Exploring the Ancient Science: How the Greeks Knew That the Earth Is Spherical

Summary

This video explores how ancient Greeks deduced the Earth's spherical shape. Around 500 BC, while most believed in a flat Earth, Pythagoras and his students proposed it was spherical. Anaxagoras supported this by observing lunar eclipses. Aristotle provided several arguments, including the varying position of the North Star and the disappearance of ships at sea, suggesting a curved surface. Eratosthenes, using observations of shadows at different locations during the summer solstice, calculated the Earth's circumference to be approximately 40,000 kilometers, offering compelling evidence for a spherical Earth.

Takeaways

• 🌐 The ancient Greeks, notably Pythagoras and his pupils, were among the first to propose that the Earth is spherical, contrary to the common flat Earth belief around 500 BC.
• 🌑 Anaxagoras supported the spherical Earth theory by observing the Earth's shadow on the Moon during lunar eclipses between 500 to 430 BC.
• 📚 Aristotle provided several arguments for a spherical Earth around 340 BC, including the varying positions of the North Star when observed from different locations.
• 🚢 Aristotle also noted that ships disappear hull-first over the horizon, suggesting a curved surface rather than a flat Earth.
• 🌕 He reasoned that if the Moon and the Sun are spherical, it's likely that the Earth is too, based on the uniformity of natural bodies.
• 🔍 Eratosthenes, working at the Library of Alexandria, provided the most accurate measurement of the Earth's size and circumference for his time.
• 📏 Eratosthenes used the difference in shadows at noon in two different locations to calculate the Earth's curvature and circumference.
• 🌞 He hypothesized that the Sun's rays are parallel and used this to determine the angle of the Sun's rays at different locations.
• 📐 From his measurements, Eratosthenes computed the Earth's circumference to be approximately 250,000 stadia, which is about 40,000 kilometers.
• 🧠 The video concludes by challenging viewers to answer a comprehension test based on the information provided about the Greeks' understanding of a spherical Earth.

Q & A

Who were the first Greeks to propose that the Earth is spherical?

Pythagoras and his pupils were the first to propose that the Earth is spherical.

How did Anaxagoras support Pythagoras's proposal of a spherical Earth?

Anaxagoras supported Pythagoras's proposal by observing the shadows that the Earth cast on the moon during a lunar eclipse.

What was one of Aristotle's arguments for a spherical Earth?

One of Aristotle's arguments was the observation that the North Star appears closer to the horizon when the Greeks traveled to places near the equator, like Egypt.

What did Aristotle infer from the observation of ships disappearing over the horizon?

Aristotle inferred that if the Earth were flat, a ship would become smaller and smaller until it disappeared, but instead, the hull disappeared first, suggesting that the Earth is curved.

How did ancient scholars attempt to calculate the circumference of the Earth?

Ancient scholars like Eratosthenes attempted to calculate the Earth's circumference through observations and mathematical calculations.

What was the significance of the difference in shadows observed by Eratosthenes between Alexandria and Syene?

The difference in shadows observed by Eratosthenes indicated that the Sun was not directly overhead in Alexandria during the summer solstice, suggesting that the Earth is curved.

What was the unit of measurement Eratosthenes used to estimate the Earth's circumference?

Eratosthenes used 'stadia' as the unit of measurement to estimate the Earth's circumference.

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

Eratosthenes computed the circumference of the Earth to be approximately 250,000 stadia, which is about 40,000 kilometers.

What was the common belief about the Earth's shape among Greeks around 500 BC?

Around 500 BC, most Greeks believed that the Earth was flat.

What was the significance of the observation that the North Star's position changes with location?

The observation that the North Star's position changes with location, being higher in the sky in northern regions and closer to the horizon near the equator, was evidence for the Earth's sphericity.

How did the Greeks' understanding of the Earth's shape evolve from a flat Earth to a spherical Earth?

The Greeks' understanding evolved through observations like lunar eclipses, the changing position of the North Star, and the disappearance of ships over the horizon, leading to philosophical and scientific arguments supporting a spherical Earth.