In the annals of scientific history, one name shines brightly for its groundbreaking contributions to understanding the shape of our planet: Eratosthenes. This ancient Greek polymath, living in the 3rd century BCE, devised ingenious methods to demonstrate the spherical nature of the Earth. His compelling evidence not only revolutionized ancient cosmology but also laid the foundation for modern astronomy and geography. In this article, we delve into the remarkable evidences presented by Eratosthenes that affirm the Earth’s spherical shape.

### The Experiment at Syene

Eratosthenes’ most famous evidence for the Earth’s sphericity stems from a simple but brilliant experiment conducted in the ancient city of Syene (modern-day Aswan, Egypt). Eratosthenes observed that on the summer solstice, the sun cast no shadow on the bottom of a deep well in Syene, indicating that the sun was directly overhead. This phenomenon occurred because Syene lies on the Tropic of Cancer, where the sun reaches its zenith during the summer solstice.

### Measurement of Sun Angles

Eratosthenes realized that if the sun was directly overhead in Syene, it must cast a shadow at a certain angle in Alexandria, located north of Syene. By measuring the angle of the shadow cast by a vertical pillar or obelisk in Alexandria during the same time, Eratosthenes could calculate the angle between the two locations and thereby estimate the curvature of the Earth’s surface.

### Calculation of Earth’s Circumference

Using trigonometric principles, Eratosthenes determined that the angle between the sun’s rays at Syene and Alexandria was about 7.2 degrees (1/50th of a full circle). Knowing the distance between Syene and Alexandria (approximately 800 kilometers), Eratosthenes calculated the Earth’s circumference by multiplying the distance by 50, yielding a remarkably accurate estimate of around 40,000 kilometers—remarkably close to the modern value.

### Celestial Observations

In addition to his experimental observations, Eratosthenes drew upon celestial phenomena to support his belief in the Earth’s spherical shape. He noted that during lunar eclipses, the shadow cast by the Earth on the moon was consistently round, suggesting that the Earth itself was spherical. Furthermore, Eratosthenes observed that as ships sailed over the horizon, their masts disappeared last, indicating the curvature of the Earth’s surface.

### Mathematical and Philosophical Arguments

Beyond empirical observations, Eratosthenes marshaled mathematical and philosophical arguments to support the notion of a spherical Earth. He reasoned that the spherical shape of the Earth was the most logical and elegant explanation for various phenomena, such as the changing positions of stars and constellations as one traveled north or south. Moreover, Eratosthenes argued that a spherical Earth provided a coherent explanation for the existence of different climates and seasons across the globe.

Eratosthenes’ evidences for the spherical Earth represent a triumph of scientific inquiry and critical thinking in the ancient world. Through astute observation, ingenious experimentation, and rigorous mathematical analysis, Eratosthenes demonstrated beyond doubt that the Earth is indeed a sphere—a concept that would reverberate through the ages and shape our understanding of the cosmos. His legacy serves as a testament to the power of human curiosity and intellect in unraveling the mysteries of the universe.