As you have seen, the Earth is tilted and direct light causes more heat than indirect light. Remembering your walk around the chair, and how the tilt is at each point, can you work it out?

This is how it works. When the Northern hemisphere is pointing at the sun, sunlight falls most directly on it. This is summer in the Northern Hemisphere. As you get around to the side of the chair, neither hemisphere is pointing toward the Sun. The light strikes both equally and directly. However, as the orbit is elliptical, the Earth is further from the Sun at this point so although the light is still direct, it is cooler, because of the distance.

 

As the Earth moves around to the other side of the Sun the Northern Hemisphere is tilted away from the Sun. Now the light falls indirectly on it. It is winter. As it moves around to the fourth side, it the light again falls directly on it, but it is further away, so it is warmer than winter but cooler than summer.

Notice that the exact opposite is happening in the Southern hemisphere. When the Northern Hemisphere has summer, the Southern Hemisphere has winter, and the other way round. So, how do you decide which has spring and which has fall? That is simple. Spring follows winter and fall follows summer!

Previous:

Introduction

Earth’s Tilt

The effects of Direct and indirect Sunlight

Earth’s Orbit