Full Idea
We speak of the equator as an imaginary line, but it is not a fictitious line; it is not a creature of thought, the product of a psychological process, but is only recognised or apprehended by thought.
Gist of Idea
The equator is imaginary, but not fictitious; thought is needed to recognise it
Source
Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §26)
Book Reference
Frege,Gottlob: 'The Foundations of Arithmetic (Austin)', ed/tr. Austin,J.L. [Blackwell 1980], p.35
A Reaction
Nice point. The same goes for the apparently very abstract and theoretical concept of a 'circle', because a perfect circle could be imagined in a very specific location, perhaps passing through three specified points.