The Second Sun
From Shousetsu Bang*Bang Wiki
The Second Sun is a story written by Kikuna Matata and illustrated by beili. It ran in Special Issue 9 at http://s2b2.livejournal.com/271056.html, and is mirrored at http://www.shousetsubangbang.com/mirror/the-second-sun/.
Summary
From fail-fandomanon.dreamwidth.org: "Set in the C19th - Isabel, disguised as a young man, travels Europe studying astronomy and trying to avoid discovery."