This is way cool. Astronomy Picture of the Day for last Friday is a very clever stereo pair image of the Mercury transit. A stereo pair is two images which, when you arrange for each eye to see one of them, will produce a 3D effect. This one is set up to work by crossing your eyes. Old time stereo viewers required a parallel view setup, so you’d need to edit the image and flip the two pictures if you wanted to see it that way. Try it, and you’ll see Mercury hovering in front of the Sun! Realistically, too. Our eyes obviously are way too close together to get any useful parallax at that range, but the relative motion of Earth and Mercury *does* give us enough parallax, so the clever thing here is that astrophotographer Stefan Seip took pictures at just the right interval to give us the right parallax to see the real distance relationship between the Sun and Mercury.
If you can, cross your eyes until these two suns appear one, and then see Mercury pop out!