The Seasons

Contrary to a student's first guess, it is not the earth's distance from the sun that dictates the seasons, rather the tilt of the earth as it rotates around the sun. In fact, the earth is closest to the sun during the Northern Hemisphere's winter.

Objectives

1. To determine the effect of the earth's tilt on the amount of incoming solar radiation throughout the year.
2. To understand the reason for the changes in seasons.

Notes to the teacher

1. This expansion is easy to do after "Angle of Incidence" and is a must for those teaching weather. Make sure that students understand how the earth is tilted with respect to the sun before or during this lab. If the earth's tilt is foreign to them, they will not understand the seasons properly.
2. This activity is best discussed as an entire class, but make time at the beginning for the students to read the questions and try to answer them individually.

PROCEDURE 1. Place the light on a table or desk and turn it on. 2. Take the tilted globe and walk around the light bulb to simulate the earth's orbit around the sun. Important: Always tilt the "earth" toward a specific wall of the classroom. It is scientifically incorrect to always tilt it toward or away from the "sun". QUESTIONS 1. Using the knowledge you have gained in "Angle of Incidence", how intense (bright) is the light at your school on December 21? On March 21? On June 21? On September 21?  2. What does your answer in #1 say about light hitting the earth from the sun throughout the year?  3. What factors determine how much light hits a point on the earth on a clear day? 4. If the earth were not tilted on its axis, would the amount of sunlight that hit a given point on the earth's surface change from season to season? Explain. 5. What causes the seasons? 6. The earth is closer to the sun in the winter in the Northern Hemisphere than in the summer. Why is it cooler in the winter if the earth is closer to the sun? 7. (Optional) If the total amount of light given off by the flashlight is given in units called Watts, why would it make sense to describe the light received at a point on the ball by Watts per square meter?

PREREQUISITES Understanding of the angle of incidence of sunlight onto the earth MATERIALS Light bulb in lamp (no shade) or socket Globe Pencil Paper CORE CURRICULUM SKILLS APPLIED IN THIS LESSON Identify similar or different characteristics in a given set of objects, organisms or events. Select descriptive (qualitative) or numerical (quantitative) observations in a given set of objects, organisms or events. Identify qualitative and quantitative changes given conditions before, during and after an event. Identify properties by which a set of objects, organisms or events could be ordered. Recognize and describe patterns.