Assignment 1

Contributions of Erastosthenes to Ancient Science

Early Life:

Erastosthenes was born in Cyrene (present Libya), an ancient Greek city in 276 BC [1]. He was very involved in learning which took him to being the chief librarian at the Library of Alexandria. He learnt about Geography, Mathematics, Astronomy, Philosophy and History. He was known to be second among the people with similar work which some people who disliked him nicknamed him as Beta which later on he proved wrong with producing accurate results better than others at that time. He discussed about lot of ideas with his friend, Archimedes and these discussions were part of Eratosthenes' inspiration to work on experimental methods [2]. This is an iteration of experimental science which in ancient times was a tool to derive theoretical results where conversely in modern science theories are used to explain experimental results. 

Science work:
Erastosthenes' contributions to science was mainly in Geography and History. He build Chronographia, which lists the dates of major events around the Greeks in his time, like the date of the "seige of Troy at 1184 BCE" with the help of the methods of Hecateus, a Historian & Geographer himself [3].  He also collected data from explorers and voyagers to describe some of the activities of the Earth tectonics activities like fire , earthquakes and volcanism which are very important even in today's geography. He also contributed with his writings in philosophy, chronology,  literary criticism, poetry and most of his work was destroyed in the Destruction of the Library of Alexandria[4]. His work in astronomy is also much notable, he "presumably" knew the correct value of distance to Sun from Earth and also that the rays from the Sun can be practically parallel at the Earth as can be observed in Figure 1 below [5].

[6] Figure 1: Parallel Rays from Sun and the distance to the Sun.

Size of Earth:
[7] Eratosthenes used his knowledge about the shadows Sun's rays of light would produce at Egypt, on the Tropic of Cancer during summer solstice.  He measured the angle between this zenith line and the line of the rays of sunlight to be 1/50th of a circle using a gnomon, which measures angle using the shadows that fall on its sundial. He used this to find the angle the vertical line from Earth's surface at Alexandria made with a line of sunlight to Alexandria. He knew that in similar time interval, at Syene (now Aswan), the Sun's rays appeared directly on top of the head, or at zenith. He assumed the Earth being perfectly spherical (360 degrees), he calculated from his measurements of angles the arc distance between Syene and Alexandria to be 7 degrees and 2 arc minutes of the entire sphere angle and it is great that his measurements precise up to degrees. He then used the data available to him, the distance between Syene and Alexandria which was about 5000 stadia (ancient Greek unit for distance) and hence interpreted the entire circumference of the Earth to be 252000 stadia which is about 46620 km and this is precise to 16.3% to the current accepted value. Some argue that with the conversion he used for stadia and meters, his value is precise to 1.6% to the current accepted value. Nevertheless, such a precision given the tools available to him at that time is indeed something remarkable. He produced his precise results in a essay named, "On the Measurement of the Earth" combining theory with experiment to maximise the accuracy of the results of measurements at that time.

[8] Figure 1: Measuring the size of the Earth

Conclusion:
Eratosthenes contributed in various disciplines of study in his time which can be considered remarkable especially the precision of his measurements. In honour of him, a crater on the moon is named after him, we have the Eratosthenian period in the lunar geologic timescale highlighting his contributions to ancient Astronomy and Geography [9]. One surviving work of his is Catasterisms, text which describes constellations in the form of stories with a count of the number of stars contained in them. Eratosthenes' old age was attributed with blindness and he is said to have "committed suicide by voluntary starvation." [10]

References:
[1] [4] [7] [8] [9] http://en.wikipedia.org/wiki/Eratosthenes

[2] [3] Eratosthenes, http://www.ancient.eu/Eratosthenes/, Cristian Volatti, April 2013

[5] [6] [10] http://www.britannica.com/EBchecked/topic/191064/Eratosthenes-of-Cyrene