SEASONS AND THE EARTH'S ROTATION

KEY CONCEPTS

  1. Climates are not constant. Climatic variation occurs over the course of a year due to regular seasonal changes.
  2. Seasons occur because our planet's axis of rotation is inclined at an angle of 23.5° to the plane in which Earth revolves around the Sun. At any given time, this tilt causes some regions on the earth to receive more direct solar energy than others.
  3. Equinoxes and solstices mark the beginning of seasons and refer to the sun's position relative to the earth.
 

MATERIALS

  • tennis ball to represent Earth
  • 15 cm wire or a knitting needle pushed through the ball to represent the Earth's axis
  • 1 sheet cardboard or poster board (2')
  • light source to represent the sun such as flashlight or lamp suspended above the board
  • (optional) QuickTime movie viewer installed on your computer
 
 

PROCEDURE

  
 
 

Activity

  1. Prepare the model by drawing a circle 40 cm in diameter on the board to represent Earth's orbit. Mark points north, south, east, and west corresponding to the equinoxes and solstices.
  2. Suspend the light source 15 cm above the center of the board.
  3. Place the ball successively at positions north, south, east, and west, while keeping its axis inclined at an angle of 23.5°. Be sure to keep this angle throughout the full revolution of "Earth" around the "Sun." Each position corresponds to an equinox or solstice, as indicated in the diagram provided below.
    • Click here to see an animation of how the tilt of Earth's rotational axis causes seasons.
    • Be aware that this animation is 3.1 MB in size!
  4. Observe how much of earth's surface is exposed to sunlight at each quarter position. Also, identity which hemisphere, if any, is receiving greater solar exposure at each quarter position. Which season does each hemisphere experience at each position?
 
 

Extension

  • Repeat the procedure with the axis perpendicular to the board and observe the results.
 
 

Assessment Ideas

  • Have students create a moving model or hyperstudio stack which can demonstrate what would happen to the equinox and solstice if the degree to which earth's axis is tilted changed.
  
 
 

CROSS-CURRICULAR IDEAS

  • Mathematics/History: Indians could tell the changes in weather and seasons with out any of the modern tools that we have. How?
  • Mathematics/History (Class Project): Using a shadow stick, calculate the angle of the sun each day for the entire school year. Record your results as a bar graph or line graph. Discuss the results each week.
 
 

VOCABULARY

  • climate: the daily and seasonal weather conditions experienced in any area over an extended period of time.
  • equinox: one of the two times (vernal and autumn) when the middle point of the sun goes across the equator and the day and night are equally long.
  • solstice: one of the two points (summer and winter) during the year when the sun is farthest from the equator.
 
 

SOURCE

  • Adapted from "Winds of Change" educational CD-ROM, Copyright Caltech and NASA/Jet Propulsion Laboratory
  

Day and night lengths vary throughout the year because of the orientation of Earth's tilted spin axis in relationship to the Sun. Associated changes in solar energy input to each hemisphere cause seasons. In the northern hemiphere, summer begins at the summer solstice when the north pole is titled toward the Sun. Winter in the northern hemisphere begins when the north pole is oriented away from the Sun, around December 21. In both hemispheres, day and night lengths are equal on two days each year, the autumnal and vernal equinoxes.

[Click here to see an animation that also demonstrates this.]