Here are a couple things I’ve learned about while doing worldbuilding research for Stars Fall Out:
The Roman month of Mercedonious. Learning more about the history of intercalation (inserting extra days into the calendar, such as in a leap year), provided the inspiration for me to fix a story problem. I have a section of Stars that, timing-wise, felt weird in relation to the rest of the story. I decided to run with the weirdness; I made them intercalated days called for on the authority of one of the Grand Oneiromancers. One of those “can you believe this law is still on the books?” kind of things.
I also learned how to convert between different number base systems. I’ve always thought it would be cool if we had a base 12 system rather than base 10, so I gave one of the cultures in my story a base 12 system, and another base 9. Now I’m trying to make it as hard as possible for them to change currency, which is adding some nice color to a scene in the bakery when a guy comes in with imperial money.
Also, history of the number zero. Unrelated, for my purposes.