Equatorial astronomical instruments

Modern astronomical observatories derive from a Chinese, not a European tradition, which makes an understanding of sky position easier. They are oriented and mounted according to what is known as the equatorial system of astronomy. This is traditionally Chinese, and it goes back to at least 2400 BC. It takes the equatoras the horizontal circle around the side of the instrument, and the pole as the top point. This may seem simple and obvious, but it was not the system used by European ancestors. In European tradition, which is called 'ecliptic', the two horizontal circles which were of importance were not the equator but the horizon and the ecliptic (the circle described by the Sun's motion in the sky, which is the same plane as the Earth's orbit around the Sun). It came to be realized in seventeenth-century Europe that the Chinese system of equatorial astronomy was more convenient and showed greater promise.

The Chinese system was really very simple. Everything was conceived of as radiating from the celestial pole, as if it were the point where the stem of an orange were attached. The sky was then divided up into twenty-eight sections rather like orange segments, known as hsiu, or lunar mansions. Each one of these hsiu contained certain star constellations which were known as given names. Since the pole star and the stars near it never set beneath the horizon at any time during the year (whereas most stars do), the Chinese gave greatest attention to them, and by noticing where the starts at the top of a sky segment were, they could then precisely specify where the stars at the bottom of the same sky segment were, even though they might be invisible beneath the horizon.

To do this sort of thing with precision required instruments. The Chinese had such superior expertise in metal casting, having after all invented cast iron, that

they made large and impressive instruments of bronze and iron. These would take the form of huge metal rings precisely graduated with the degrees of the circle. Different rings representing different sky-circles would then be joined together at the two pionts where they crossed one another, forming what looked like the skeletons of spheres. These are called armillary spheres, from the Latin word armilla, meaning 'bracelet'. One ring would obviously represent the equator. Another would represent what is called the meridian, which is a great sky-circle that passes directly over one's head and also through the pole.

These instruments also had sighting-tubes, through which one could peer at particular stars. The sighting-tube was moved along the equator ring until a star was found.Then one counted the number of degrees marked on the ring back to the meridian ring, which stood up from it vertically. As soon as the degrees had been counted, the exact position of the star along the equator would become clear and one could tell what sky segment it was in. By such means as these, star maps were drawn with great precision, and positions of stars were recorded. The sky became not a maze of points of light, but a sensibly ordered arrangement of constellations. And to make sense of the night sky is, after all, what astronomy is.