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Review key Eratosthenes: Syene, Alexandria & Earth Circumference exam facts and rate your mastery to track revision.
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#1
Ptolemy III Euergetes appointed Eratosthenes as the third head librarian of the Library of Alexandria around 245 BCE.
#2
Eratosthenes coined the term geography, derived from the Greek words ge (Earth) and graphein (to write or describe).
#3
His comprehensive three-volume geographic treatise Geographika divided the known terrestrial world into five climatic zones.
#4
He was nicknamed Beta by contemporaries because he was regarded as the second-best scholar across multiple intellectual disciplines.
#5
Eratosthenes conducted his planetary circumference measurement around 240 BCE using midsummer solstice observations.
#6
At local solar noon on the summer solstice, sunlight reached the bottom of a deep vertical water well in Syene without shadow.
#7
Syene was historically situated close to the Tropic of Cancer, where the sun reaches its zenith during the June solstice.
#8
In Alexandria, Eratosthenes measured the solstice shadow using a vertical gnomon mounted within a graduated hemispherical bowl.
#9
The angular shadow cast by the gnomon in Alexandria measured 7.2 degrees, exactly one-fiftieth (1/50) of a 360-degree circle.
#10
The calculation assumed parallel incoming solar rays based on the immense distance separating the Sun from the Earth.
#11
Euclidean geometry established that the alternate interior angle at Earth's center equaled the zenith angle of the gnomon's shadow.
#12
The method presumed that Alexandria and Syene shared an identical longitudinal meridian, despite Syene lying three degrees farther east.
#13
Professional survey pacers called bematists measured the overland distance between Alexandria and Syene as 5,000 stadia.
#14
Multiplying 5,000 stadia by 50 yielded a total planetary circumference estimate of 250,000 stadia, later adjusted to 252,000 stadia.
#15
Using the Egyptian stadium of approximately 157.5 meters, his 252,000 stadia estimate equates to 39,690 kilometers.
#16
The modern accepted meridional polar circumference of the Earth is 40,008 kilometers, demonstrating an error margin under two percent.
#17
He developed the Sieve of Eratosthenes, an ancient algorithmic method for identifying prime numbers by systematically sieving out multiples.
#18
Eratosthenes calculated Earth's axial tilt (obliquity of the ecliptic) as 11/83 of a circle, translating to approximately 23 degrees 51 minutes.
#19
Later Greek astronomer Posidonius calculated Earth's circumference using the star Canopus, which yielded a smaller figure that misled Columbus.
#20
Eratosthenes constructed an armillary sphere around 255 BCE, providing a mechanical model of celestial coordinates for astronomical observation.
Subject Specialist Commentary
Analytical perspective & practical exam advice from the Master10 academic board
Eratosthenes proved that you do not need modern satellites to measure an entire planet; simple geometry and careful observation are enough. By noticing that sunlight reached the bottom of a well in Syene without casting any shadow at noon on the summer solstice, while a vertical stick in Alexandria cast a shadow of roughly seven degrees at that exact same time, he recognized that Earth's curved surface explained the difference.
In UPSC CSE and SSC examinations, questions frequently focus on the scientific assumptions behind his experiment. Examiners test whether candidates know that Eratosthenes assumed the Sun's rays hit Earth in parallel lines and that Syene and Alexandria shared the same longitudinal meridian. Candidates often forget the role of bematists, who were professional foot-pacers hired to measure the overland distance. To remember his two key cities and instruments, recall the phrase 'Sun at Syene, Shadow at Alexandria' alongside the gnomon rod.
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