Harry Uses the Sun as radar
So I'm looking for a fic that's a crossover between Harry Potter and Stargate SG1. I don't remember most of it, but I do remember that they were trying to get advanced warning when the bad guys showed up in the solar system and someone said since magic interaction with light was weird to just use a spell to read the light of the sun.
Harry casts the spell because he's the only one with the power. And rather than just the Sol system, the spell infected everything the light touches at the same time, including the other stars and they wind up with a real time map of the galaxy.