[antognini]

There is a really nice surviving water clock from the early Imperial era in China that was discovered in the 1970s [1]. The Imperial bureaucracy was sophisticated enough at this point that official timekeeping equipment like this was carefully tracked. The water clock has an inscription that details where it was made (Qianzhang), when it was made (27 BC), and how heavy it is 32 "Jin").

There is a text written a few decades after this water clock was made that provides enough detail to approximately reconstruct how astronomers used it for their measurements. In essence they would calibrate the water clock against the motion of the Sun so that they could correspond some volume of water to a 24 hour period. Then they would measure the amount of water that flowed from the transit of one star to another to figure out the separation of those stars in right ascension. The measurements were sophisticated enough that they apparently took into account factors like humidity and temperature when using the water clock.

[1]: https://www.reddit.com/r/ArtefactPorn/comments/150024l/the_z...