Eratosthenes

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

Eratosthenes is famous for being the first person to calculate the Earth's circumference and the Earth's axial tilt. He made the first global projection of the world using parallels and meridians. He also helped start the study of scientific chronology by using old records to guess when events happened.

In number theory, Eratosthenes created the sieve of Eratosthenes, a way to find prime numbers. People called him “Pentathlos” because he knew a lot about many subjects.

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.

Later, Eratosthenes traveled to Athens to study under famous teachers. He explored many areas of learning, including philosophy, mathematics, and literature.

In 246 BC, Eratosthenes moved to Alexandria in Egypt, where he became the chief librarian. He worked closely with the famous mathematician Archimedes. 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 discovered a lot about the size of our planet. He used a clever idea: by measuring the sun's shadow in two different places, he could work out the distance between them and then guess the whole Earth's circumference. He chose two cities, Alexandria and Syene (now Aswan). On one special day, the sun shone straight down in Syene but made a small shadow in Alexandria. By measuring this shadow and knowing how far the cities were apart, he worked out that the Earth's circumference was about 40,000 kilometers. This was very close to what we know today!

Sun measurements

Eratosthenes also tried to measure how far the sun is from Earth. He guessed it was much farther away than it really is, but his idea showed how smart ancient scientists were. He also tried to guess how wide the sun looked compared to Earth, though he was not quite right.

Obliquity of the ecliptic

Eratosthenes studied the sun's path from Earth. He measured the angle between the sun's path and Earth's equator, which helps us understand why we have seasons. He also helped make an early calendar with 365 days, adding an extra day every four years — this is where our leap years come from 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 added lines to his maps to help people 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 smart 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 work together. He even wondered about the soul, thinking it might have both invisible and visible parts.

Works

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

When he was 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.

Later, 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.