Circle Wikipedia

Unit circle - Wikipedia.

In mathematics, a unit circle is a circle of unit radius--that is, a radius of 1. Frequently, especially in trigonometry, the unit circle is the circle of radius 1 centered at the origin (0, 0) in the Cartesian coordinate system in the Euclidean plane.In topology, it is often denoted as S 1 because it is a one-dimensional unit n-sphere.. If (x, y) is a point on the unit circle's ....

https://en.wikipedia.org/wiki/Unit_circle.

Great circle - Wikipedia.

A great circle, also known as an orthodrome, of a sphere is the intersection of the sphere and a plane that passes through the center point of the sphere. A great circle is the largest circle that can be drawn on any given sphere. Any diameter of any great circle coincides with a diameter of the sphere, and therefore all great circles have the same center and circumference as each ....

https://en.wikipedia.org/wiki/Great_circle.