Ask most people why it is cold in the winter and warm in the summer and the response is usually that the Earth must be closer to the Sun in the summer and farther away in the winter. After all, this makes sense as this phenomenon is in line with what we experience in our everyday lives. As you get closer to a fire, for instance, you get warmer.
While this is a keen observation, it doesn't tell the whole story. It doesn't even scratch the surface really. The reason can be most readily identified by remembering one key fact about the Earth. When it is summer in the northern hemisphere, it is winter in the southern hemisphere, and visa versa.
How can that be? If the reason for the seasons was solely due to our proximity to the Sun, then it should be warm in both the northern and southern hemispheres at the same time. Something else must be the primary cause.
It's a Matter of Tilt
The biggest reason for the seasons is that the Earth's axis is tilted relative to its orbital plane, perhaps as a result of a large impact in our planet's history that may, in fact, be responsible for the creation of our Moon.
Why does that matter? Well, there is a simple experiment that is often used to illustrate this. Grab a flashlight and a piece of paper.
Holding the flashlight perpendicular to the plane of the paper, turn the flashlight on. You will notice that the light makes a circle on the surface of the page.
Now, begin tilting the paper while holding the flashlight still. You will notice that the light on the page no longer traces out a circle, but rather an ellipse.
What's more, the area of the ellipse is greater than the area of the circle that we started with. That means the same amount of light coming from our flashlight must be spread out over a greater part of the paper.
Since light from the Sun not only brings about illumination but deposits energy, the tilt of the Earth will determine how much energy (heat) a given area receives.
So when the northern hemisphere is tilted toward the Sun, summer is experienced on Earth. While during the same time the southern hemisphere gets less light over that area of the planet, so winter occurs.
It's Hotter at High-Noon Too
There is another part of the equation as well. The tilt of the Earth also means that the Sun will rise and fall in different parts of the sky during different times of year.
In the summer time the Sun will peak almost directly over head, and generally speaking will be above the horizon (i.e. there will be daylight) during more of the day.
This means that the Sun will have more time to heat the surface of the Earth in the summer, making it even warmer.
Back to Proximity
So does it matter how close the Earth is to the Sun? Well, yes.
The problem is that the Earth's orbit around the is only slightly elliptical. The difference between its closest point to the Sun and its most distant is little more than a 3 percent.
This translates to a difference of a few degrees Celsius on average. That actually is quite a bit. But the temperature difference between summer and winter is a lot more than that.
Interestingly, the northern hemisphere is closest to the Sun (perihelion) during winter. This means that the northern hemisphere gets slightly warmer winters (though it doesn't feel like it to me) than the southern hemisphere, while our friends to the South get slightly warmer summers.