(Mild spoilers for Project Hail Mary) if the Eridians from PHM weren’t aware of the existence of radiation or light waves, then how did they map the stars?

I’m not sure if I missed an explanation at some point of the book or movie but u never really understood how the Eridians actually made it to Tau Ceti, let alone figure out that other stars exist in general (or their own for that matter). I’m aware that rocky creates that little device that lets him translate light waves into a physical surface for him to observe, but to my knowledge he only was able to create it after Grace taught him about light waves.

Again I’m not sure if I missed something so if anyone could help me out with this I’ll be able to sleep soundly again