This question asks what would happen to the solar system if the sun vanished for about three days, the mass and energy of the sun simply ceasing to exist. This answer (as of this question, anyway) notes that for the most part, the solar system will remain unchanged.
If Ragnarök has already happened, and the sun has already been eaten and replaced, is it possible to use the current eccentricity, positions, and speeds of the various planets in our solar system to determine how long ago that occurred?
As for an explanation of why this is in World Building rather than Physics: The events in Ragnarök result in sentient life on Earth being effectively wiped out; after the cataclysmic event, only two people remain, and civilization restarts through them and their children. There would be no written history of the event, and oral tradition is unreliable at best. There may not even be any archaeological evidence. However, the physics would remain, and "proving" that the sun vanished for three days is a pretty good step towards building a magical, unknown history into a normal, mundane world - through science, no less!