The Historical and Scientific Journey to Measure the Distance from Earth to the Sun

To accurately determine the Earth-Sun distance, it is essential first to establish the size of the Earth. This pivotal discovery traces back to approximately 200 B.C., attributed to the Greek philosopher Eratosthenes in Alexandria, Egypt. Eratosthenes observed that at noon on the first day of summer in Syene (modern-day Aswan), the Sun was directly overhead, casting no shadow. Conversely, in Alexandria, the Sun cast a shadow due to its position at a slight angle. By measuring the angle of the shadow in Alexandria, which was approximately 7 degrees, and knowing the distance between Alexandria and Syene (approximately 5000 stadia or about 800 kilometers), Eratosthenes employed simple geometric calculations to estimate the Earth’s circumference to be around 40,000 kilometers, a figure remarkably close to the modern value. This measurement proved crucial for subsequent astronomical calculations. For instance, by observing the transit of Venus across the Sun from various locations on Earth, astronomers could utilize parallax to determine the Earth-Sun distance. Achieving these measurements necessitated the development of calculus, Isaac Newton’s laws of gravity, Johannes Kepler’s laws of planetary motion, and the heliocentric model supported by Galileo’s studies. Collectively, these advancements significantly enhanced our understanding of the cosmos and the Earth’s position within it.