c. 135-c. 51 B.C.

Stoic philosopher and astronomer who improved on Hipparchus's (c. 170-c. 120 B.C.) estimate of Earth's distance to the Sun. He also disagreed with Eratosthenes's (c. 276-c. 294 B.C.) excellent calculation of 252,000 stades for Earth's meridian circumference. Posidonius's poorer and much smaller value of 180,000 stades encouraged Christopher Columbus to undertake his voyage of discovery. Posidonius was also the first to draw attention to the spring and neap tides. On a more philosophical note, he emphasized the hypothetical nature of scientific knowledge.