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The Birth of Astronomy in the Far East - China

Preface
I decided to take a quick journey through history of astronomical knowledge of the 'ancient Asian people because it had a cultural growth independent of the developed West in ancient times. A hypothetical history of scientific thought would be incomplete omitting astronomical studies, tools and measurements of the ancient Chinese observers.
The Chinese civilization is much older than the Greek and Roman even if it was earlier civilizations that developed along the Nile and in Mesopotamia. It 'difficult to prove that Chinese astronomers made important measurements before the Egyptians and Babylonians; it is important to consider at least a comparison between China and Western civilization as regards the study of the movements of the stars on the celestial sphere, because to a certain point in time it has evolved in parallel and independently.
We realize that the Chinese astronomy, unlike the Greeks, for example, has developed from studies of a small number of solitary individuals but it was a discipline in the service of empire. The great importance of the imperial astronomers was directly proportional to the thoroughness of their comments, whose primary purpose was to draw up a timetable. The precision with which they were made is that they assume the best record of ancient astronomy, at least until the time of the Arabs.
Astronomy China has never developed a predictive theory mathematization of motion of the stars as it has been in the West, since the purpose of the observations had a purely practical.

astronomical survey instrument
The Chinese were the first astronomers to make observations on the sun using special rods embedded perpendicular to the ground, the gnomons . Subject
of measurements was the same length of the shadow cast by the gnomon on a horizontal plane when the star on the meridian that passes to the south. This direction was easily deduced from the fact that the length of the shadow of the gnomon falls gradually until the dawn of the day at a certain point and then resume rising.

The direction of the shadow at the lowest point is the direction of the south. Then, having noticed that this direction down the length of shadow at meridian passage varied within a certain range and had a length and a maximum. Those days are, respectively, the summer solstice and the winter.

The gnomons were provided with a horizontal scale, in terms of which were carried out measurements of the shadow. If the gnomon is short on the scale divisions should be extremely tight and precise, if it is long it is easier to read the subdivisions but the shadow is unclear and poorly defined. Astronomers avoid this fact by using a rod parallel to the land located at the top of the gnomon and a sheet of metal with a central pinhole in the horizontal plane. The definer shadow could be rotated and moved back and forth to make sure that the light could pass through the hole. This' which was projected on the horizontal plane was a small bright spot in the middle with the bar positioned sideways: the gnomonic hole had the function to focus like a lens, the image dell'asticella placed in the top of the gnomon. The measure
gnomonic shadow was also used to measure the latitude of a place to establish territorial boundaries. With this practice Chinese astronomers also were able to calculate the obliquity of the ecliptic. In fact, once you know the maximum height of the sun in the days of the winter solstice and summer solstice simple calculations lead to the determination of both quantities.

If the Earth in its motion of revolution around the sun was not inclined, as indeed it is, the height of the sun on the horizon to the meridian passage is always the same on any day of the year. In a reference system integral with the sun, we could always see the Earth's axis perpendicular to the plane of the orbit. This actually happens only during the equinoctial. Also in a solidarity with the sun we could imagine seeing the earth move along the plane of the meridian passage of time on the day of the winter solstice at the same time on the day of the summer solstice. The direction of the Earth would vary with respect to the sun as a kind pendulum with the highest of oscillations in the days of the solstices.

So to measure the obliquity of the ecliptic is enough to calculate half the angular difference between the height of the sun at meridian passage on the day of the winter solstice and summer. The book chou them, dating from around 1100 BC, gives the value 23.88
The passage on the meridian was the subject of the measurements for the night sky observation. Water clocks for determining the time interval between the rising or setting of the sun and the passage of a star on the meridian. The use of measuring time with the flow Water is really the ancient Chinese civilization. At the court of the Emperor, there was a ministry official, whose office was hereditary, which dealt exclusively with the construction and calibration.
Chinese astronomy was mainly based on observation of the circumpolar stars and had a polar-equatorial character. You immediately notice the difference with the West, in which the positions of the stars were reported to the ecliptic. The Chinese model was correct for the fact that the observation of stars placed near the north pole is easier to put the observations of stars along the ecliptic, because under a certain height in the sky they are no longer visible due to atmospheric turbulence.
The distribution of the sky was completely different from the West, and consisted of about 250 small constellations, the most famous of which, the Dragon has come down to our skies. This constellation, which has become a national symbol, once held the pole star, as can be seen by considering the motion of precession of the equinoxes and the resulting shift of the north celestial pole.

E 'can then calculate the exact position of the north celestial pole at any historical period. In the year 2500 BC is calculated by the following map Equatorial