Navigation When a traveler is in unfamiliar territory, such as a featureless desert or ocean, he or

6.2.2 Navigation When a traveler is in unfamiliar territory, such as a featureless desert or ocean, he or

she needs to find a method for identifying the right direction to reach the

211 destination. Celestial navigation depends on observing the positions of the sun and

6.2 WATER TRANSPORTATION

the stars. The sun is always in the south at noon. The sun rises in the east and sets in the west, but this is strictly accurate only during the equinoxes (March 21 and September 21). During the summer solstice (June 21), the sun actually rises in the northeast, and during winter solstice (December 21), it rises in the southeast. The further north the traveler is in latitude, the greater is the seasonal angular correction. It is simpler to rely on the Polaris star of the night sky, which is at the tip of the handle of the Little Dipper, and always at the north. Since Polaris is not visible in the southern hemisphere, the nearest substitute is the Southern Cross, which is inconveniently located at 60 S. However, should the traveler need directions at an inconvenient time of the day, in any season, or in cloudy weather, then these sky watch methods would fail.

In 585 BCE, Thales of Miletus discovered the property of lodestone of attracting iron and other lodestones, and named this phenomenon after Magnesia where the lodestones were found, but did not mention its application in naviga- tion. The compass was first mentioned around 250 BCE in China, and is one of the greatest inventions to benefit travelers. According to legends, around 2600 BCE the Yellow Emperor had a battle in a thick fog, during which his troops were getting lost. He introduced the South Pointing Chariot, which always pointed to the south, and he won the battle. Historical records have been found from the sixth century, on the use of the South Pointing Chariot for travelers over boundless plains, with a figure on the roof that always pointed toward the south, no matter which way the chariot turned. This chariot made its way to Japan by the year 720.

The lodestone is a piece of iron oxide ore that has been magnetized, and has a tendency to rotate to the north–south direction and align with the earth’s natural magnetic field. The rotation force is feeble, so the needle in the compass must be free to rotate without friction forces. In ancient China, the solution was to carve the lodestone into the shape of a spoon or ladle, and place it on a smooth polished surface to let it rotate freely (see Fig. 6.5). Then the handle of the spoon would point south, which gave it the name of zhi-nan-zhen or the south-pointed needle. The ladle shape was chosen to resemble the Bear or the Dipper. In the year 1086, Shen Gua described in his Dream Pool Essays that, “Alchemists rubbed the needle with a lodestone, making it capable of pointing to the south.” He also pointed out that often the needle points slightly east of true south, which today is called magnetic declination. This correction was noticed by Christopher Columbus during his voyage. This correction is small near the equator, and can be enormous near the North or South Magnetic Poles. Shen Gua pointed out that this magnetic needle could be attached to a piece of wood, and left to float in a bowl of water; it could also be placed on a fingernail or on the rim of a bowl; but his favorite design involved suspending the needle from a thread in a place without wind.

The compass may have passed from China to the Middle East via the Silk Road, and then introduced to Europe. There is another theory that Europeans independently invented the compass, but its ability to point north–south was not discovered in Europe till Alexander Neckam in 1180, who wrote about mag- netic directions that led to its use in navigation. The magnetic compass played a