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.

Outlines

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🌐 Understanding Earth's Sphericity in Ancient Greece

This video explores how ancient Greeks deduced that the Earth is spherical. Initially, around 500 BC, the common belief was that the Earth was flat. However, Pythagoras and his students were the first to suggest a spherical Earth. Anaxagoras later supported this idea by observing the Earth's shadow on the Moon during lunar eclipses. Aristotle, around 340 BC, provided several arguments for a spherical Earth, including the varying positions of the North Star at different latitudes and the observation that ships disappear hull-first over the horizon. Eratosthenes, working at the Library of Alexandria, calculated the Earth's circumference by comparing the shadows cast by the sun at different locations on the summer solstice, leading to a remarkably accurate estimation for his time.

Keywords

💡Spherical Earth

The concept of a 'Spherical Earth' refers to the Earth's shape being round and three-dimensional, like a sphere. This idea was revolutionary during ancient times when many believed the Earth to be flat. In the video, the Greeks' understanding of the Earth's shape is central to the narrative, as they were among the first to propose and later prove that the Earth is spherical. The video discusses various observations and arguments that led to this conclusion, such as the changing position of the North Star and the way ships disappear over the horizon.

💡Pythagoras

Pythagoras was an ancient Greek philosopher and mathematician who, along with his pupils, first proposed the idea of a spherical Earth around 500 BC. His contribution is foundational to the video's theme as it marks the beginning of the scientific inquiry into the Earth's shape. The video mentions that Pythagoras's proposal was later supported by observations during lunar eclipses.

💡Anaxagoras

Anaxagoras was a Greek philosopher who supported Pythagoras's spherical Earth theory between 500 to 430 BC. He observed the shadows that the Earth cast on the Moon during lunar eclipses, providing early evidence for the Earth's round shape. His observations are highlighted in the video as a key piece of evidence that helped to shift the Greek understanding of the Earth's shape.

💡Aristotle

Aristotle, a prominent Greek philosopher, is mentioned in the video for listing several arguments for a spherical Earth around 340 BC. His observations included the varying positions of the North Star when traveling to different locations and the disappearance of ships over the horizon. Aristotle's work is significant as it further solidified the spherical Earth theory and provided additional evidence beyond what was offered by Pythagoras and Anaxagoras.

💡North Star

The 'North Star', also known as Polaris, is noted in the video as a fixed point in the sky used by Aristotle to argue for a spherical Earth. The star appears at different altitudes depending on the observer's latitude, which would not be possible if the Earth were flat. This observation is crucial as it provides geographical evidence for the Earth's curvature.

💡Lunar Eclipse

A 'lunar eclipse' is an astronomical event where the Moon passes into the Earth's shadow. The video describes how Anaxagoras used observations of lunar eclipses to support the spherical Earth theory, noting how the Earth's shadow on the Moon is round, indicating the Earth's spherical shape.

💡Eratosthenes

Eratosthenes was an ancient scholar who is highlighted in the video for his accurate calculation of the Earth's circumference. His method involved observing the angle of the sun's rays at different locations during the summer solstice, which led him to deduce the Earth's curvature and calculate its size. Eratosthenes' work is a significant achievement in the video's narrative as it demonstrates the Greeks' advanced understanding of geometry and geography.

💡Circumference

The 'circumference' refers to the measurement of the distance around a circle or spherical object. In the context of the video, the Greeks' calculation of the Earth's circumference was a way to quantify the size of the spherical Earth. Eratosthenes' calculation of approximately 40,000 kilometers is mentioned, which was remarkably close to the modern value.

💡Stadia

Stadia, as mentioned in the video, was a unit of measurement used in ancient Greece, equivalent to the length of a typical stadium. Eratosthenes used this unit to express his calculation of the Earth's circumference, which was approximately 250,000 stadia. The use of stadia in the video helps to connect the ancient measurements to the modern understanding of the Earth's size.

💡Ship Disappearance

The observation of 'ship disappearance' over the horizon is used in the video as evidence for a spherical Earth. Aristotle noted that as ships sail away, their hulls disappear before their sails, which would not be the case if the Earth were flat. This phenomenon is explained by the curvature of the Earth, which obstructs the view of lower parts of objects at a distance.

💡Parallel Light Rays

The concept of 'parallel light rays' is introduced in the video to explain Eratosthenes' method for calculating the Earth's circumference. He hypothesized that the sun's rays are parallel, which, combined with the observed angles of shadows at different locations, allowed him to deduce the Earth's curvature. This concept is essential to understanding how the Greeks used simple observations and geometric principles to arrive at complex conclusions about the Earth's shape.

Highlights

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

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

Anaxagoras supported the spherical Earth theory by observing lunar eclipses.

Aristotle provided several arguments for a spherical Earth around 340 BC.

The position of the North Star was an argument for a spherical Earth.

Aristotle also argued that if the moon and sun were spherical, Earth might be too.

The disappearance of ships at sea was another argument for a spherical Earth.

Ancient scholars attempted to calculate the Earth's circumference.

Eratosthenes provided the most accurate measurement of the Earth's size during his time.

Eratosthenes used the angle of the sun's rays at different locations to calculate the Earth's curvature.

He computed the Earth's circumference to be approximately 250,000 stadia.

The stadia was a unit of measurement used to describe a typical stadium at that time.

Eratosthenes' measurement is about 40,000 kilometers, which is remarkably close to the actual value.

The Greeks' understanding of the Earth's shape was based on a combination of observations and logical reasoning.

The video concludes with an eight-item test to check comprehension.

Answers to the test will be provided in the video description.

Transcripts

in this video we are going to talk about

how the Greeks knew that the Earth is

spherical at the end of the lesson you

will be able to explain how the Greeks

knew that the Earth is spherical so

around 500 BC

most Greeks believe that the Earth was

flat okay

but it was fittagoras and his pupils who

were the first to propose a spherical

Earth

and then n500 to 430 BC

anaxagoras supported Pythagoras's

proposal by observing the Shadows that

the Earth cast on the moon during a

lunar eclipse

so around 340 BC

Aristotle again listed several Arguments

for a spherical Earth one of his

arguments was about the position of the

North Star

okay so the North Star was believed to

be at a fixed position in the sky

but when the Greeks traveled to places

near the equator like Egypt

they noticed that the North Star is

closer to the horizon

okay

Aristotle also argued that if the moon

and the sun were both spherical

then perhaps the Earth was also

spherical and if the Earth were flat

then a ship traveling away from an

observer should become smaller and

smaller until it disappeared

but the Greeks observed that the ship

become smaller

and then its Hull disappeared first

before the sale as if it was peeing

enveloped by the water until it

completely disappeared

so these are the arguments made by

Aristotle now let's talk about the size

of the spherical Earth

ancient Scholars tried to provide the

proof of a spherical earth and its

circumference through calculations it

was eratosthenius who gave the most

accurate size during their time

so while he was working at the Library

of Alexandria in third in Egypt

he received correspondence from cyan in

certain Egypt

which stated that a vertical object did

not cast any Shadow at noon time during

summer solstice

but this was not the case in Alexandria

where at noon during the summer solstice

a vertical object still casts a shadow

so these observations could only mean

that the Sun during this time in

Alexandria was not directly overhead

then determine the angle the sun made

with a vertical Direction by measuring

the shadow that a vertical stick casts

he found out that in Alexandria the sun

makes an angle of 7.2 degrees from the

vertical while zero degrees in cyan to

explain the difference he hypothesized

that the light rays coming from the Sun

are parallel and the Earth is curved so

from his measurements he computed the

circumference of the earth to be

approximately 250

000 stadia stadia means a stadium okay

it's a unit of measurement used to

describe a typical Stadium at that time

so this is about 40 000 kilometers now

okay so those are the reasons or things

that made the Greeks believe that the

Earth is spherical

okay so again before we end this video I

want you to answer this eight item test

just the tester comprehension

again the answer will be posted in the

description box