# Why Is It Summer After Spring, not Winter? | DFRobot Science Lab Season 2 EP05

### Standards

NGSS

4-PS3-2 Make observations to provide evidence that energy can be transferred from one place to another via sound, light, heat, and current.

3-ESS2-1 Present data in charts to describe the expected typical weather conditions in a specific season.

5-ESS1-2 Present data in charts to illustrate how the length and direction of shadow change throughout the day, the alternation of day and night, and seasonal changes of certain stars.

### Overview

This lesson will mainly focus on seasonal alternation, discussing the question "why is spring followed by summer, not winter?". It will lead students to construct a simple "earth-sun" model to explore the distribution of sunlight on the earth, study the direct and oblique light knowledge, and learn the revolution of the earth and the formation of the four seasons.

### Materials

Play and Learn with Boson Science Design Kit: https://www.dfrobot.com/product-2176.html

Engage

In most places on the earth, people experience spring, summer, autumn and winter every year. Do you feel the differences between different seasons?

We feel different temperature during different seasons. The climate is pleasant in spring and autumn, hot in summer and cold in winter. Think about it, why are there four seasons in a year? Why is it summer after spring, not winter?

Explore

Last lesson, we talked about how the sun brings light and heat to the earth, and makes the earth full of vigor. Think about it, does the distribution of sunlight on the earth vary during the year? Let's perform an experiment to find it out!

This experiment will be divided into two parts. The first part is to learn the direct and oblique light, and the second part is to explore the distribution of the direct and oblique light of the sun on the earth.

Part 1 Observation: Direct and Oblique Light

The effect is different when the light hits at different angles. A beam of light that shots vertically is called direct light, and one that shots at an oblique angle is called oblique light.

What is the difference between direct and oblique light? Try filling in your guess in the table below.

### Step

Let us observe the direct and oblique light with a flashlight. The operation is as follows.

### Activity Conclusion

The referable results are as follows.

As we can see from the above experiment, the direct light is concentrated, while the oblique light is scattered. The greater the angle of oblique light is, the more scattered the light is. With what we've learned about thermal radiation, we know that the energy is higher when the light is concentrated, and the temperature is higher.

### Conjecture

Think about it, is the sunlight direct or oblique? Will the distribution of the sun's direct and oblique light on the earth change? Come up with your conjecture!

Part 2 Experiment：The Distribution of The Sun's Direct and Oblique Light on the Earth

In many places on the earth, people will experience spring, summer, autumn, and winter every year. The seasons alternate and repeat. Do you know why the cycle of the four seasons is one year? Will the distribution of the sun's direct and oblique light on the earth change during the year? Let's learn it together!

Let’s learn about the revolution of the earth first. The earth revolves around the sun, which is the revolution of the earth, and the time for one revolution is one year. During the year, the earth will revolve to different positions. The following figure shows the relative position of the earth and the sun in March, June, September, and December. During the revolution, the earth is slightly tilted.

As the figure below, we can use a globe to represent the earth and a flashlight to represent the sun. Let the globe rotate around the flashlight to simulate the revolution of the earth. For example, in March, when the earth is in front of the sun, we turn on the flashlight and shine it on the globe, just like the sun shines on the earth.

Now, let’s find a place with four seasons and observe the changes of light here as the earth revolves. For example, we select Guangzhou, China as the location. Find the location of Guangzhou on the globe (it is located near the Tropic of Cancer in China), and use the BOSON light sensor to detect the light intensity.

Finally, we choose March, June, September, and December as the observation points, when the earth revolves to the front, left, back, and right positions of the sun, observe whether the light in Guangzhou is direct or oblique, and whether the oblique angle is large or small. Record the results in the table below.

### Materials

Prepare the following BOSON modules and other materials.

### Step

1. First, build an experimental circuit, and detect the light intensity through the light sensor.

2. After finishing the circuit connection, use the "Globe-Flashlight" to build the "Earth-Sun" revolution model.

### Activity Conclusion

The referable experimental data are as follows:

The referable line chart is as follows:

From the data, we can see that in Guangzhou, it is direct light in June, when the light is the strongest; In March and September, it is oblique light with a small oblique angle, and moderate light intensity; In December, it is also oblique light, with a larger oblique angle. The light is the weakest during this period. According to the information, Guangzhou will enter the hot summer in June, spring and autumn in March and September, and cold winter in December.

It can be concluded that the change in the angle of light irradiation corresponds exactly to the seasonal temperature change. When it is direct sunlight, the light is strong and the temperature is high; when it is oblique sunlight, the light is weak and the temperature is low. As the earth revolves, the angle of illumination in Guangzhou changes, causing temperature change, forming the four seasons of spring, summer, autumn and winter.

Now you know why spring is followed by summer, not winter!

Science Background

In the experiment, we assume that the sunlight is a parallel beam like the flashlight. Is the sunlight really a parallel one? Let's have a look!

### Is Sunlight a Parallel One?

The sun is like a shining fireball, radiating light equably around. The sunlight is not parallel theoretically. But because the distance between the earth and the sun is very far, about 150 million kilometers, the sunlight on the earth can be approximately regarded as parallel light.

### The Direct and Oblique Light on the Earth

If you regard sunlight as parallel light, since the earth is a sphere, there are some places where sunlight is direct light, while there are some places where sunlight is oblique light, it depends on the angle between the light and the surface of the earth. For example, in the picture below, in the equatorial region, the light is perpendicular to the surface of the earth, so it is direct light, and when going to the south or north pole form the equator, the light gradually obliques, so it is oblique light.

### The Rotation and Revolution of the Earth

As everyone knows that the earth is not stationary. There are two forms of motion, one is revolving around the sun, the other is rotation around its axis of rotation.

Most celestial bodies in the universe rotate, and the earth rotates for about 24 hours a turn, which is one day. The plane formed on the equator when the earth rotates is called the equatorial plane.

Because of the gravity of the sun on the earth, the earth will also revolve around the sun. One revolution of the earth takes about 365 days, that is, a year. The plane on which the earth revolves around the sun is called the ecliptic. There is a fixed angle between the equatorial plane and the ecliptic plane, called the obliquity of the ecliptic, which is about 23.4°.

### The Formation of Four Seasons

Since the earth always revolves around the sun in a tilting condition, the direct light from the sun is centered on the equator and constantly sweeps north and south with the Tropic of Cancer as the boundary once a year, thus forming the four seasons, and the sequence of alternation.

Conclusion

In this project, we first observed the direct and oblique light phenomenon, and then constructed a simple "earth-sun" model, simulated the process of the earth's revolution in the experiment, and learned the formation of the seasonal alternation.

The project is going to an end, please remove cables, and put all the BOSON modules back into the kit according to the corresponding position.

Extend

1. Why are the seasons opposite in the northern and southern hemispheres? Is there seasonal alternation near the equator?

2. The orbit of the earth’s revolution is actually an ellipse, and there are perihelion and aphelion on it. Will they influence the seasonal alternation?

Appendix – Explore “Why are the seasons opposite in the northern and southern hemispheres?”

There are three crucial lines on the globe, namely the Tropic of Cancer, the Equator, and the Tropic of Capricorn. North of the equator is the northern hemisphere, and south of the equator is the southern hemisphere. During one year, the seasons are opposite in the northern and southern hemispheres. Why is this? Let's explore it together!

Use the "Globe-Flashlight" model to simulate the earth's revolution. During the revolution, we use the BOSON light sensor to detect the light intensity values at the Tropic of Cancer, Equator, and Tropic of Capricorn in March, June, September, and December. Check the distribution of light in different places on the earth at the same time.

### Step

1. Continue to use the previous experimental circuit and model.

2. Record the light intensity at the Tropic of Cancer, Equator, and Tropic of Capricorn in March, June, September, and December. Fill them in the table.

### Activity Conclusion

The referable experimental data are as follows:

The referable line chart is as follows:

Through experiments, we can know that in March the sun shines directly on the equator. By this time, it is spring in the northern hemisphere and autumn in the southern hemisphere; then the point of direct light gradually moves northward, and the sun shot directly on the Tropic of Cancer in June, and it is summer in the northern hemisphere and winter in the southern hemisphere. ; Then the direct shot point gradually moved south, and in September the sun shot directly on the equator, and by this time, it is autumn in the northern hemisphere and spring in the southern hemisphere; Then the point of direct light continued to move southward and the sun shot on the Tropic of Capricorn in December, and by this time, it is winter in the northern hemisphere and summer in the southern hemisphere.

When the earth revolves, the direct light always sweeps back and forth between the Tropic of Cancer and the Tropic of Capricorn, so the seasons are opposite in the northern and southern hemispheres. In the equatorial region, the light intensity in a year does not change much, so there will be no obvious seasonal alternations.