Ulkire, welcome. Since it's the second time you're trying to hide links to other unrelated websites in an attempt of multiplying external references, I think you should know it's against Forum rules and probably you'll be banned next time you do that.
That's not possible, you have to confuse me. Although I followed the development of Celestia for years, I've never been active in the forum.
Mirrors are just sites that host the thing you want to download. There are many mirrors as to give you options (not all work the same for different countries), so you just have to download SE from one mirror (any would work in general but if don't then try another one)