Reasons we know the Earth is round


1. Ships moving over the horizon disappear hull first, top last. They can turn around and sail back and reappear top first, hull last. Ships do not sail off the edge of the Earth.

2. The shadow of the Earth on the Moon during a lunar eclipse is round. The geometry of the situation is vital to making sense of this proof: a picture would help.

3. Eratosthenes and the technique of measuring the radius of the Earth using the length of shadows in two locations on the Earth's surface of known separation. Subtracting one angle from the other gives the angle between the two locations as measured from the Earth's center. Trigonometry can be used to find the radius or (simpler) a proportion can be used to find the circumference of the Earth directly. For example a measured angle of 9deg between two locations 1000 km apart gives the following proportion: 9/360 = 1,000/x. Where x is the circumference of the Earth and it equals 40,000 km. This technique only works if the Earth is round since for a flat Earth the parallel rays of the Sun would make the same angle with all poles everywhere.

4. Not all stars are visible from all locations. For example, stars near the north celestial pole are invisible to people who are south of the equator.

5. Sunset and Sunrise are not simultaneous everywhere as they would have to be on a flat Earth. Similarly, eclipses of the Sun and Moon are not observed at the same time everywhere on the Earth.

6. Send a rocket far enough away from the Earth to take a picture of it. Make sure to take pictures from several directions. The only shape that looks like a circle from every direction is a sphere. Ergo, the Earth is round.

7. The distance between meridians of longitude converge as both the North and the South Poles are approached. In the case of at least one version of a flat Earth the meridians would converge toward one pole but must diverge at the other.

8. Others?