Eratosthenes

Around 500 BCE, many ancient Greeks recognized that the Earth was round. Pythagoras initially suggested this, considering the sphere as the ideal shape. Aristotle provided several reasons for this belief: the way ships disappear over the horizon, the round shadow Earth casts on the moon during a lunar eclipse, and the varying constellations observed at different latitudes. However, the actual size of Earth remained a mystery until 240 BCE, when Eratosthenes estimated its circumference.

Eratosthenes was not just a scholar; he was also a librarian, geographer, mathematician, astronomer, historian, and poet. He was so accomplished that his peers dubbed him Pentathlos, which refers to an athlete who competes in five different events, symbolizing his wide-ranging expertise. Although most of his works are lost, other scholars have documented his extensive findings.

Born in the Greek colony of Cyrene, now known as Shahhat, Libya, Eratosthenes later traveled to Athens to study at Plato’s Academy. After gaining recognition for his scholarship, the Greek ruler of Egypt invited him to tutor his son in Alexandria. By age 40, he took over as the chief librarian at the renowned Library of Alexandria.

In Alexandria, Eratosthenes carried out his most notable experiment. He had learned that at noon on the summer solstice, the Sun illuminated the bottom of a well in Syene (now Aswan), a city south of Alexandria, casting no shadows. On the same day, he measured the shadow a stick cast in Alexandria and found the Sun’s angle to be 7.2 degrees or 1/50th of a circle.

With the understanding that the Earth was round, Eratosthenes deduced that if he knew the distance between Syene and Alexandria, he could estimate Earth’s entire circumference. He determined it to be about 250,000 stades, approximately 28,500 miles (45,867 km), which is impressively close to today’s known value of 24,901 miles (40,075 km).

A few decades later, Posidonius, applying a similar method but using the star Canopus as a light source and Rhodes and Alexandria as reference points, miscalculated Earth’s circumference to be about 18,000 miles (28,968 km) due to an inaccurate distance between the two cities.

This miscalculation found its way into Ptolemy’s geographical treatise in the 2nd century CE, influencing explorers like Christopher Columbus to underestimate the Earth’s size and overestimate the feasibility of westward routes. Knowledge of Eratosthenes’ larger and more accurate measurement may have altered Columbus’s plans.

Motivated by a passion for geography—a term he coined—Eratosthenes crafted a system of longitude and latitude and mapped the known world, earning him the posthumous title “father of geography.” He also devised a technique, the “Sieve of Eratosthenes,” to identify prime numbers, a method still utilized today.

Eratosthenes was the first to accurately calculate the tilt of the Earth’s axis—a finding later documented by Ptolemy. Furthermore, he attempted to calculate the distances from the Earth to the Moon and the Sun, albeit with less precision, and compiled a catalog encompassing 675 stars.

Fearing impending blindness, Eratosthenes chose to end his life at around 82 by starving himself.