Seasonal Science: The Reasons for the Seasons

 

An astronomical activity from Science Buddies

Feb 20, 2014 |By Science Buddies

George Retseck

Introduction
Have you ever lived somewhere where you get to experience the full glory of all four seasons? If so, you know well the full blossoms and dramatic skies of spring; the long, sun-drenched days of summer; the trees shaking in crimson and gold in fall; and the sparkling snows of winter. But do you know why we have these seasons over and over again in a cycle as predictable as sunrise and sunset? It actually has to do with Earth's tilt. In this science activity you'll investigate how this tilt affects how the sun's rays strike our planet and create seasons.

Background
In Earth’s Northern Hemisphere summers are hot and filled with many hours of strong sunlight whereas winters are cold due to shortened daylight hours and weak sunlight. Why is this? One big part of the answer is that Earth is tilted on its axis. To visualize this axis, picture an imaginary stick piercing Earth from its North to South poles. Earth spins once around this axis every 24 hours. While spinning like this our planet also circles the sun in a big orbit, completing this loop in about 365 days.

This axis isn't straight up and down as Earth orbits around the sun, however. Instead, it is tilted at approximately 23 degrees and also remains fixed, always aligned in the same direction in space. This tilt changes how the sunlight hits Earth at a given location in its yearly orbit. When it is summer in the Northern Hemisphere, the top part of the axis (the North Pole) points more toward the sun, and the sun's rays shine more directly on the Northern Hemisphere (where the continents of North America, Europe and Asia as well as the northern parts of Africa and South America are located); at the same time in the Southern Hemisphere (Antarctica, Australia, most of South America and the southern third of Africa), where it's winter, the South Pole end of the axis is tipped away from the sun, and its rays hit that half of Earth on a slant.