This is the first Esperanto sentence in Helen Fryer's "The Esperanto Teacher: A Simple Course for Non-Grammarians", which was, as far as I can tell, the first Esperanto textbook written for regular people in the English language.
I was marked wrong for trying "The sun is brilliant", thinking that it could be a verbed adjective. Was I wrong, or should I report this as "my answer should have been accepted"?
"Brili" comes from the Italian "brillare" and Spanish "brillar"- both of which mean to shine. Those words come from the Latin "beryllus" which referred to a certain kind of gem which shines.
The English "brilliant" comes from the same Latin word, but with a different meaning.