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Mechanism.
The initial 'fixing' alignment is made by lookiing through center of 'axle' of "E" wheel - along the two parallel 'spokes' of "D" wheel - through the small "sighting ring" - to the Pole Star.
The whole massive structure is then bolted down / bedded in, on that bearing.
A 'sighting' of a star can now be made, steered to,
i) by turning "E" wheel to find the East-West (equatorial) direction,
ii) then swivelling the "observation tube" on the "D" wheel to find the "N-S" (declination) direction.
The `instrument' has quite a height - the massive base plate measures 18 feet by 12 feet - so even a tall adult could stand at left - below the "E" - wheel - to check alignment with the Pole Star (which would be somewhere to right - and way up high).
To sight on a star one would look up through the "observing tube" which swivels on axis of central "D" wheel.
The "Instrument" was made in 1270 AD, for the astronomer Kuo Shou-Ching, for use at Linfen, Shansi.
And, because of its construction obviously it will be accurate only at the latitude of Linfen, in Shansi.
In Nanjing, where the "Instrument" was taken many hundreds of years ago during the Ming dynasty, and where it is photographed, the whole edifice is useless.
[Story told by Robert Temple in "The Genius of China" Prion Books 1999 ISBN 1-85375-292-4]
Those rulers were not unique,
it seems all authority tends to be stupid.
Check this item from - Lost Science
Some thousand or so years earlier, when the Romans sacked Syracuse, they captured a sundial and took it back to Rome.
Well, like Kuo's "observing instrument", all sundials are only 'right' at one latitude - because the Sun's at a different angle when seen from a point north or south of that line.
But apparently, there was "incomprehension at the highest political level" when the looted prize failed to show the right time.
We're told Syracuse is hundreds of kilometers south of Rome, about 5 degrees
Story told by Colin Ronan in Astronomy before the Telescope British Museum Press 1999 ISBN 0-7141-2733-7 pbk edition
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