Where most people live on Earth, summers are hot and filled with many hours of strong sunlight, while winters are cold due to shortened hours of daylight and weak sunlight. You might think that the extreme heat of summer and the icy cold of winter have something to do with how close Earth is to the Sun, but actually, Earth's orbit is almost circular around the Sun, so there is very little difference in the distance from Earth to the Sun throughout the year. So, what are the reasons for the seasons, if it's not the distance from the Sun? One big part of the answer is that Earth is tilted on an axis.

What is an axis? Picture an imaginary stick going through the north and south poles of Earth. Earth rotates about this axis every 24 hours. However, this axis isn't straight up and down as Earth goes through its orbit about the Sun. Instead, it is tilted approximately 23 degrees. The degree of tilt varies by about 1.5 degrees every 41,000 years, which you can read more about in the Bibliography, below. We can thank our relatively big Moon for keeping this degree of tilt so stable. Without the influence of our Moon's gravity, the tilt would vary dramatically, like that of a wobbling top, resulting in rapidly changing seasons that would make it difficult for life to exist on Earth. Planetary scientists think that our relatively big Moon, and the axis tilt itself, were created by enormous collisions Earth experienced early in its formation 4.5 billion years ago.

How does the tilt of the axis create seasons? The tilt changes how the sunlight hits Earth at a given location. As shown in Figure 1, Earth's axis (the red line) remains fixed in space. It always points in the same direction, as Earth goes through its orbit around the Sun.

When it is summer in North America, the top part of the axis (the north pole) points in the direction of the Sun, and the Sun's rays shine directly on North America; while in South America, the axis is tipped away from the Sun and the Sun's rays hit Earth on a slant. So, when it is summer in North America, it is winter in South America. When it is winter in North America, the north pole is tipped away from the Sun, and the Sun's rays hit the Earth on a slant there; meaning it is summer in South America, because the Sun's rays hit Earth more directly in that hemisphere. As for the intermediate seasons, spring and fall, these are seasons when neither the top, nor the bottom, of Earth's axis are pointed in the direction of the Sun, days and nights are of equal length, and both the top half and the bottom half of Earth get equal amounts of light.

Slanted rays are weaker rays because they cover a larger area and heat the air and surface less than direct rays do. You can see this if you shine a flashlight on a large ball. If you point the flashlight directly at the ball, it makes a bright, circular spot on the ball; however, if your point the flashlight at the edge of the ball, the light makes a duller, more oval-looking spot on the ball. The same thing happens with Earth and the Sun—imagine the ball is Earth and the flashlight is the Sun. In this astronomy science fair project, you'll investigate how tilting a surface affects how light rays hit that surface.