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.

Outlines

00:00

🌐 Early Theories on Earth's Spherical Shape

This paragraph delves into the historical understanding of Earth's shape, revealing that the concept of Earth being spherical dates back over 2,000 years. Ancient civilizations likely assumed Earth's roundness without concrete evidence. Pythagoras is credited with proposing the idea in 500 BC, but it was Aristotle who provided the first substantial evidence in 350 BC through observations such as the round shadow of Earth during lunar eclipses and varying star positions across different locations. Aristotle's work laid the foundation for understanding Earth's spherical shape and its relatively small size.

05:01

📏 Eratosthenes' Method for Measuring Earth's Circumference

The second paragraph focuses on the ancient Greek scholar Eratosthenes, who not only confirmed Earth's roundness but also calculated its circumference using simple observations and calculations. Eratosthenes, a prominent scholar and chief librarian of the Great Library of Alexandria, used the angle of the sun's shadow at noon during the summer solstice in two different locations to estimate the Earth's circumference. Despite the challenges of accurately measuring distances in 240 BC, he employed a bemelist to determine the distance between Alexandria and Syene (modern-day Aswan). By recognizing that 7.2 degrees is 1/50th of a full circle, he multiplied the distance by 50 to estimate Earth's circumference, coming remarkably close to the modern measurements. The paragraph also discusses the assumptions and inaccuracies in his calculations, which were later refined with more precise measurements.

Keywords

💡Spherical Shape

The term 'spherical shape' refers to the geometrical form of a sphere, which is perfectly round and symmetrical in every dimension. In the context of the video, it is the ancient understanding of Earth's shape, which was a foundational concept in early astronomy and geography. The script mentions that the Earth's spherical shape was discovered over 2,000 years ago, indicating its significance in the history of human knowledge.

💡Pythagoras

Pythagoras was an ancient Greek philosopher and mathematician, credited as the first to propose the idea that the Earth is round, around 500 BC. His concept is fundamental to the video's theme as it marks the beginning of the intellectual journey towards understanding Earth's true shape, even though he did not provide empirical evidence for his claim.

💡Aristotle

Aristotle, another Greek philosopher, is noted in the script for providing hard evidence supporting the spherical shape of the Earth through observations such as the round shadow during lunar eclipses and the changing positions of stars based on geographic location. His contributions are pivotal to the video's narrative as they represent a significant leap from philosophical speculation to empirical science.

💡Lunar Eclipse

A lunar eclipse occurs when the Earth comes between the Sun and the Moon, casting its shadow on the Moon. In the video, Aristotle's observation of the round shadow during a lunar eclipse is used as evidence of Earth's spherical shape. This natural phenomenon is a key example illustrating how ancient scholars used celestial events to deduce the Earth's shape.

💡Eratosthenes

Eratosthenes was an ancient Greek scholar known for his work in various fields, including the measurement of Earth's circumference using only the Sun and a stick. His method, as described in the script, involved observing the angle of the Sun's rays at different locations and calculating the distance between them. His achievement is a highlight of the video, showcasing the ingenuity of ancient science.

💡Circumference

Circumference refers to the total distance around a circle or spherical object. In the video, Eratosthenes' calculation of Earth's circumference is a central point, demonstrating an early attempt to quantify the size of our planet. His method and the resulting estimate are directly related to the video's theme of ancient efforts to understand Earth's dimensions.

💡Summer Solstice

The summer solstice is the day with the longest period of daylight in the year, when the Sun is at its highest position in the sky. In the script, Eratosthenes used the summer solstice to observe the Sun's position and the resulting shadows, which was crucial for his measurement of Earth's circumference. This astronomical event is integral to the video's explanation of his methodology.

💡Tropic of Cancer

The Tropic of Cancer is the northernmost latitude where the Sun can be directly overhead at noon. In the video, the city of Alexandria's position relative to the Tropic of Cancer is used to explain the angle of the Sun's rays and the resulting shadow, which helped Eratosthenes in his calculations. This geographical marker is essential for understanding the video's discussion of Eratosthenes' experiment.

💡Bemetist

A bemetist, as mentioned in the script, was a professional walker in ancient times who measured distances by counting steps. Eratosthenes hired a bemetist to measure the distance between Alexandria and Syene (modern-day Aswan), which was critical for his calculation of Earth's circumference. The role of the bemetist is significant in the video as it illustrates the methods of distance measurement in ancient times.

💡Stadia

Stadia was an ancient Greek unit of length, based on the circumference of a typical sports stadium. In the video, the bemetist used stadia to measure the distance between Alexandria and Syene, which was then converted into kilometers for Eratosthenes' calculations. The use of stadia is an example of the units of measurement employed in the script to convey the distances involved in Eratosthenes' experiment.

💡Polar Circumference

Polar circumference refers to the circumference of Earth measured from pole to pole, as opposed to the equatorial circumference. The script notes that Eratosthenes' calculation was close to the modern measurement of Earth's polar circumference. This term is relevant to the video's theme as it highlights the accuracy of Eratosthenes' method despite the limitations of his time.

Highlights

The concept of Earth's spherical shape dates back over 2,000 years, predating advanced satellite technology.

Ancient civilizations likely assumed Earth was round before having concrete evidence.

Pythagoras was the first to propose the idea of Earth's round shape in 500 BC.

Aristotle provided the first hard evidence for Earth's round shape in his book on the heavens written in 350 BC.

Aristotle used lunar eclipses and the round shadow of Earth on the Moon as evidence for its spherical shape.

Stars' varying positions depending on location on Earth provided further evidence for its spherical shape.

Eratosthenes, a Greek scholar, proved Earth's roundness and measured its circumference using the Sun and a stick.

Eratosthenes was appointed as the chief librarian of the Great Library of Alexandria in 240 BC.

He aimed to create a world map and needed to determine the size of Earth first.

Eratosthenes used the phenomenon of sunlight at noon during the summer solstice to measure Earth's circumference.

He used the angle of the Sun's shadow in Alexandria to calculate Earth's circumference.

Eratosthenes hired a bematist to measure the distance between Alexandria and Syene with high precision.

The distance between Alexandria and Syene was determined to be roughly 5,000 stadia or 800 kilometers.

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

Modern measurements confirm Eratosthenes' calculation was remarkably close to the actual polar circumference.

Eratosthenes made several assumptions that affected the accuracy of his measurements.

A 2012 experiment using modern tools calculated Earth's circumference with a 0.16% error margin.

Advanced satellite technology now allows for the exact measurement of Earth's circumference down to the meter.

Transcripts

knowledge of the earth's spherical shape

is nothing new well at least for most of

us that is

you might even be surprised to learn

that the earth shape was discovered over

2 000 years ago well before any advanced

satellites were able to make the exact

measurements that we have today to be

fair many of the ancient civilizations

likely assumed that the earth was round

even before having the evidence the

first credited to propose such an idea

was philosopher and mathematician

pythagoras back in 500 bc

but when it came to proving that the

earth is round it wasn't until aristotle

came around that hard evidence was first

laid out

when aristotle wrote his book on the

heavens in 350 bc

he laid out several pieces of evidence

for why he believed the earth assumed a

spherical shape

one thing he noticed and pointed out was

that each time there was a lunar eclipse

the shadow of the earth upon the moon

could be seen

and since that shadow is always round

regardless of where the earth is in its

rotation the earth must be round as well

another point he made was that the stars

are constantly in different positions

depending on where you are on the earth

aristotle noticed that there were stars

that he could see in egypt that could

not be seen from cyprus some 1 000

kilometers away and that as you move

further and further away from the

equator the visible constellations in

the sky would dramatically change

that proved as he wrote that not only

that the earth is circular in shape but

also that it is a sphere of no great

size for otherwise the effect of so

slight a change of place would not be

quickly apparent

several years later another greek

scholar eratosthenes would go a step

further and not only prove that the

earth was round but also measure its

circumference with only the power of the

sun and a stick

known as one of the most prominent

scholars of his time

eratosthenes studied in athens and

produced impressive works in astronomy

mathematics geography and philosophy

in 240 bc he was even appointed as the

chief librarian of the great library of

alexandria one of the largest and most

significant libraries of the ancient

world

one of eratosthenes many ambitions was

to create a world map and to do that he

knew that he first needed to determine

the size of the earth

eratosthenes lived in the city of

alexandria near the mouth of the nile

river in present-day modern egypt

he had first heard about a well located

in the ancient city of cyene which is

now present-day modern aswan he realized

that at noon on the summer solstice when

the sun was directly overhead

sunlight illuminated the entire bottom

portion of the well without casting any

shadows on the sides like during other

days of the year

he then checked to see if the same

phenomenon was happening in alexandria

by placing a stick upright in the ground

at noon on the day of the summer

solstice

here he noticed that the sun did cast a

shadow at an angle of 7.2 degrees

this is because the city of alexandria

is north of the tropic of cancer or also

known as the northernmost point where

the sun will be directly overhead

eratosthenes realized that he could then

perform some simple calculations with

this measurement and easily determine

the circumference of the earth

however one important factor in this

calculation was determining the distance

between alexandria and cyene

with modern technology this would be no

big deal but back in 240 bc this was

much more difficult to perform and

especially perform accurately

eratosthenes hired what was known as a

bemetist who essentially was a

professional walker trained to measure

distances by counting steps

although not much is known about their

measurement methods they were known to

have a high degree of precision and were

thought to have accompanied alexander

the great on his campaign throughout

asia

these bemidjis which measured distances

and length of stadia an ancient greek

unit of length based on the

circumference of a typical sports

stadium determined that the distance

between alexandria and saini was roughly

5 000 stadia or 800 kilometers in

today's terms

since eratosthenes knew that the sun hit

straight on in saini and at a 7.2 degree

angle in alexandria the distance between

them should be a 7.2 degree piece of the

360 degree sphere that is the earth

conveniently 7.2 degrees is 1 50th of a

full circle meaning that the distance

from alexandria to saini equates to

roughly 2 percent of the total earth's

surface

by taking this estimated distance of 800

kilometers and multiplying it by 50 he

was able to determine that the earth has

a total polar circumference of roughly

40 000 kilometers

so was he right well he was really

really close

today we know the earth has a

circumference of forty thousand

seventy-five kilometers around its

equator and forty thousand and eight

kilometers when measuring the polar

circumference

this is due to the slight bulge that the

earth has around its middle due to the

spin of the earth it is important to

remember as well that he had to make

many important assumptions which at the

time were not so accurate

number one that the distance between

alexandria and saini was exactly 5 000

stadia or 800 kilometers we know today

that this distance is measured at

841 kilometers

number two he assumed that alexandria is

due north of saini which although it is

north it is not directly due north as he

originally assumed

number three he assumed that saini lies

on the tropic of cancer today we know

that present-day aswan is actually 73

kilometers away from the tropic of

cancer

number four that light rays emanating

from the sun are parallel

and finally that the earth is perfectly

spherical in shape

with proper measurement tools and

accurate distance measurements between

alexandria and cyeni

eratosthenes would have likely been able

to get a much closer result

in fact in 2012 a similar experiment was

conducted with more accurate measuring

tools and it was calculated that the

distance was 40 074 kilometers which is

only 0.16 percent off from the currently

accepted polar circumference of the

earth

it is certainly amazing to know what was

accomplished so long ago with such a

limited tool set today with advanced

satellite measurement capability we are

able to determine the exact

circumference down to the meter

eratosthenes would go on to create his

map as well as make many other

breakthroughs but that will have to be

saved for another video

if you would like to stay updated with

more videos like this one be sure to

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as always thank you so much for watching

this video