I can see a few problems right off the bat.
I don't know the exact php files you have, so you'll have to track these down on your own.
NOTE: close your img tags properly for your DOCTYPE. XHTML requires self-closing img tags. Also, XHTML requires img alt="some description of the image here"
1) You have no /HEAD or BODY tags. Not your fault I checked the theme as posted, and it didn't COME with any. You'll need to add those.
2) You'll see a sequence that looks like this:
<div id="top_strip_left">
<ul><li id="top_active_tab">
<div></ul>
</div>
<div id="top_strip_right">
This is just plain bad coding.
Try, instead:
<div id="top_strip_left">
<ul>
<li id="top_active_tab">
<div> </div>
</li>
</ul>
</div>
<div id="top_strip_right">
Or you can remove the empty "top_active_tab" list for formatting. Play with it a little.
3) In your sidebar, you have an UL with class="sidebar_boxes" with all of your nice google ads in it. That needs to be moved into a div.
So, DIV class="sidebar_boxes" then you UL and SCRIPTS and whatnot, then /UL and /DIV.
Without that DIV in place, you end up with ...
4) Your sidebar DIV class="sidebar_internals" is actually OUTSIDE of your sidebar DIV.
There are more, but I think I managed to fix the page. It now shows up fine in my Firefox 3, Linux machine.
Check my version of your page at: http://catesintheloop.com/earthascope.html. Then you can view the source for yourself. If you have any questions, I included my own email address in my copy of your site. Just look for the big EMAIL link.
Hope I can help.