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The length per one degree of longitude at the equator.
Literal
Equator [at/in-における] longitude 1 degree per-当たり [of-の] length.
Noun-ending definitional phrase, not a full sentence. ~における is a formal/written equivalent of ~で for 'at, in, regarding,' almost exclusively used in formal written contexts. ~当たり as a suffix means 'per (unit)' (e.g., 一人当たり 'per person'). This phrase would typically head a dictionary entry, reference text, or exam question.