Expert: Philip Stahl - 12/8/2011

Question
How does the tilt of earth's axis affect the seasons?

Answer
Hello,

Interestingly, I was certain I'd answered this question already several years ago, but on going through the answer archives there's no sign of it! Anyway, I have appended a diagram which pretty easily shows how the tilt of the Earth's axis afects the seasons.

At point A in the diagram, we have the commencement of summer in the northern hemisphere (summer solstice) because the Earth's axis is tilted toward the Sun hence the N. hemisphere receives more direct sunlight, radiation. Also at point A, we have the longest day. Summer lasts up to point B which marks the autumnal equinox, or equal lengths of day and night. From point A onwards the days continually grow shorter since the Sun's altitude in the sky is becoming smaller and its path in the sky (diurnal circle) less. This extends to point C, or the winter solstice, which marks both the shortest day (again for the N. hemisphere) and the beginning of winter. Note the tilt of the Earth's axis is *away* from the Sun here, indicating less direct solar heat, radiation - while the S. hemisphere now receives more - hence it is now summer in that hemisphere.

The winter season extends from point C to point D, with the days now increasing all the while, until the vernal equinox is reached around or about March 21, the first day of Spring. Spring then extends from March 21 to the summer solstice, with the length of days increasing further throughout as the Sun now gains greater altitude in the sky, and thus longer diurnal circles as it approaches the summer solstice (A).

In summary then, the tilt of the Earth modulates the extent or the amount of sunlight-radiation (including heat) a hemisphere receives at a particular time, or over a given interval. Onset of winter and summer are defined respectively by the extreme axial tilts of "fully away" from the Sun, and "fully toward", while Fall, Spring are defined by neutral tilts - ie neither away nor directly toward.

Hope this helps!