equinox'
      - compound aequus (Latin), 'equal'
        inherited aiquos (Sabellic)
        inherited *aikʷos (Proto-Italic)
      - compound nox (Latin), 'night (period of time)'
        inherited *nokts (Proto-Italic), 'night'
        inherited *nókʷts (Proto-Indo-European), 'night (or possibly 'evening')'
      - suffix -ium (Latin), 'Suffix used to form abstract nouns, sometimes denoting offices and groups. May no longer be productive.'

Ancestors

aequinoctium (Latin)
*nókʷts (Proto-Indo-European)

Cognates and doublets

équinoxe (French)
efnniht (Old English)

Descendants and derived words

equinox flower (English)
equinoxial (English)
equinoxial (English)
precession of the equinoxes (English)

Sources

Updated September 2026

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Etymology data from Wiktionary, CC BY-SA 4.0 · About Miraris