Seasonal Science: The Reasons for the Seasons
Have you ever lived someplace where you get to experience the full glory of all four seasons? If so, you know well the heady blossoms and dramatic skies of spring; the long, sun-drenched days of summer; the trees shaking in crimson and gold in fall; and the sparkling, brittle snows of winter. But do you know why we have these seasons, over and over again, in a cycle as predictable as the rising and setting of the sun? It actually has to do with the Earth’s tilt and the Moon. In this science activity, you’ll investigate how the Earth’s tilt affects how the Sun’s rays strike the Earth and create seasons.
Where most people live on Earth, in the Northern hemisphere, summers are hot and filled with many hours of strong sunlight, while winters are cold due to shortened hours of daylight and weak sunlight. Why is this? One big part of the answer is that Earth is tilted on its axis. To visualize the Earth’s axis, picture an imaginary stick going through the North and South poles of Earth. Earth does a complete rotation 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, the axis is tilted by approximately 23 degrees. Earth’s axis remains fixed in space, always pointing in the same direction, as Earth goes through its orbit around the Sun, and the tilt changes how the sunlight hits Earth at a given location. 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, where it’s winter, the axis is tipped away from the Sun and the Sun’s rays hit Earth on a slant.
- Small cardboard box, stepping stool, brick, or large block of wood
- Masking tape
- Large, firm book or cutting board
- Sheet of paper
- Pen or pencil
- Helper (optional)
- If you are using a cardboard box, tape it so it is sealed shut.
- Place the sealed-shut cardboard box, stepping stool, brick or block of wood on a table or on the floor.
- Lay the flashlight on its side on top of the cardboard box (or other object). Line up the light-emitting end of the flashlight with the edge of the box. Use masking tape to tape the flashlight down so it can’t roll around. The flashlight will represent the sun.
- Tape a sheet of paper to the book (or cutting board) so that the paper will be stiff enough to tilt and so that you can draw on it. This paper will represent part of the Earth’s surface.
- Turn on the flashlight.
- Put the book (with the taped-on paper) vertically in front of the flashlight and move the book closer or farther away from the flashlight until the light on the paper forms a medium-sized, sharp circle that’s about two to three inches in diameter. Make sure that there’s at least two inches of paper above the top of the circle of light that’s shining on the paper. If needed, raise the paper (by taping it higher on the book) and/or lower the flashlight (by taping it to a shorter cardboard box, etc.).
- While you are holding the book (or having a helper hold it for you) vertically in front of the flashlight, look at the light that is shining on the paper. How bright is the light on the paper? Using the pen or pencil, draw around the outline of the light on the paper.
- Without changing the distance between the book and the flashlight, tilt the book back, away from the flashlight, by about 45 degrees. In other words, the book should roughly form a 45 degree angle with the surface it is on. How did the light on the sheet of paper change? How bright is the light on the paper now? Again, draw around the outline of the light on the paper. Tip: If the top of the outline goes off the top of the paper, you can tilt the book back towards the flashlight a little until the entire outline fits on the paper.
- If you want, try moving the book back and forth between the vertical position and the 45 degree angle position a few times. Be sure not to change the actual distance between the book and the flashlight. How does the light on the paper change as you change its tilt angle?
- Turn off the flashlight and look at your sheet of paper. How did the outline change when the book was tilted by 45 degrees? How did this correlate with a change in light intensity? What degree of tilt do you think is most similar to the light that North American experiences in summer? What about in winter?
Extra: Repeat this activity using a larger range of degrees, such as 20, 30, 40, etc. You will want to do this using a protractor at the base of the book as you tilt it. How does the light outline change as you increase the angle of the book?
Extra: Try repeating this activity but instead of a blank sheet of paper use graph paper. When you are done making your outlines, you can count how many squares are filled by light when the book isn’t tilted compared to when it’s at a 45 degree angle. Using these numbers, just how different are the two outlines in size?
Extra: You could repeat this activity using a light meter, which would let you quantify your brightness observations. Just how much brighter is the light on the paper at one angle compared to another angle?
Observations and Results
Was the light on the paper much brighter when the book was vertically in front of the flashlight compared to when the book was at a 45 degree angle away from the flashlight? Did the outline get bigger and elongated when the book was tilted away from the flashlight?
In this activity you should have seen that when the flashlight was shown on the sheet of paper placed vertically in front of it, the light formed a crisp, bright circle. When the book was tilted back by 45 degrees, away from the flashlight, it should have made an oval shape that was much dimmer and larger (almost twice the size of the first outline). Basically, as the book is being tilted away from the flashlight, the light rays that hit the paper’s surface become more slanted. Slanted light rays are weaker rays because they cover a larger area and heat the air and surface less than direct rays do. The same thing happens with the Earth and Sun. When Earth’s North Pole is tilted towards the Sun, the direct rays make it generally warmer and sunnier in North America – causing it to be summer time – compared to when the North Pole is tilted away from the Sun (then North America gets less direct rays and more slanted ones, causing it to be winter time).
Ask an Expert
- What causes the seasons?, from NASA
- Moon May Save Earth from Chaotic Tilting of Other Planets, from John Noble Wilford, The New York Times
- Fun, Science Activities for You and Your Family, from Science Buddies
- The Reasons for the Seasons, from Science Buddies