How the Ancient Greeks Proved that the Earth was Round
Summary
TLDRThis 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
🌐 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.
📏 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.
Mindmap
Keywords
💡Spherical Shape
💡Pythagoras
💡Aristotle
💡Lunar Eclipse
💡Eratosthenes
💡Circumference
💡Summer Solstice
💡Tropic of Cancer
💡Bemetist
💡Stadia
💡Polar Circumference
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
click on the subscribe button below and
as always thank you so much for watching
this video
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