Chapter 14 Section 4 Celestial Navigation

Sailing on the vast sea, without any characteristic scenery around, how to judge the position of the hull?Before the invention of modern navigation technology, we could only rely on observing the stars. "Huainanzi·Qi Su Xun" said: "Those who are confused by boats don't know what things are, but they will be fooled when they see the Big Dipper." It means that when you can't distinguish the direction when sailing in the sea, you can use the Big Dipper to distinguish it.This is the earliest astronomical record of navigation in China, and it shows that as early as the early Han Dynasty, the technology of celestial navigation had been widely used.

Zhu Yu (yuyu) of the Song Dynasty wrote in "Pingzhou Ketan": "The boat master knows geography, he can observe the stars at night, the sun during the day, and the compass when it is dark." Judging the direction of the boat, but it is impossible to know the geographic latitude and longitude. In the Ming Dynasty, Zheng He sailed to the west, passed through the South China Sea and the Strait of Malacca, and reached the west coast of the Indian Ocean. "Zheng He's Voyage Chart" describes the whole process of Zheng He's voyage in the form of a map.We know from the picture that different positioning methods are used in three stages. The first stage is from Suzhou to the northern tip of Sumatra, Indonesia. Since the fleet is sailing on the right side of the coast, it can be positioned only with a compass.In the second stage, from Sumatra to the west to Ceylon (now Sri Lanka), the fleet has been going west, the latitude has not changed much, and the distance is not too far, so the compass is mainly used, and astrology observation is supplemented.The third stage is from Ceylon to the east coast of Africa, across the Indian Ocean. If the ship deviates slightly, it will be far away from the destination. At this time, it can only be positioned completely by astrology. "Zheng He's Nautical Map" also has a set of drawings called "Crossing the Ocean and Pulling the Stars Map".The map shows in detail the azimuths and altitudes of the stars seen when the fleet crossed the Indian Ocean.

Before the compass was used for navigation, the direction was judged by the appearance and disappearance of the sun, moon, and stars.As for the altitude of the horizon, since the hull fluctuates with the waves, the horizon theodolite, which must be placed horizontally, cannot be used, so the crew generally use their palms or fingers to measure.Of course, this method is very crude.