Great circle of the earth (7)

I believe the answer is:

equator

(Other definitions for equator that I've seen before include "Imaginary line dividing the earth's hemispheres" , "Imaginary line round the earth" , "Longest line of latitude" , "Line around earth equidistant from poles" , "Earth's north-south dividing line" .)

I've seen this clue in the Sydney Morning Herald.

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