In this Bonus Episode, Jim and Janet take closer look on the themes that run through the story of the Bonds and the Mitchells and dive a little deeper into topics that were just touched upon in the regular season, such as the history of solar noon and why longitude was such a tough nut to crack.
This has been a production of the Nantucket Atheneum.
Written, edited and narrated by Janet Forest
Special thanks to the Atheneum’s Reference Library Associate Jim Borzilleri
If you want to know more about this topic, check our this very thorough list of resources that Jim has compiled: FURTHER READING
More about Eraosthenes
Two centuries after Aristotle, an Egyptian mathematician and astronomer, named Eraosthenes, (Aira-Tass-Ta-Knees), became head of the Library of Alexandra. This was a winning combination of skills: besides having access to the scientific works in the library, his training enabled him to identify key information as new material arrived. One day he received a letter from a colleague in Syene, a city directly to the south, and whose distance from Alexandria had been measured. The letter said that on a certain day of the year, at solar noon the sun not only on the east-west meridian, but also north-south meridian. It was directly overhead, and the shadows had no angle. (Mathematically the angle of the shadows was 0 degrees)
On the same date the following year, at solar noon, Eraosthenes measured the angle of the shadows cast at Alexandra and, using trigonometry and the information from the letter, calculated the earth’s circumstance. Even though some of his information was inaccurate, the calculation was only off by 15 per cent.
With the size of the earth established, if you measured the angle of the sun at your location at solar noon, and you also knew the latitude where the sun was on the north-south meridian that day, you could determine your latitude even at sea. Experienced sailors had long done this to estimate their position on familiar routes, but now it could be precisely calculated in unfamiliar waters.
Eraosthenes is also credited with inventing a coordinate system – forerunner of the today’s latitude and longitude – to create a map of the known world, again using the Library’s resources, that was said to show the location of over 400 cities.
Over the centuries his techniques and coordinate systems were refined by many others, slowly increasing their precision. Latitude coordinates were eventually fixed, with the equator set to zero degrees, reaching to ninety degrees at the North and South poles.
Mathematicians and astronomers computed table of the sun’s latitude for every day of the year. Similar tables were computed for other celestial bodies. After completing the necessary angular observations, mariners used these tables to determine their latitude (knowledge of geometry and trig was also very helpful).