An example of planetary rotation that caused astronomers quite a bit of confusion was the planet Mercury. Because Mercury is located so close to our Sun, it was difficult to observe the planet closely. In 1974, NASA sent the Mariner 10 space probe to investigate Mercury. The plan was to have Mariner 10 orbit our Sun in such a way that it passed by planet Mercury three times for every two orbits. Unfortunately for the astronomers, Mercury has the same rotation around our sun, three times every two orbits, resulting in Mariner 10 photographing the SAME SIDE of Mercury every time!
Another planet that is difficult to directly observe rotate is our own planet Earth. This is because we live on Earth and rotate with it. Historically, it was not until after Copernicus that the idea of the Earth's rotation was accepted. Several experiments have been conducted to demonstrate directly the rotation of Earth; one interesting example is the Foucault Pendulum Experiment (ref ). Recently, NASA scientists were finally able to get great movies of Earth rotating when the Galileo probe flew past Earth on its way to Jupiter in 1994.