The equator.
The equator and all lines of longitude are called great circles because the represent the circumference of the earth. The other latitude lines along the globe are smaller then the actually circumference.
Lines of latitude, or parallels, are imaginary circles that run parallel to the equator. As you move from the equator towards the poles, these circles decrease in circumference due to the Earth's spherical shape. At the equator, the latitude is 0 degrees and the circumference is largest, while at the poles, the lines converge to a single point at 90 degrees north and south, resulting in their diminishing size. This geometric property is a direct consequence of the Earth's curvature.
circumference = 2*pi*radius or pi*diameter
It is an arc
An arc
The circumference of a circle = pi*diametre = 2pi*radius
Then they are similar.
Pi times the diameter of a circle gives its circumference.
The ratio of the circumference of a circle to its diameter is Pi. (3.14159)
The planet with the largest circumference is Jupiter. About its equator, the circumference measure about 449,197 kilometers. Jupiter is an outer planet that also is the largest in the solar system.
The equator, around Earth's total circumference as a sphere.
Circumference is a special name for a circles perimeter. I think the answer you are looking for is radious.