The Reasons for the Seasons

[International Desk, DIU]

Objective

To investigate how axial tilt affects how the Sun's rays strike Earth and create seasons.

Introduction

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.



Figure 1. This drawing shows how Earth's axis remains fixed in space (pointing 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.



Figure 2. This drawing shows the different shapes and brightness produced by rays of sunlight that hit Earth more directly (in summer), and rays that hit Earth at a slant (in winter)

Experiment

Materials and Equipment

1) Stepping stool, brick, or large block of wood
2) Flashlight
3) Masking tape
4) Scotch® tape
5) Pieces of graph paper (4)
6) Large, firm book or a cutting board
7) Ruler
8) Protractor
9) Optional: Camera
10) Optional: Light meter, such as the Light Meter LX1010B,50,000 Lux Luxmeter with lcd display.
11) Helper
12) Lab notebook
13) Graph paper


Experimental Procedure

Preparing the Light Source

1) Place the stepping stool, brick, or block of wood on a table, or on the flat, firm floor.

2) Lay the flashlight on its side on top of the stepping stool, brick, or block, and line up the edge of the flashlight so it is close to the edge of the stepping stool, brick, or block. Use masking tape to tape the flashlight down so it can't roll around.

Preparing the Surface

1) Tape a sheet of graph paper to a firm surface, like a large book or a cutting board, so that the paper will be stiff enough to tilt, and so that you can draw on it. Ask your parents if it's okay if you use Scotch tape on the surface you have chosen.

2) Turn on the flashlight.

3) Put the graph paper vertically in front of the flashlight, as shown in Figure 3. Move the graph paper closer or farther away from the flashlight, until the light on the paper forms a medium-sized, sharp circle 2–3 inches in diameter. Have a helper help you measure the distance from the edge of the graph paper to the block of wood and write down this starting distance in your lab notebook. You will keep the graph paper at this starting distance for all testing.



Figure 3. This drawing shows how to set up your flashlight and graph paper for testing


Testing the Surface

1) Have a helper hold the graph paper vertically (straight up and down) at the starting distance in front of the flashlight.

2) Use your pencil to draw around the outline of the light on the graph paper. Draw a line from the circle and note that the graph paper is at 0 degrees for this outline (no tilt). An alternative to drawing around the outline is to take a picture of the graph paper with a camera.

3) Observe the brightness of the light inside this outline and record your observation in your lab notebook, or (optionally) measure the brightness with a light meter held at a fixed distance from the graph paper.

4) Place the protractor next to the graph paper, at the spot shown in Figure 3, and tilt the graph paper 10 degrees (tip the cutting board from the 90-degree mark to the 100-degree mark).

5) Use your pencil to draw around the outline of the light on the same piece of graph paper. Again, draw a line from this outline and note the angle for this outline on the graph paper. An optional alternative to drawing around the outline is to take a picture of your graph paper with a camera.

6) Observe the brightness of the light inside this outline, and record your observation in your lab notebook, or (optionally) measure the brightness with a light meter held at a fixed distance from the graph paper. Compare the brightness to the previous outline.

7) Repeat steps 4–6 for tilt angles of 20, 30, and 40 degrees.

8) Remove the sheet of graph paper and attach a new one.

9) Repeat steps 1–8 two more times.

Analyzing the Graph Paper

1) If you used a camera instead of drawing around the light outlines, print out your photographs so you can analyze them.

2) For each sheet of graph paper, count the approximate number of squares inside each light outline. For partial squares, estimate how much of the square is lit up; for example, if it looks like one-fourth of the square is lit up, add 0.25; if it looks like half of the square is lit up, add 0.5; if it looks like three-fourths of the square is lit up, add 0.75. Enter your counts in a data table.

3) Calculate the average number of squares inside each outline for each degree of tilt and enter your calculations in the data table.

4) Plot the degree of tilt on the x-axis and the average number of squares illuminated on the y-axis. You can make the line graph by hand or use a website like Create a Graph to make the graph on the computer and print it.

5) How did the numbers of squares inside the outline change as the degree of tilt increased? How did the brightness change? What degree of tilt produces light similar to what North America experiences in summer? What degree of tilt produces light similar to what North America experiences in winter?

Variations

Investigate the axial tilts and the presence or absence of seasons on other planets. Can you predict which planets have seasons, based on their axial tilts?
Plot the degree of tilt on the x-axis and the illuminance (in lux), from light meter readings, on the y-axis.

Credits

Kristin Strong, Science Buddies

The procedure for this science fair project was adapted from an activity outlined in the following source:

Harcourt Science. (2000). Lesson 3: What Causes Earth's Seasons? pp. B16-B21.
Scotch® is a registered trademark of 3M.