“The dance between darkness and light will always remain — the stars and the moon will always need the darkness to be seen, the darkness will just not be worth having without the moon and the stars.” -C. JoyBell C.
What do you do when you discover a moon around Saturn that's only visible during half of its orbit? If you're Giovanni Cassini, you postulate that half of the moon is darker than the other, and you work tirelessly to improve the telescope so that you can see it when it's in "faint mode." After more than 3 decades, he succeeded.
We now know that Iapetus is not only two-toned, with one side more than 10 times less reflective than the other, but it orbits out of the plane of all the other moons, has a giant equatorial ridge and is unlike any other world in the Solar System. The last of these are mysteries still, but the two-toned conundrum, thanks to Cassini (the NASA mission), has been solved!