But, to determine that the Earth is a sphere, and to accurately compute the circumference of the earth, you need only simple tools and extremely simple math, to deduce a solution. So easy in fact that it was done over 2200 years ago.

Norwood and the French Geodesic Mission acheived the same, with differing methods, in the 1600s and 1700s respectively, arriving at, within reasonable margin of error for the methods, the same result as each other.

As a double-check, if you take the orbit of the ISS to be a perfect circle around a globe, use the published and observed orbital time, and published speed to calculate the circumference travelled by the craft, if you then subtract the stated orbital height above the surface, and recalculate the circumference of that surface, you end up with a figure broadly the same as those derived by the three methods outlined above.