 |
 |
|
Eratosthenes of Kyrene, fl. 200 BC
Eratosthenes of Kyrene (ca. 273 - ca. 192), librarian at Alexandria and tutor there to Ptolemy IV, is often credited with the first measurement of the circumference of the Earth. Dikaiarchos of Messina (fl. 326 - 296), the pupil of Aristotle, probably calculated it first by the same method, if not Eudoxos of Knidos (c. 408 - 355) yet earlier.
All of these mathematicians apparently measured the distance from Syene (modern Aswan) north to Alexandria, perhaps by having soldiers march it off in measured paces. At summer solstice in Syene, the Sun casts no shadow on a sundial (and will illuminate the bottom of a well) since it shines from directly overhead. By measuring the angle between the gnomon and the Sun's shadow on the ground at Alexandria at the same moment, and applying simple Euclidean geometry (particularly, the theorem that alternate interior angles are equivalent), one can determine the angle at the center of the Earth pointing to Syene and Alexandria. This angle was estimated at one fiftieth of a circle; by multiplying the measured distance of 5000 stadia by fifty an accurate value for the earth's circumference was obtained.
Review the summary of Eratosthenes' calculation of the circumference of the Earth you already encountered in Week 1 in the Shape of the Earth video (about 33 minutes into the video).
|
HSCI 3013. History
of Science to 17th century |
spellcheck.net | wordcounter.net
Many thanks to the pedagogical model developed in Mythology and Folklore and other online courses by Laura Gibbs, which have been an inspiration for this course.
|
|
This course is currently undergoing major reconstruction to bring it into alignment with the new version of the course at Janux |
|||
