What About Closeness to the Sun?
"Summer is when the Earth is close to the Sun, and winter is when the Earth is farther away."
The above statement is false, and one of the most common misconceptions about the causes of the seasons. Where does it come from?
The orbital path of the Earth around the Sun is an ellipse, rather than a perfect circle. At some points along its path, the Earth is closer to the Sun, and at other points farther away - that part is true. But often, diagrams that illustrate the ellipse will exaggerate it to demonstrate their points, making it much more elongated than it actually is.
From there, it's reasonable to assume that something close to a heat source will be warmer than something far from it, and since summer is warm and winter is cold, the seasons must be due to closeness to the Sun along the ellipse.
When we sit close to a heat source, we're warmer than when we sit farther away. But stop and think about that for a moment too. Where are you warm? The parts that directly face the heat source? What about the parts off to the side? Or do you have to rotate back and forth a bit to warm up your sides and keep your front from getting too toasty? The same thing happens for the Earth, as measured by the Sun's angle of incidence, which depends on the Earth's axial tilt (see page 1).
Another common misconception is that the solstice points happen when the Earth is at its closest to the Sun (perihelion) and at its farthest from the Sun (aphelion). This is also false. The pattern of Earth's axial tilt doesn't match up quite so neatly with the orbital ellipse.