Visit Georgia's Great Lake 45 Minutes North of Atlanta


N 34° 14.750 W 083° 56.253

UTM: 17S E 229463 N 3793318

Puzzle cache using ancient Chinese technique to determine longitude.

The cache is not located at the posted coordinates, which indicate a spot in Lake Lanier. The true cache coordinated must be computed using a technique for determining longitude developed by Chinese navigators and astronomers in the early 15th century.

By the year 1400 the Chinese had nearly 800 years of experience sailing their trading fleets in Asian waters and into the Indian Ocean. They had the magnetic compass, accurate water clocks, and they could determine latitude by observing Polaris. They obviously knew that the earth was spherical and could compute its diameter with some accuracy. They had accurate star maps, and could predict future solar and lunar eclipses.

However, determining the longitude of any point was not possible. To sail to a distant port, they had lists of “sailing instructions”, which gave approximate compass bearings or celestial information from point to point. Some attempts were made to determine longitude by dead reckoning, as they could measure a ship’s speed through the water with logs. However, this method was inaccurate because the Chinese were unaware of ocean currents, which moved the ships with undetectable speed and direction.

In the 1420s, Chinese astronomers devised a method for determining the longitude of a given point. Lunar eclipses occur at least twice a year, and the dates could be calculated far in advance. During an eclipse, it is visible anywhere on the globe where it is night; that is, over most of a hemisphere. A ship would be sent to the point to be measured so as to arrive well before the eclipse. If luck were with them, the eclipse would be visible at the distant point as well as at Beijing. During the eclipse, both the ships crew and the astronomers back in China would determine which star was directly overhead (i.e., at its local noon or meridian passage). The time of this measurement is essentially simultaneous in both places.

The ship then returned to China with its information. The Chinese astronomers would wait until “their” star is directly overhead (assuming the point to be measured is west of Beijing), and then use their accurate water clocks to measure the interval until the “ship’s” star was overhead. The measured interval directly allowed them to determine the number of degrees east to the point of measurement.

In reality the Chinese sent many ships at the same time, and from the information would be able to draw reasonably accurate maps of the coastlines where the points were measured.

To determine the location of the cache, you must first determine the identity of a small town somewhere in the US from the following data, in a manner similar to what the Chinese would have done.

Knowing that a lunar eclipse was imminent, I anchored my boat in Lake Lanier at the posted coordinates. When the eclipse occured, I determined that the bright star Arcturus was directly overhead. My Chinese collaborator, Long Wei, also observed the eclipse and reported that at his location the star Regulus was overhead. He also reported that a sextant reading of Polaris showed that it was 60 degrees, 20.1 minutes above the horizon.

Using this information, determine the town where Long Wei was located, and then determine its 5-digit post office zipcode, ABCDE. The cache container will be found at N34 0A.00B W84 XX.YYY, where XX = CD-41 and YYY = ABC-108.

Container is a .30 ammo box hidden in the woods in a small Fulton County park.

Official Geocache Code: GC13XMY

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