Courtesy: Wikipedia

Eratosthenes of Cyrene (Ἐρατοσθένης ὁ Κυρηναῖος) [276 BC – 195/194 BC]
was a Greek mathematician, geographer, poet, astronomer, and music theorist. He was a man of learning, becoming the chief librarian at the Library of Alexandria. He invented the discipline of geography, including the terminology used today.

He is best known for being the first person to calculate the circumference of the Earth, which he did by applying a measuring system using stadia, a standard unit of measure during that time period. His calculation was remarkably accurate. He was also the first to calculate the tilt of the Earth’s axis (again with remarkable accuracy). Additionally, he may have accurately calculated the distance from the Earth to the Sun and invented the leap day. He created the first map of the world, incorporating parallels and meridians based on the available geographic knowledge of his era.
Source: Wikipedia – Eratosthenes of Cyrene

Here is a short 2 minute video depicting in simple terms Eratosthenes accurate calculation of the circumference of the earth, 2,200 years ago, with just a stick and his brain:

Sources: “Cosmos” Carl Sagan, NASA

Here is another video, in more detail, from the Physical Science class by Derek Owen,
how Eratosthenes measured the distance around the Earth.