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In the words of Euclid:

In a circle straight lines are said to be equally distant from the center when the perpendiculars drawn to them from the center are equal.

(The Elements: Book $\text{III}$: Definition $4$)

And that straight line is said to be at a greater distance on which the greater perpendicular falls.

(The Elements: Book $\text{III}$: Definition $5$)