Astronomy Practice Exam 2026 – 400 Free Practice Questions to Pass the Exam

Eratosthenes is renowned for his remarkably accurate measurement of the Earth's circumference in the 3rd century BCE. He achieved this by using a simple yet ingenious method involving the angles of shadows cast by objects in different locations. By comparing the angle of the sun's rays at noon in Alexandria and Syene, a city located directly south of Alexandria, he deduced that the Earth was a sphere and calculated its circumference with impressive accuracy.

Using the distance between the two cities and the difference in shadow angles, Eratosthenes was able to estimate the perimeter of the Earth to be about 39,375 kilometers, quite close to the modern value of approximately 40,075 kilometers. This accomplishment not only showcased his advanced understanding of geometry and geography but also laid foundational principles for the field of astronomy and earth sciences.

In contrast, the other options focus on measurements or concepts that were not a part of Eratosthenes' work. The distance to the moon and the size of the solar system involve measurements made with more advanced technology, and while the angle of Earth's tilt is crucial for understanding climate and seasons, it was not Eratosthenes' contribution. Thus, his achievement in measuring the Earth's circumference stands out as a pivotal