{Inspired by Bad Horse's far superior work Moments, which you should read first, because it's better.}
Luna knows what is about it happen. Celestia and Twilight have failed to find an alternative, and she is not willing to risk the world she has grown to love on the off chance that another solution can be found in time.
Nooooooo Luuuuuuunnnnnnaaaaaaaaa
The second I realised what Luna was doing... Excuse me I need to go cry some more
They could still make it move slower and it would miss.