This actually is a picture of an astrolabe, but it is probably not dissimilar to a stellar compass, which works like a regular compass (North, South, East, West and so forth, indicated by one brass band) except that it also includes Up and Down (indicated by the other brass band,) and it orients towards the crystal shell, calling the centre of the sphere "North". If the indicators of the instrument spin in circles, you know you're at the centre of the sphere.
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