How the Ancient Greeks Proved that the Earth was Round

Summary

This video script delves into the historical discovery of Earth's spherical shape, dating back over 2,000 years. Pythagoras first proposed the idea in 500 BC, but it was Aristotle who provided evidence through lunar eclipse observations and star positions in 350 BC. Eratosthenes, in 240 BC, ingeniously measured Earth's circumference using the angle of the sun's shadow and the distance between Alexandria and Syene, remarkably close to modern measurements despite the limitations of his time.

Takeaways

• 🌐 The concept of Earth's spherical shape has been known for over 2,000 years, predating advanced satellite technology.
• 📚 Pythagoras was the first to propose the idea of Earth being round in 500 BC, but it was Aristotle who provided the first evidence in 350 BC.
• 🌑 Aristotle used the round shadow of Earth on the Moon during lunar eclipses as evidence for Earth's spherical shape.
• 🌠 He also noted the changing positions of stars and constellations depending on one's location on Earth, indicating Earth's spherical form.
• 📏 Eratosthenes, a prominent scholar, used the angle of the sun's shadow at noon to calculate Earth's circumference without modern tools.
• 🏛 As the chief librarian of the Great Library of Alexandria, Eratosthenes was well-positioned to pursue his ambition of creating a world map.
• 🌞 He observed the difference in the sun's position at noon in two cities, Alexandria and Syene (now Aswan), to determine Earth's circumference.
• 👣 Eratosthenes employed a bematist, a professional distance measurer, to find the distance between the two cities using steps.
• 🔢 He calculated Earth's circumference to be approximately 40,000 kilometers, which is remarkably close to the modern measurement.
• 🧭 Despite the accuracy of his method, Eratosthenes made several assumptions that affected the precision of his calculation.
• 🛰️ Modern technology allows for even more precise measurements of Earth's circumference, down to the meter.

Q & A

When was the Earth's spherical shape first proposed?

The Earth's spherical shape was first proposed by the philosopher and mathematician Pythagoras around 500 BC.

Who provided the first hard evidence that the Earth is round?

Aristotle provided the first hard evidence that the Earth is round in his book 'On the Heavens' written in 350 BC.

What evidence did Aristotle use to support the Earth's spherical shape during a lunar eclipse?

Aristotle noted that during a lunar eclipse, the shadow of the Earth upon the Moon is always round, indicating that the Earth must be round as well.

How did Aristotle's observations of stars contribute to the understanding of Earth's shape?

Aristotle observed that the stars' positions changed depending on the location on Earth, and the visible constellations changed as one moved away from the equator, suggesting that the Earth is not only circular but also of a manageable size.

Who was the Greek scholar that measured the Earth's circumference using the sun and a stick?

Eratosthenes was the Greek scholar who measured the Earth's circumference using the sun and a stick in 240 BC.

What phenomenon did Eratosthenes observe in Cyene that helped him measure the Earth's circumference?

Eratosthenes observed that at noon on the summer solstice, sunlight illuminated the entire bottom of a well in Cyene without casting any shadows, indicating the sun was directly overhead.

How did Eratosthenes use the angle of the sun's shadow to calculate the Earth's circumference?

Eratosthenes compared the angle of the sun's shadow in Alexandria (7.2 degrees) to the direct overhead sun in Cyene and used this angle to calculate that the distance between the two cities was approximately 2% of the Earth's surface.

What was the role of a bematist in Eratosthenes' measurement of the Earth's circumference?

A bematist was a professional walker trained to measure distances by counting steps. Eratosthenes hired a bematist to determine the distance between Alexandria and Cyene, which was crucial for his calculation of the Earth's circumference.

What was the approximate circumference of the Earth that Eratosthenes calculated?

Eratosthenes calculated the Earth's polar circumference to be roughly 40,000 kilometers.

What are some of the assumptions Eratosthenes made that affected the accuracy of his measurement?

Eratosthenes made several assumptions, including the exact distance between Alexandria and Cyene, the direct northward position of Alexandria relative to Cyene, the location of Cyene on the Tropic of Cancer, the parallel nature of sunlight, and the perfect sphericity of the Earth.

How close was Eratosthenes' measurement to the currently accepted polar circumference of the Earth?

Eratosthenes' measurement was very close to the modern value; a 2012 experiment using more accurate tools calculated the distance to be 40,074 kilometers, which is only 0.16 percent off from the currently accepted polar circumference.