The progression of seasons is marked by four distinct points in Earth's orbit:

As Earth orbits, these positions swap. This constant shifting of light and energy is why, while one half of the world is shoveling snow, the other is headed to the beach.

At two points in the orbit—the vernal (spring) and autumnal equinoxes—neither hemisphere is tilted toward or away from the Sun. Both hemispheres receive equal amounts of sunlight, and day and night are approximately 12 hours each. These transitional seasons mark the shift between the extremes of summer and winter.