We are all familiar with seasonal changes, such as longer and warmer days in summer and shorter and cooler days in winter. But why do seasons occur? In this section, we’ll explore the seasons and their cause, and how seasons can differ around the world and even on other planets.
Teacher Notes for Section 2.2: In principle, students have already learned about seasons in elementary school. However, after covering Chapter 1 and Section 2.1, they should be much better prepared to truly understand how the seasons occur, why they are due to Earth’s axis tilt (and not to any variation in our distance from the Sun), and how seasons differ around the world (by latitude).
Also very important: Because most people using this book are likely to live in the Northern Hemisphere, the examples and discussion generally refer to that, though everything we say is still accurate for the Southern Hemisphere as well. But if you happen to live in equatorial regions where you don’t have the same pattern of 4 seasons as at temperate latitudes, you’ll need to help your students understand some of the differences. For example, at the equator the length of time the Sun is above and below the horizon is always 12 hours each, so statements such as the summer Sun having a “longer and higher path” through the sky would not be valid at the equator.
Section Learning Goals
By the end of this section, you should be able to answer the following questions:
- What causes the seasons?
- How do seasons differ around the world?
- Does the orientation of Earth’s axis ever change?
Before you continue, take a few minutes to discuss the above Learning Goal questions in small groups or as a class. For example, you might discuss what (if anything) you already know about the answers to these questions; what you think you’ll need to learn in order to be able to answer the questions; and whether there are any aspects of the questions, or other related questions, that you are particularly interested in.
Journal Entry
Your Seasons
Answer these two questions in your journal.
- Make a list of at least 5 things that distinguish summer from winter for you personally. For each one, write a sentence or two to describe how the difference affects you. Example: If you live in a climate that gets winter snow, one list item might be “It snows in winter but not in summer,” and your sentence might then explain what you like (or don’t like) about snow.
- What do you think is the reasons why seasons occur? Write down your best guess about the cause of the seasons and explain why you think this is the cause. Later, you will be able to see whether or not you were correct.
Teacher Notes: The goal for this journal entry is to get students to think about how the seasons affect them personally before you have students discuss them as a group. The list will vary both with your location and with students’ individual backgrounds.
Group Discussion
What are Seasons?
For the journal entry above, you should have already made your own list of things that distinguish summer from winter. Now we will build from that as a group.
- As a group, make a list of as many things that distinguish summer from winter as you can think of.
- For each item on your list above, categorize it into one of the following three categories:
Category 1: Weather phenomena
Category 2: Seasonal effects on living organisms
Category 3: Astronomically observable effects (such the Sun’s position in the sky or effects due solely to that position) - What season is it for you now? Are there places on Earth where it is a different season right now? How do you know?
Teacher Notes: This discussion is best done as a full class with you making the list at the front of the classroom, since that is likely to yield a longer list of phenomena than you would get from smaller groups. Other notes:
- For (1): Since we will categorize the phenomena in #2, for #1 you can just write them down in the order that students come up with them.
- For (2), here are some examples to help you and your students decide which category an item belongs in:
- Category 1: Weather phenomena: it’s warmer in the summer, it only snows in winter, and so on.
- Category 2: Seasonal effects on living organisms: many flowers bloom only in summer, many animals hibernate in winter, different crops grow better in different seasons, etc.
- Category 3: Astronomically observable effects: The Sun follows a longer and higher path through the sky in summer, there are more daylight hours in summer, midday shadows are longer in winter, the Sun’s path varies with latitude, etc.
- Important note: Don’t worry if some items are difficult to categorize; in that case, you might just have students vote on which category they’d place the item in. As explained below, the only real purpose to the categorization is so that students will realize that as we go forward in this section, we will be focusing only the astronomical origin of seasons; they’ll learn more about weather later in this book, and seasonal effects on living organisms are generally covered in life science courses.
- For (3): In principle, students should already know from elementary school that the seasons are opposite in the northern and southern hemispheres. This question is designed to confirm that they are aware of this fact, and also to help them think about how they know it. For that, they should realize that rather than just taking it as a given, they could check it by looking up, for example, sunrise/sunset times or weather reports from different parts of the world to confirm that the general seasonal patterns of the moment are opposite in the two hemispheres.
Seasonal changes in weather and seasonal effects on living organisms are very important, but these are what we might call “secondary effects,” meaning that they occur only because of the more basic “primary effects” that are astronomically observable. For that reason, we will focus our attention in this section on the astronomical reasons for seasons.