 To understand the changing of the seasons, we have to understand the role of the changing sun angle and the tilt of the Earth on its axis. To look at that, we're going to start with a virtual flashlight. We'll begin with a situation where our virtual flashlight is pointed directly at an object, so the light comes in perpendicularly and strikes the object. You can see that the light is fairly bright and it's spread over a small area, so the amount of light per unit area is maximized and the light is intense as it strikes the object. But if we change the angle of our flashlight and make the light come in at a lower angle and strike the surface, the light now gets spread out over a larger area. So the amount of light per unit area is reduced and the light is dimmer on the surface. It's not as intense. So even though the same amount of light is coming out of the flashlight, it's dimmer on the surface because of the lower angle of the light that is striking the surface. You could try that experiment at home with a real flashlight if you wanted, but in a nutshell, you've seen the reason for the change of seasons. Earth is tilted on its axis at a 23.5 degree angle and as it revolves around the sun, the location that gets the direct beam of light changes throughout the year. The direct beam of light from the sun is always between 23.5 degrees north latitude and 23.5 degrees south latitude. For example, the equator gets the direct beam of light twice a year on the spring equinox and on the autumnal equinox. On the other days of the year, the direct beam is over other various locations between 23.5 degrees north and 23.5 degrees south. To better see how the changing sun angle impacts the seasons, we have an animation here of the earth revolving around the sun and the earth is tilted on its axis at the 23.5 degree angle. The seasonal designations here are for the northern hemisphere. If we click on summer, we can see the situation in the northern hemisphere and why it's warmer in the northern hemispheric summer. When it's summer in the northern hemisphere, the northern hemisphere is tilted toward the sun so the sun's light is striking the surface more directly and the heating power of the sun is greater in the northern hemisphere during summer. Meanwhile, in the southern hemisphere, it's tilted away from the sun and the sunlight is coming in at a lower angle and spreading out over a larger area so the heating power of the sun is reduced in the southern hemisphere. That's why the southern hemisphere is having its winter while the northern hemisphere has its summer. The situation during northern hemispheric winter is just the opposite. During the northern hemisphere winter, the sun angle is low in the northern hemisphere and the light is spreading out over a larger area so the heating power of the sun is reduced because the northern hemisphere is tilted away from the sun and the light is coming in at a low angle. Meanwhile, the southern hemisphere is pointed more directly at the sun so the sun is coming in more directly and the heating power of the sun is stronger in the southern hemisphere. That's why when it's winter in the northern hemisphere, it's summer and warmer in the southern hemisphere. Finally, in spring and fall, the tilt of the earth's axis isn't directed toward or away from the sun so the sun's rays strike similar latitudes at the same angle in both hemispheres, which means the radiation per unit area is the same and temperatures end up being similar. Ultimately, this setup in spring and fall gives us an idea of what would happen if the earth was not tilted on its axis at all. We would not experience any seasonal variation as the earth revolves around the sun.