Читать книгу 1434: The Year a Chinese Fleet Sailed to Italy and Ignited the Renaissance - Gavin Menzies, Gavin Menzies - Страница 17

In the thirteenth century, the astronomer Guo Shoujing 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. The farther north one goes, the more of the heavens one can see.

In 1964 I was navigator of HMS Narwhal, a submarine operating under the polar ice cap. Now and then we would find clear-water “lakes,” called polynyas, where we would surface in order to fix our position by the stars. The heavens appeared like a vast globe above us. As we approached the North Pole, we seemed to be inside a bowl looking at a hemisphere of stars spreading in an arc down to the horizon all around us.

Diagram showing the positions of ships A and B on a globe.

Ship A is at 20° N 20° W, Ship B is at 0° N 20° E.

Ships A and B discovering new lands at point C will have the same

position for the new land: 10° N 0° E.

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 one another.

The Chinese divided the sky into twenty-eight segments or mansions. Picture an orange with its skin sliced; the cuts start where the orange was fixed to its tree and continue vertically downward. They called each mansion a hsiu. They fixed the position of stars at the top of each of the twenty-eight mansions relative to the Pole Star (ABC).

Then they fixed stars in the lower part (DEF) of each segment relative to those in the upper part (ABC). Because stars never change their position relative to one another, even if the Chinese were not near the North Pole and hence could not see the stars in the lower part of each segment (because these stars were below the horizon), they always knew the stars’ positions. So they could produce star maps.

The Chinese fixed the position of stars at the top of each

of the 28 lunar mansions relative to the Pole Star.

They noted the vertical positions of each star below Polaris (none can be above Polaris) and the horizontal position of each hsiu relative to Nanjing (longitude). The Chinese called the vertical height of each star below Polaris “declination” and its position around the equator from Nanjing “right ascension.” So for the stars in the sky, the Chinese had the same system of measurement they used to determine latitude and longitude. This system was called the equatorial system—vastly simpler than the equinoctial system, used in medieval times before Guo Shoujing, which relied on the ecliptic or the horizon. After 1434, Europeans adopted the Chinese system, which remains in use today.

Next, the Chinese needed precise instruments to mea sure each star’s position. Guo Shoujing provided the tools. A sighting tube was first positioned by pointing it at Polaris at precisely the angle of the observer’s latitude—that is, if the observer was at the North Pole, the sighting tube would be at 90° elevation (see page 34). On this diagram, the instrument is aligned to Polaris at 39°49 N, the latitude of Beijing. Once positioned, the instrument was bolted down—because if the angle changed from the latitude of the observer, it became useless.

The observer then selected a star, looking at it through another tube attached to a circle marked in degrees. The movement of the tube along the circle gave the number of degrees below Polaris of the selected star (the arc y-z), which is the star’s declination.

The horizontal angle, the angle from Nanjing, was found by rotating the ring around the equatorial circle, which gave the horizontal angle of the star from Nanjing (its right ascension). The position of the star then was entered in the star tables. 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 recognize each star.

A torquetum based on the equatorial system, as used by Zheng He’s

navigators and pioneered by Guo Shoujing.