When viewed from Earth, the moon appears flat. Stereo vision requires each eye to see an object from slightly different angles, but the intervening 239,000 miles squash the moon’s deep craters and spherical shape into a uniform, luminous pancake. You could build a spaceship to get close enough for a respectable 3-D view—or just trick your brain using the 2.5 inches of space between your pupils as well as photos that reveal the moon’s subtle orbital wobble. (The motion, called libration, creates different lunar perspectives over time). Make this stereoscope for a stellar view of the moon.