Eratosthenes and the Sphere

The ancient Greeks were among the first to propose that the Earth was round, and they developed several arguments and observations to support this view. Here are some of the ways in which the ancient Greeks provided evidence for a spherical Earth:

1. Lunar eclipses: The ancient Greeks observed that during a lunar eclipse, the Earth casts a curved shadow on the Moon. This curved shadow can only be produced by a round object, such as a sphere.
2. Ship observations: The Greeks noticed that when a ship sails away from an observer on the shore, the ship appears to gradually sink below the horizon until only the top of the ship’s mast is visible. This phenomenon can be explained by the curvature of the Earth’s surface, which causes objects to appear to sink below the horizon as they move farther away.
3. Star observations: The ancient Greeks noticed that the stars appear to move in circular paths around a central point in the sky, known as the celestial pole. This motion can only be explained by a spherical Earth, as the stars would not move in circles if the Earth were flat.
4. Travelers’ accounts: The Greeks also gathered information from travelers and explorers who had journeyed to distant lands. These travelers reported that they had observed different constellations in the sky, depending on their location. This observation is only possible if the Earth is round.

Eratosthenes is credited with providing one of the earliest known proofs that the Earth is round. He was a Greek mathematician, geographer, and astronomer who lived in the 3rd century BCE.

Eratosthenes’ method for measuring the size of the Earth involved measuring the angle of the Sun’s rays at two different locations on the same day. He knew that on the summer solstice (the day with the most sunlight) at noon, the Sun would be directly overhead at the Egyptian city of Syene (modern-day Aswan). He also knew that at the same time, the Sun would be at an angle of about 7.2 degrees from the vertical in his hometown of Alexandria.

By measuring the angle of the Sun’s rays in Alexandria and knowing the distance between Alexandria and Syene, Eratosthenes was able to calculate the circumference of the Earth. He estimated the distance between Alexandria and Syene to be 5,000 stadia (about 800 kilometers), and using his measurements of the Sun’s angle, he calculated the Earth’s circumference to be about 39,375 kilometers. This was remarkably close to the modern value of 40,075 kilometers.

Eratosthenes’ method and calculations provided strong evidence that the Earth was round, as only a spherical Earth would produce the observed angles of the Sun’s rays at different locations on the same day. His work was highly influential in the development of astronomy, geography, and cartography, and his ideas were widely adopted by later scholars and thinkers.

there were several other ancient philosophers who proposed that the Earth was round. Here are a few examples:

1. Pythagoras (6th century BCE): The Greek philosopher Pythagoras is believed to have been one of the first to propose that the Earth was a sphere. He argued that the sphere was the most perfect shape and that the Earth, as a perfect object, must be a sphere.
2. Plato (4th century BCE): Plato, another Greek philosopher, also believed that the Earth was round. He argued that the sphere was the most natural shape for celestial bodies, and that the Earth was a celestial body like the stars and planets.
3. Aristotle (4th century BCE): Aristotle, a Greek philosopher and scientist, also proposed that the Earth was round. He observed that during a lunar eclipse, the Earth casts a curved shadow on the Moon, and argued that this could only be explained if the Earth were round.
4. Ptolemy (2nd century CE): The Greek astronomer and mathematician Ptolemy also believed that the Earth was round. He used observations of the positions of stars and planets to develop a detailed model of the solar system, which assumed that the Earth was at the center of the universe and that all celestial bodies moved in circular orbits around it.

These and other ancient philosophers and scientists used a variety of methods, including observation, deduction, and mathematical calculation, to support their belief in a spherical Earth. Their ideas were influential in the development of astronomy, geography, and other sciences, and their work laid the foundation for much of our modern understanding of the universe.