Happy Monday, and a riddle!

Just thought I'd say a quick hello and happy Monday!

This morning was the highlight of my day, and here's why: I was listening to the radio this morning and they were having a riddle contest. I happened to have my phone on me and wrote a very quick riddle to send to them, and guess what? They read it a few minutes later, and I stumped them! Ah the power of technology.

Here's the riddle: I am in the middle of a river, the center of gravity, I hold a capital position in Vienna, and I am invisible. What am I?

Comment if you know the answer!

And here's a video I found you might like: