Here’s something unusual: an astrolabe plate engraved with constellation images instead of the usual lines and labels.
(Image here: Constellation plate)
It is part of this lovely astrolabe in the Greenwich collection – and I’ve never seen another plate like it. Indeed, its existence is a bit puzzling considering the rete is already a star map. Unless, perhaps, the astrolabist was trying to add celestial globe functionality?
Here’s the thing: each of the labelled pointers on the rete (rendered as leaves and blossoms on this and many other Islamic astrolabes) indicates a star. But in order to use the rete, you need to already know how to recognise the stars it represents in the night sky. You can’t just look back and forth between the rete and night sky thinking that the arrangement of stars on the former will match up with the latter. This is partly because retes tend to show only 15-30 stars, but also because of the stereographic projection that makes it possible to map the sky onto an astrolabe. While this preserves certain mathematical relationships between the stars so that that astronomical calculations can still be done, it severely distorts the visual layout of the stars. (To give you an idea, compare these stereographically projected photos to how we actually see the world.)
That’s where celestial globes and star manuals come in: they actually show the stars laid out as they appear, with memory-aiding illustrations. Star manuals let you learn individual constellations in detail whereas celestial globes show you how the constellations relate to each other in the sky. But celestial globes are bulky – harder to carry around. So I can see why someone might make a plate giving their astrolabe the same functions.
All this said: I don’t know if that’s actually what’s going on here. I have just discovered this object and I have to do more work on it (among other things, I need to find a larger picture so I can try to read the inscriptions). But it is certainly a fascinating – and beautiful – find!
EDIT: I’ve learned considerably more about such plates since; there are two other astrolabes that have them, by the same astrolabist. All this will be the subject of another post.