In the 1760's Charles Mason and Jeremiah Dixon surveyed a line of the same name. They found larger than normal systematic errors, which Henry Cavendish attributed to the gravitational pull of the Allegheny Mountains. Several years later Cavendish did a series of sensitive experiments to determine the gravitational constant, which is in every physics textbook. The journey from a line in the sand to the event horizon of a black hole started more the 2,000 years ago.
Eratosthenes's MethodAssume that the yellow rays of sunlight are parallel. One line strikes the equator and there is no shadow. Farther north, the black line casts a shadow. The black line crosses two parallel lines so that the shaded angles are congruent to each other.
Example: The Tropic of Cancer is about 23.4 degrees north. The distance to the equator is 1617 miles. Circumference of the Earth is 1617 x 360 / 23.4 = 24,877 miles