Chapter summary
Latitude and longitude
Calendars enabled longitude to be calculated by using the slip between solar and sidereal time, by eclipses of the moon, or by the angular distance between the moon and selected stars or planets. Thus Zheng He was able to provide Europeans with maps, navigational tools and an astronomical calendar beyond anything they had yet been able to produce on their own. Supplied with this revolutionary knowledge, the barbarians would be able to make their way to the Middle Kingdom, appropriately “with deference.”
There are no signposts in the open ocean. The only way a navigator can determine his position is by using the stars, planets, sun and moon. As a first step, a navigator must have a system of providing markers across the oceans. This system of markers, adopted by all seafaring civilisations for millennia, is called Latitude and Longitude. It involves drawing imaginary horizontal and vertical lines over the globe. Latitude lines are parallel with the equator; each longitude line passes through both the North and South Poles. So a navigator’s precise position can be fixed on the globe using a common system.
In order to have produced an accurate map of the world by 1418, the Chinese fleets must have had such a system to determine their positions at sea. Without an accurate system, captains could not have known the true locations of newly discovered lands, and any map derived from their disparate calculations would have been an incoherent mess. Unlike the Europeans, who followed Babylonian astronomers with 360 degrees of longitude, the Chinese employed 365 ¼ degrees. The Chinese used latitude degrees below Polaris (at 90 degrees elevation) the Europeans used latitude above the equator (Polaris 00 elevation). The results are the same for both systems. After establishing a common system for the earth, the Chinese next had to establish a common map of the heavens; each navigator would have had to use the same name for the same star in the sky as well as the same star map from which longitude would have been determined.
In the 13th century, the astronomer Guo Shou Jing fixed the positions of key stars relative to Polaris (the Pole Star). Polaris appears on an extension of the earth’s axis, billions of miles away above the North Pole. Because of the earth’s rotation, the heavens appear to rotate around Polaris. At the North Pole, the Chinese could fix the position of every star in the northern hemisphere relative to Polaris. The stars are so far away that to an observer on earth they never change their positions relative to each other. This way they always knew the stars’ positions so they could produce star maps.
The Chinese entered 1,461 stars in their tables, a process that required many astronomers and hundreds of years. Tables were printed and, along with a star map, given to each navigator. Thus all navigators possessed a common system of latitude and longitude to fix their positions on the globe, and an identical map of the heavens, which enabled them to recognise each star.
Recently discovered star maps of Zheng He’s voyages show which stars Zheng He’s fleet actually used to determine latitude and longitude on their passage to India. It is my submission that Zheng He’s navigators were able to calculate latitude within half a degree, or 30 miles, and longitude to within two seconds, or 3 degrees. When the fleets arrived in Venice and Florence, their methods of calculating latitude and longitude were transferred to Europeans. In due course, Columbus and Vespucci used them to reach the New World
Further reading
To read Robert Cribbs presentation on Using the Slip between Solar and Sidereal Time
to Measure Longitude as detailed at the Library of Congress, May 2005, please visit www.marcopolovoyages.com and go to Library of Congress presentation. We will post this article up soon.
Tai Peng Wang – click here
Robert Cribbs’ method – click here
Chinese knowledge of the spherical earth – click here
Determining longitude by the equation of time of the moon – click here
From the vaults of the heavens:
http://www.emis.de/journals/NNJ/Jaff.html
The published writings of Nathan Sivin:
http://www.uni-tuebingen.de/uni/ans/eastm/back/cs13/cs13-7-sivinbib.pdf
Ancient navigators could have measured longitude, Sanders
http://www.21stcenturysciencetech.com/articles/fall01/navigators/navigators.html
Gutenberg and the Koreans: Did East Asian Printing traditions influence the European Renaissance?
http://www.rightreading.com/printing/gutenberg.asia/gutenberg-asia-1-introduction.htm
A study of the Shou Shi Li Cheng, by Lee, Eun-Hee and Jing Bing:
http://max.lis.unt.edu/~bjing/documents/LIJING_ENG.pdf
Ancient Chinese astronomer Gan De discovered Jupiter’s satellites 2000 years earlier than Galileo, by Rosa Mui and Paul Dong- click here
Instruments and observation at the imperial astronomical bureau during the Ming dynasty: Thatcher E. Deane
http://www.jstor.org/pss/302002
Calendars, Interpolation, Gnomons and Armillary Spheres in the Work of Guo
Shoujing (1231-1314) by Ng Say Tiong and Helmer Aslaksen
http://www.math.nus.edu.sg/aslaksen/projects/nst-abstract.pdf
