Winter, spring, summer, and fall—Earth experiences them all! But what about Mars? Scholars compare the planets in terms of distance, tilt, and rotation during a lesson from PBS's Space series. Great visual models of Earth and Mars, plus...

This photo shows an astronaut researching in the zero gravity environment of space.

An astronaut experiments in gravity free space with an eye tracking device. The complicated equipment rests on the astronauts head as he floats around the shuttle.