The Transit of Venus was yesterday - that is, Venus could be seen moving across the sun (for those with the proper eye protection to look at the sun). This event will not occur again until 2117. Although it was cloudy, I was able to catch a small glimpse of it at one point. I spent several hours watching the live feed NASA had up, which also included a lot of mini-documentaries on the transit, and related topics, as well as the scientists at the observatory talking about it.
At some point the topic came up about viewing the transit from Mars - and then, that if you were on Mars you could also view the transit of Earth when it happened. Like Venus going across the sun's face here, the Earth would just be this little dot traveling across the sun to someone standing on Mars. It really puts into perspective just how large our solar system is... which puts into perspective just how large our galaxy is... and of course, just how large the universe is.