Previously, we discussed parallax and determining distances to stars. Now, we need to address a critical prerequisite: knowing the distance between the Earth and the Sun. Understanding this distance has been a challenge for centuries. Today, we use radar reflections off Venus and geometry to calculate it, but historically, people made good estimates long before modern technology. To determine the Earth-Sun distance, we first need to know the Earth’s size. This discovery dates back to around 200 B.C. with the Greek philosopher Eratosthenes in Alexandria, Egypt. He realized that at noon on the first day of summer in Syene (modern Aswan), the Sun was directly overhead, casting no shadow. Simultaneously in Alexandria, the Sun cast a shadow due to its position at a slight angle. By measuring the angle of the shadow in Alexandria (about 7 degrees) and knowing the distance between Alexandria and Syene (approximately 5000 stadia or about 800 kilometers), Eratosthenes used simple geometry to estimate the Earth’s circumference to be around 40,000 kilometers, very close to the modern value. This measurement was crucial for later astronomical calculations. For instance, by observing the transit of Venus across the Sun from different points on Earth, astronomers could use parallax to determine the Earth-Sun distance. NASA's Lunar Eclipse page: Venus Transit SDO: Using the Transit of Venus to Determine the Astronomical Unit:.