Suppose you are still a two-dimensional being, living on the same sphere of radius R. Show that if you draw a circle of radius r, the circle’s circumference will be C= 2(pi)Rsin(r/R) (3.62) Idealize the Earth as a perfect sphere of radius R = 6371 km. If you could measure distances with an error of ± 1 meter, how large a circle would you have to draw on the Earth’s surface to convince yourself that the Earth is spherical rather than flat?

Ryden, Barbara. Introduction to Cosmology (pp. 47-48). Cambridge University Press. Kindle Edition.