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Eratosthenes of Cyrene (276-194 BC)

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Eratosthenes of Cyrene(276-194 BC)

Finding Earth’s Circumference

January 21, 2013

Math 250

- Knowing also that the arc of an angle this size was 1/50 of a circle, and that the distance between Syene and Alexandria was 5000 stadia, he multiplied 5000 by 50 to find the earth's circumference.His result, 250,000 stadia (about 46,250 km), is quite close to modern measurements: The circumference of the earth at the equator is 24,901.55 miles (40,075.16 kilometers). But, if you measure the earth through the poles the circumference is a bit shorter - 24,859.82 miles (40,008 km).

- Eratosthenes also determined the obliquity of the Ecliptic, measured the tilt of the earth's axis with great accuracy obtaining the value of
23° 51' 15", prepared a star map containing 675 stars, suggested that a leap day be added every fourth year and tried to construct an accurately-dated history.

- He developed the “Sieve of Eratosthenes” method of finding prime numbers smaller than any given number, which, in modified form, is still an important tool in number theory research.

Hipparchus-Ptolemy Models for Planetary Orbits

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Uniform-Circular-Motion Model

The following equation represents the circular motion of a planet, P around the Earth, E.

,

,

= radius of orbit

= period

= phase parameter

P at z1(t)

E

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Epicycle-on-Deferent Model

This equation demonstrates the observed retrograde motion of a planet as seen from Earth. The planet, P, undergoes uniform circular motion about a point that undergoes uniform circular motion about the earth, E.

,

P at z2(t)

z1(t)

E

epicycle

deferent

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E

epicycle

deferent

Three-Circle Model

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E

epicycle

deferent

Connection to Fourier Series

- This motion is periodic only when T1, T2, … , Tn are integral multiples of each other
- Hipparchus and Ptolemy found that if you shift the position of Earth and keep the orbits where they are, an even more accurate depiction of the orbital motion can be obtained

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Note: Only a truncated Fourier Series if

are all rational numbers.

Hipparchus-Ptolemy Model Using Cartesian Coordinates

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Mathematica 5.1

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