Sidebar and Floating

I assume that this is a question about CSS and floating, but I’m not exactly sure what the problem is.

https://www.brianjleung.com/journal

If you view it in Firefox, it comes out fine. But when you use IE, the sidebar does not get displayed at the top next to the post, but instead is below it. I assume there’s something wrong with a float tag somewhere along the way, but I can’t figure out where. Can someone help please? Thanks.