Eratosthenes

Eratosthenes, or Eratosthenes of Cyrene, (c. 276 BCE - c. 194 BCE), was a Greek scientific writer, astronomer, and poet, who was reputed for making one of the first measurements of the size and circumference of Earth. Although his original manuscripts were lost, a version described by Cleomedes was used[1] to popularize the measurement.

Round Earth Circumference Experiment
Erasthonese measured the angle of the Sun from two North-South points separated by a distance of 500 miles, Syene and Alexandria. While the sun was directly overhead at one location, it was angled at 7°12' south of zenith at the other city. This allowed him to compute the circumference of the Earth under the assumption that it was round, the sun was distant, and that the displacement of the sun at the second location was due to the observer standing at different angle upon the curved Earth.

https://www.britannica.com/biography/Eratosthenes

Eratosthenes' method of measuring Earth's circumference

Assumptions
Millersville University explains that Eratosthenes assumed that the earth was a sphere, the sun was very far away, and that light traveled in straight lines.

https://www.millersville.edu/physics/experiments/058/index.php

Agreement With Modern Value
It is often claimed that Eratosthenes's circumference agrees with the modern value for the circumference of the Earth, and that this is evidence in favor of the globe. However, it should be noted that the 'modern value' is based on a repetition of the original experiment made with the same assumptions, with updated methods for accuracy:

https://www.britannica.com/topic/Measuring-the-Earth-Modernized-1673316

Measuring the Earth Modernized

Displacement of the Sun
Under the Flat Earth Theory the cause of the displacement of the sun in the sky at two different locations is often attributed to either Electromagnetic Acceleration or a perspective theory. See: Sunrise and Sunset

Monopole Model Interpretation
It's a common misconception that Eratosthenes was measuring the circumference of the round earth in his shadow experiment. Eratosthenes had simply assumed that the earth was a sphere in his experiment, based on the work of Aristotle. He was actually measuring the diameter of the flat earth (distance across), which is a figure identical to the circumference of the round earth (distance around).

We can use Eratosthenes' shadow experiment to determine the diameter of the Flat Earth Monopole Model.

Syene and Alexandria are two North-South points with a distance of 500 miles. Eratosthenes discovered through the shadow experiment that while the sun was exactly overhead of one city, it was 7°12' south of zenith at the other city.

7°12' makes a sweep of 1/25th of the FE's total longitude from 90°N to 90°S (radius).

Therefore we can take the distance of 500 miles, multiply by 25, and find that the radius of the flat earth is about 12,250 miles. Doubling that figure for the diameter we get a figure of 25,000 miles.

The earth is physically much larger, of course. A circle with a diameter of 25,000 miles across is simply the area of land which the light of the sun affects, and represents the area of our known world.