What causes the seasons to change? The first answer many people give when asked why the Earth gets cold in the winter and warm in the summer is because the Earth’s distance from the sun can vary during its orbit. Astronomers have carefully measured the distance between the Earth and Sun and have discovered that the Earth is closest to the Sun in January, and farthest away in July.
But wait a second! January is the peak of winter in the Northern hemisphere, and July is the peak of summer. There is another problem with the distance/season misconception: Why is it that when it is summer in the Northern hemisphere, it is winter in the Southern hemisphere? Something else besides varying distances from the sun must be causing the seasons.
Eventually, humans figured out that the varying seasons were caused by the earth’s tilt in combination with its orbit around the sun. You might have learned that the Earth spins like a top. It does not spin straight up and down, though—it spins at a 23 ½ degree tilt. The imaginary line that the earth is spinning around on is called its axis. When the top of the Earth is tilted towards the sun, it is summer in the northern hemisphere. Since the bottom half of the Earth (the southern hemisphere) is tilted away from the sun, it’s winter there.
On around June 21st, the northern hemisphere is at its max tilt towards the sun. This is called the summer solstice. This is also the longest period of daylight of the year in the northern hemisphere. On around December 21st, the Northern hemisphere is at its max tilt away from the Sun, which is known as the winter solstice and the shortest period of daylight. June 21st is the winter solstice for the southern hemisphere, and December 21st is its summer solstice. Just think about it: kids in Australia get to enjoy long summer days and winter holidays at the same time!
In addition to solstices, our planet also experiences equinoxes. During its journey around the sun, the Earth reaches two points in its orbit where the tilt isn’t towards or away from the sun. The length of day and night are equal. These are called the equinoxes. On about March 21st, it is the spring equinox in the northern hemisphere, and the fall equinox in the southern hemisphere. On about September 21st, it is the spring equinox in the southern hemisphere and fall equinox in the northern hemisphere.
The tilt correctly predicts the seasons, but how does the tilt cause warmer or colder temperatures? You can see for yourself!
How does the tilt of the Earth cause the seasons?
- Graph paper
- Room you can make dark
- Tape a piece of graph paper over the Northern hemisphere of your globe.
- Tape another piece of graph paper over the Southern hemisphere of your globe.
- Using the protractor, have you partner tilt the Northern hemisphere of your globe toward you 23 ½ degrees (Note: some globes might already be tilted on the correct axis).
- Have your partner continue to hold the globe in position.
- Standing about one foot away from the globe, shine the flashlight at a point just above the equator.
- Congratulations! You just modeled the way Earth and Sun are positioned during the summer solstice in the Northern hemisphere, which occurs on June 21st.
- Now, make the room dark.
- Ask your partner to trace the circle of light made by the flashlight. He or she should be tracing on the paper, not the globe itself.
- Next, ask your partner to lower the tilted globe (without changing its tilt) so that circle of light is now on the middle of the Northern hemisphere of the globe. Keep the flashlight in the same position.
- Ask your partner to trace the circle of light made by the flashlight in this region. Do you notice anything about how the brightness and shape of the circle of light changes?
- Next, ask your partner to lower the tilted globe so that circle of light is now on the upper part of the northern hemisphere of the globe. Again, describe how the shape of the light circle has changed.
- Next, ask your partner to raise the tilted globe so that the circle of light is now just below the equator. Make sure that this hemisphere is still tilted away from the flashlight.
- Ask your partner to trace the circle of light made by the flashlight.
- Next, ask your partner to raise the tilted globe so that circle of light is now on the middle of the southern hemisphere of the globe. Keep the flashlight in the same position.
- Again, ask your partner to trace the circle made by the flashlight.
- Finally, ask your partner to raise the tilted globe so that the light is nearest to bottom of the globe. What do you notice about the light circle?
- Have your partner do his or her best to draw the shape of light on the graph paper.
- Turn on the lights.
- Compare the number of squares in the light tracings your partner drew.
The number of squares you count will vary depending on the size of your globe, graph paper squares, and flashlight. When you moved the flashlight over the surface of the globe, you probably noticed that the circle of light emitted by the flashlight was brighter and rounder near the equator. The circle of light became bigger and not as bright as you moved it towards the poles. There should be more graph paper squares in those circles. The light tracings also should start to look less like circles and more like stretched-out ovals. When you moved the flashlight along the middle part of your globe’s southern hemisphere, you were likely to notice that the circle of light was not as bright as it was in the northern hemisphere. When you got to the bottom of the globe, the light from the flashlight shouldn’t have reached the globe’s South Pole at all.
In addition to demonstrating how seasons are caused, this experiment shows why some parts of the earth are warmer than others year-round. Remember that the flashlight always emits the same amount of light. When the light shines on the equator, the circle is bright and small, meaning lots of sunlight is concentrated in a small area. This is why the parts of the Earth near the equator are hot and have tropical vegetation. As you moved the light along the curve of the Earth towards the middle of either hemisphere, the same amount of light was spread out over a larger area. These parts of Earth’s surface get only a medium amount of light energy, explaining why these parts of the Earth are not as warm as parts located near the equator. Your light tracings near the poles were the biggest, meaning the sun’s light was spread very thin. That is why the North and South Poles are so much colder than the equator.
The Earth’s tilt creates seasonal differences in light intensity. Since the northern part of the Earth was tilted towards the sun, the light circles were smaller and brighter. This causes these parts of the Earth to be warmer during the summer. By the time your light reached the North Pole, the light circle was bigger (meaning the light is more spread out), but since the pole was tilted toward the light, it can still experience daylight in the summer. When you shined the light toward the South Pole, it probably didn’t reach the South Pole at all because this pole is tilted away from the sun during the winter. This is why the poles experience almost total darkness in the winter.