Greek astronomer
Using elegant mathematical reasoning and limited empirical measurement, in approximately 240 b.c., Eratosthenes of Cyrene (in current-day Libya) made an accurate measurement of the circumference of Earth. In addition to providing evidence of scientific empiricism in the ancient world, this and other contributions to geodesy (the study of the shape and size of the earth) spurred subsequent exploration and expansion. Ironically, centuries later the Greek mathematician and astronomer Claudius Ptolemy's erroneous rejection of Eratosthenes' mathematical calculations, along with other mathematical errors, resulted in the mathematical estimation of a smaller Earth that, however erroneous, made extended seagoing journeys and exploration seem more tactically achievable.
Eratosthenes served as the third librarian at the Great Library in Alexandria. Serving under Ptolemy III and tutor to Ptolemy IV, the head librarian post was of considerable importance because the library was the central seat of learning and study in the ancient world. Ships coming into the port of Alexandria, for example, had their written documents copied for inclusion in the library and, over the years, the library's collection grew to encompass hundreds of thousands of papyri and vellum scrolls containing much of the intellectual wealth of the ancient world.
In addition to managing the collection, reading, and transcription of documents, Eratosthenes' own scholarly work concentrated on the study of the mathematics related to Platonic philosophy. Although Eratosthenes actual writings and calculations have not survived, it is known from the writings of other Greek scholars that Eratosthenes' writings and work treated the fundamental concepts and definitions of arithmetic and geometry. Eratosthenes' work with prime numbers, for example, resulted in a prime number sieve still used in modern number theory. Eratosthenes' also contributed to the advancement of scientific knowledge and reasoning, including the compilation of a large catalogue of stars and the preparation of influential chronologies, calendars and maps. So diverse were Eratosthenes' scholarly abilities that he was apparently referred to by his contemporaries as "Beta," a reference to the fact that although Eratosthenes was well-grounded in many scholarly disciplines, he was seemingly second-best in all.
Eratosthenes is, however, best known for his accurate and ingenious calculation of the circumference of the earth. Although Eratosthenes' own notes regarding the methodology of calculation are lost to modern historians, there are tantalizing references to the calculations in the works of Strabo and other scholars. There are extensive references to Eratosthenes' work in Pappus's Collection, an a.d. third century compilation and summary of work in mathematics, physics , astronomy , and geography. Beyond making an accurate estimate of the earth's circumference, based on observations of shifts in the zenith position of the Sun , Eratosthenes also made very accurate measurements of the tilt of the earth's axis.
Apparently inspired by observations contained in the scrolls he reviewed as librarian, Eratosthenes noticed subtle differences in accounts of shadows cast by the midday summer sun. In particular, Eratosthenes read of an observation made near Syene (near modern Aswan, Egypt) that at noon on the summer solstice, the Sun shone directly on the bottom of a deep well and that upright pillars were observed to cast no shadow. In contrast, Eratosthenes noticed that in Alexandria on the same day the noon Sun cast a shadow on a stick thrust into the ground and upon pillars.
Based upon his studies of astronomy and geometry, Eratosthenes assumed that the Sun was at such a great distance that it could be assumed that its rays were essentially parallel by the time they reached the Earth. Although the calculated distances of the Sun and the Moon , supported by measurements and estimates made during lunar eclipses, were far too low, the assumptions made by Eratosthenes proved essentially correct. Utilizing such an assumption regarding the parallel incidence of light rays, it remained to determine the angular variance between the shadows cast at Syene and Alexandria at the same time of day. In addition, Eratosthenes needed to calculate the distance between Syene and Alexandria.
Given modern methodologies, it seems intuitive that Eratosthenes would set out to establish the values of angle and distance needed to complete his calculations. In the ancient world, however, Eratosthenes' empiricism, reflected in his actual collection of data, reflected a significant break from a scholarly tradition that relied upon a more philosophical or mathematical approach to problems. Moreover, Eratosthenes' solution relied upon a world-view, especially reflecting the spheroid shape of the world, that itself was subject to philosophical debate.
Eratosthenes ultimately determined the angular difference between the shadows at Syene and Alexandria to be about seven degrees. Regardless of how he obtained the distance to Syene (some legends hold that he paid a messenger runner to pace it off), Eratosthenes reasoned that the ratio of the angular difference in the shadows to the number of degrees in a circle (360 degrees) must equal the ratio of the distance (about 500 mi, or 805 km) to the circumference of the earth. The resulting estimate, about 25,000 mi (40,200 km), is astonishingly accurate.
In making his calculations, Eratosthenes measured distance in units termed stadia. Although the exact value of the stadia is not known, modern estimates place it in the range of 525 ft (160 m). Depending on the exact value of the stadia, Eratosthenes' estimate varied only a few percent from the modern value of 24,902 mi (40,075 km) at the equator. It is necessary to specify that this is the circumference at the equator because the earth is actually an oblate sphere (a slightly compressed sphere with a bulge in the middle) where the circumference at the equator is greater than the distance of a great circle passing through the poles.
Although Eratosthenes' calculations were disputed in his own time, they allowed the development of maps and globes that remained among the most accurate produced for
more than a thousand years. The interest in geography and geodesy emboldened regional seafaring exploration using only the most primitive navigational instruments. Moreover, Eratosthenes' calculations fostered the persistence of belief in the sphericity of the earth that ultimately allowed for the development of the concept of antipodes and an early theory of climatic zones dependent on distance from the earth's equator. Eratosthenes' work, encapsulating many of his theories, was reported to have been the first to use the term geography to describe the study of Earth.
