Ah… found it. It’s your sliding plugin (which I believe is the jCarousel) you have at the bottom of your page because it has an inline style of width: 1180px…it’s not really responsive. To maintain full responsiveness of a theme, any third party plugins and scripts should also be responsive as well.
Side note: you may also want to consider updating Celestial Reloaded which is now version 2.4.0 and fixes and adds a lot of changes. Before doing so, I would recommend reading the new version’s changelog.txt file and make a copy of your existing theme before updating it (if you do).