Pbs Learning Media: Crash Course Astronomy: Cycles in the Sky

Take a look at the cyclical phenomena at work in the universe. Learn about the subtle changes in the sky that take place over time as the Earth circles the sun, following along the ecliptic. Learn how the tilt of the Earth's axis has a profound effect on the planet by creating the seasons. The Earth's axis is slowly moving as well, in a process called precession. [9:28]