It is also used in Mark Twains book "A Connecticut Yankee in King Arthur's Court" from 1889 when the main character is about to be burned at the stake. So that might be even more of an inspiration for Hergé. The chapter is available at:
With some nice illustrations.
Surely though, shouldn't cultures connected to the sun have knowledge about ecplises in their lore. I mean if Rascar Capac sort of can use the lightning as a tool then the incas should also be aware that the moon sometimes move in front of the sun. Maybe it is because the moon doesn't shine at an eclipse which it normally does that sun followers doesn't recognise it as the moon. Makes the incas seem a bit simpleton in my point of view unfortunately.
Looking through the scene in the book made me noticed the very straight and even cut logs that make up the stake but there is not any trees in sight. The trees in the jungle seem to quite crooked and these are straight as an arrow, where did the incas get them?
Later when Tintin and Haddock are taken down to the treasure room the Thompsons suddenly appear in Antarctica. Is that because they misread the pendulum as that they were going down (south) or what?