late 14c., from Medieval Latin aequator (diei et noctis) "equalizer (of day and night)," agent noun from Latin aequare "make equal" (see equate). When the sun is on the celestial equator, twice annually, day and night are of equal length. Sense of "celestial equator" is earliest, extension to "terrestrial line midway between the poles" first recorded in English 1610s.
adjectival word-forming element, variant of -al (1) with connective -i-. From Latin -ialis, in which the -i- originally was from the stem of the word being attached but later came to be felt as connective.
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Definitions of equatorial from WordNet
of or relating to or at an equator;
of or relating to conditions at the geographical equator;
of or existing at or near the geographic equator;
a telescope whose mounting has only two axes of motion, one parallel to the Earth's axis and the other one at right angles to it;