Device Held by Ptolomy -- Layout Echo?

[Linda]

I think Aga and JKP are right, it is an astrolabe. 



I don't understand "too big", seems like a common size, and "too detailed", seems to me like it is stylized because there is no detail. If you fill that in, in your mind's eye, with further breakdowns of measurements, i don't see why it is not realistic. 

What i see is a circle with a floral design in the middle, a double pointer on top of that which would point to things in the outer circular area, which is broken down into squares here, obviously these would contain markings to know what the pointer was pointing at. I see it as absolutely analogous with this volvelle albeit the floral design is now the moon.





The middle example includes 2, 3 to give the idea that numbers would be found in the boxes.
In the last example it is a cross instead of a flower, and the dimensions of the outer band and the pointers are changed but it is the same idea.

As to whether it mimics the zodiac section layout, it does insofar as being circles within circles, but i don't see the zodiac pages as measuring things per se.