Eratosthenes

Eratosthenes of Cyrene was an Ancient Greek polymath who lived from about 276 BC to 195 or 194 BC. He was a philosopher, scholar, mathematician, geographer, poet, astronomer, and music theorist. Eventually, he became the chief librarian at the Library of Alexandria.

Eratosthenes is best remembered for being the first person to calculate the Earth's circumference and the first to calculate Earth's axial tilt. He created the first global projection of the world using parallels and meridians. He also helped start the study of scientific chronology by using records from Egypt and Persia to estimate when important events, like the Trojan War, happened. He dated the sack of Troy to 1184 BC.

In number theory, Eratosthenes created the sieve of Eratosthenes, a smart way to find prime numbers and other kinds of numbers. People called him “Pentathlos” because he knew a lot about many different subjects, but some critics called him “Beta” because they thought he always came in second place.

Life

Eratosthenes was born around 276 BC in Cyrene, a city in what is now Libya. Cyrene was a Greek settlement with a rich culture and good resources. As a young boy, Eratosthenes studied in the local gymnasium, learning reading, writing, arithmetic, poetry, and music.

Later, Eratosthenes traveled to Athens to study under famous teachers. He explored many areas of learning, including philosophy, mathematics, and literature. He wrote works on the dates of important events like the Trojan War and lists of Olympic winners.

In 246 BC, Eratosthenes moved to Alexandria in Egypt, where he became the chief librarian. He worked closely with the famous mathematician Archimedes and wrote on many subjects, including geography, philosophy, and poetry. Eratosthenes served as an advisor to royalty and lived to be about 80 years old.

Contributions

Astronomy

Measurement of Earth's circumference

Main article: Earth's circumference § Eratosthenes

Eratosthenes made a famous discovery about the size of our planet. He used a simple idea: by measuring the angle of the sun's shadow in two different places, he could figure out how far apart those places were and then calculate the whole Earth's circumference. He picked two cities, Alexandria and Syene (now Aswan). On a certain day, the sun shone straight down in Syene but made a small shadow in Alexandria. By measuring the angle of that shadow and knowing the distance between the cities, he estimated the Earth's circumference to be about 40,000 kilometers. This was very close to the actual measurement we use today!

Sun measurements

Eratosthenes also tried to measure how far the sun is from Earth. He thought it was much farther away than we now know, but his idea showed how clever ancient scientists were. He also guessed how wide the sun appeared to be compared to Earth, though his guess wasn't quite right either.

Obliquity of the ecliptic

Eratosthenes studied how the sun's path looks from Earth. He measured the angle between the sun's path and Earth's equator, which helps us understand seasons. He also helped create an early calendar that had 365 days, with a extra day every four years — that's where we get leap years today!

Geography

Eratosthenes loved drawing maps. He made one of the first big maps of the world, showing places from the Library of Alexandria. He divided Earth into climate zones — cold near the poles, warm near the equator, and temperate in between. He even placed lines on his maps to help find places, which is how we still navigate today.

Mathematics, music theory and metaphysics

Eratosthenes was also a great thinker about numbers. He created a clever way to find prime numbers — numbers that can only be divided by 1 and themselves — by crossing out multiples. This method is still taught in schools today. He also thought deeply about music, using math to understand how notes fit together. He even wondered about the soul, thinking it might have both invisible and visible parts.

Works

Eratosthenes was a very smart scholar who wrote about many different subjects. Sadly, most of his work was lost when the Library of Alexandria was destroyed.

During his time in Athens, he wrote about math, literature, and poetry. One of his math works, called Platonikos, tried to explain Plato's ideas more clearly. He also wrote about old Greek plays and poems, including stories about gods and heroes.

Later, when he worked at the Library of Alexandria, Eratosthenes wrote about stars, Earth, and history. He made one of the first maps of the world and tried to measure how big Earth is. He also wrote about important events in history, using the Olympic Games to help count the years.

He created many other works too, like a way to find prime numbers, studies of music, and thoughts about philosophy and the stars.