Key concepts
Astronomy
The sun
Light
The seasons
Earth's orbit

Introduction
Have you ever lived somewhere where you get to experience the full glory of all four seasons? If so, you know well the full 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 snows of winter. But do you know why we have these seasons over and over again in a cycle as predictable as sunrise and sunset? It actually has to do with Earth's tilt. In this science activity you'll investigate how this tilt affects how the sun's rays strike our planet and create seasons.

Background
In Earth’s Northern Hemisphere summers are hot and filled with many hours of strong sunlight whereas winters are cold due to shortened daylight hours and weak sunlight. Why is this? One big part of the answer is that Earth is tilted on its axis. To visualize this axis, picture an imaginary stick piercing Earth from its North to South poles. Earth spins once around this axis every 24 hours. While spinning like this our planet also circles the sun in a big orbit, completing this loop in about 365 days.

This axis isn't straight up and down as Earth orbits around the sun, however. Instead, it is tilted at approximately 23 degrees and also remains fixed, always aligned in the same direction in space. This tilt changes how the sunlight hits Earth at a given location in its yearly orbit. When it is summer in the Northern Hemisphere, the top part of the axis (the North Pole) points more toward the sun, and the sun's rays shine more directly on the Northern Hemisphere (where the continents of North America, Europe and Asia as well as the northern parts of Africa and South America are located); at the same time in the Southern Hemisphere (Antarctica, Australia, most of South America and the southern third of Africa), where it's winter, the South Pole end of the axis is tipped away from the sun, and its rays hit that half of Earth on a slant.

Materials

  • Small cardboard box, stepping-stool, brick or large block of wood
  • Flashlight
  • Masking tape
  • Large, firm book or cutting board
  • Sheet of paper
  • Pen or pencil
  • Helper (optional)

Preparation

  • If you are using a cardboard box, tape it so it is sealed shut.
  • Place the sealed-shut cardboard box, stepping-stool and 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 Earth's surface.

Procedure

  • 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-size, sharp circle that is about two to three inches in diameter. Make sure that there are at least two inches of paper above the top of the circle of light that is 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, etcetera).
  • 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 floor or tabletop. 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 toward 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 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 America experiences in summer? What about in winter?

Extra: Repeat this activity using a larger range of degrees, such as 20, 30, 40, etcetera. 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. (You can print it from a free graph paper Web site, such as Incompetech.com, or you can draw your own graph paper using a ruler, pen or pencil and two sheets of 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 with 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 with another angle?

Observations and results
Was the light on the paper much brighter when the book was vertically in front of the flashlight compared with 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 shined on the sheet of paper placed straight in front of it, the light formed a crisp, bright circle. When the book was tilted back 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). As the book is tilted away from the flashlight, the light rays hitting the paper's surface become more slanted. Slanted light rays are weaker because they cover a larger area and heat the air and surface less than direct rays do. The same thing happens with Earth and the sun. When Earth's North Pole is tilted toward the sun, the direct rays make the sunlight stronger and thereby warmer in North America—causing it to be summertime—compared with when the North Pole is tilted away from the sun. In that instance North America gets less direct rays and more slanted ones, causing it to be colder—what we know to be wintertime. The middle of the planet, the equator, never gets tilted too far from the sun, which is why most places closer to the equator, such as Florida, have seasons that are defined less by temperature change than do places farther away from the equator, such as Minneapolis.

More to explore
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
 

This activity brought to you in partnership with Science Buddies