XJTA

In Celebrant the Fused speaking to the registrar seems to refer to Pattern as the wrong kind of spren.

Quote

There are two or three humans, one honorspren, and one inkspren.

Sanderson, Brandon. Oathbringer: Book Three of the Stormlight Archive (p. 958). Tom Doherty Associates. Kindle Edition. 

 

Since they can look at physical objects from Shadesmar I figured they determined the details of that system by observing the work of astronomers on First.