From Wikimedia, via EarthSky.org
I was collapsing a cereal box last Wednesday for recycling when I noticed an explanation of the seasons on the back. It had been quite a while since I'd noticed "educational packaging" of this sort, and here it was on a box of Shaws store brand (Essential Everyday) cold cereal.
Does anyone pay attention to these things? In this case I hope not. Alas, Shaws disappointed: they got the importance of the axial tilt, but explained that the northern hemisphere warms in summer because it is "closer to the sun" than the southern hemisphere. Out of a total earth-sun distance of 93,000,000 miles, that's only a difference of 0.0086% from one side of the earth to the other. And that's the maximum possible distance; the actual difference between distances to the poles would be less than a tenth of that.
Here's a better explanation.
The rest of the article--which isn't really about the seasons--interests me more. It first explains the slow wobble in earth's axis, and its consequences. Then it mentions seasons elsewhere in the solar system. Finally it discusses the common misconception that our changing distance from the sun causes seasons.
Three comments on these.
First, Uranus has extreme seasons because its axis is tipped nearly 90 degrees, so that its poles alternately face the sun directly--well and good; but what does it mean that Venus (with no seasons) has a tilt of 177 degrees? Although Venus' axis is nearly upright, we know it was tipped completely over early in its history because it rotates in a direction opposite that of Earth and other planets. That is, Earth, Mars, etc., both rotate and revolve in the same direction (counterclockwise viewed from above the north pole), while Venus rotates clockwise!
Second, although Mars has a similar tilt to earth, unlike Earth its seasons really are caused by changing distance from the sun! The reason is Mars' orbit is less circular than ours, a longer ellipse, so that the greater difference in distance overwhelms any seasonality related to axial tilt.
Finally, two facts help refute the idea that earth's seasons are related to its distance from the sun: (1) the seasons are opposite in the northern and southern hemispheres, and (2) the earth is actually closest to the sun in early January. The difference in energy received is simply overwhelmed by the difference in angle and daylength.
It does make me wonder, though, whether southern hemisphere seasons are more extreme--with the effects added together--than those in similar latitudes and situations in the north.