NAME ______DATE______

PARTNERS______

A REASON FOR THE SEASONS

Why is it warmer in the afternoon in summer compared to an afternoon in winter?

LET'S EXPLORE!

1. Securely tape or rubber band a pencil or stick with the point towards the lens end of a flashlight so that the pencil sticks out about 6 cm from the front of the flashlight and will not slip. This will help you set the angles for the light source (fake sun) and keep a constant distance from the recording surface.

2. Place a piece of graph paper on the table as the recording surface. We will make the assumption that the amount of light coming from the flashlight is constant and that the same total amount of light strikes the paper each time.

3. With the room darkened and the flashlight on, hold the flashlight perpendicular to the graph paper with the pencil touching the paper. Trace the bright area. This is the 90 measurement. Move to another spot on the graph paper and tilt flashlight 10o from perpendicular to 80o using a protractor to set an accurate angle. Trace the bright area. Repeat for positions at 70o, 60o, 50o, and 40o. Trace the areas each time. Make a note of the relative brightness (low, medium, high) of each of the areas as you are tracing and record them in the table on the next page.

4. Determine the illuminated area for each angle by either adding up the number of blocks (and parts of blocks) on the graph paper or using the formulas below. Record the areas for each angle in the chart below.

For a circle: A =  r2

For an ellipse: A =  ab

(a= 1/2 longest axis, b=1/2 shortest axis)

Angle / #4. Area (# Blocks or calculated value) / Relative Brightness or Intensity
by observation
(high, low…) (#3) / by area (#7)
(1.0-0)
90o
80o
70o
60o
50o
40o

5. Does the bright area vary with the angle? Explain.

Sketch a graph of angle versus area. Be sure to label the axes. What type of relationship do you see (direct or indirect, linear or curved)?

6. Is there any regular graphical relationship between the relative brightness of the illuminated surface as the angle changes? Explain.

7. If you compare your areas and brightnesses with another group you may find that the values differ. This occurs because the outputs of the bulbs vary and the areas of the lens may differ. You can eliminate these variables by finding relative intensity. This compares the area at each angle with the area at 90o. Everyone's ratios should be about same.

Calculate the relative intensity and enter the values in the previous table.

Plot the angle versus relative intensity. What type of relationship exists between relative intensity and angle?

8. Suppose you are an ant standing on the first area (90o). You will feel some warmth from the flashlight. If you walk to the next area (80o) will it feel warmer, cooler or the same? Why?

We know that the same amount of light is coming from the flashlight at each angle. When it strikes a larger area, the energy is dispersed more. There is less energy hitting each unit (block). If you are an ant, the larger the area, the cooler each point in that block will feel.

9. Now, think about the Sun. Is its angle with respect to the Earth consistent during the day? How can you tell?

At what Sun angle will you expect the warmest temperature during the day? What time of day does this correspond to?

Let's check it out.

Locate an Earth globe with an object attached at the surface. Hold the flashlight so that it is perpendicular to the object. What time of day would this be?

Gradually rotate the Earth holding the flashlight in the same position. Note the size of the illuminated area covering the attached object and the approximate angle of the light to the object on the surface of the globe. When is the light the most intense? Least intense?

How does the area of light relate to time of day?

What causes the differences in temperatures in your locale during the daytime?

10. Now let’s explore why it is cooler in the winter than it is in summer.

Set the globe on the desk so that the northern hemisphere is tilted toward you. Hold the flashlight perpendicular to the equator over the northern hemisphere and note the area of the illuminated surface that falls on the northern hemisphere. Turn the globe so that the northern hemisphere is tilted away from you. Hold the flashlight the same way and note the size of the illuminated area on the northern hemisphere.

When is the illumination most intense (smallest area)? What season does this correspond to?

When it is summer in the northern hemisphere, what season should it be in the southern hemisphere based on the Sun's intensity?

What is the Earth's tilt oriented with respect to the Sun during spring and fall?

11. Hold the flashlight steady and look at the pattern of light on the globe. Is it a perfect ellipse? Explain.

Is the light more intense closer or farther away from the equator?

Based on this where will it be cooler, at higher or lower latitudes? Why?

You notice that the rays of the Sun, like the flashlight, strike the latitudes closer to the equator more directly than rays striking the more northern or southern latitudes. As you already know, it is warmer near the equator than near the poles. When the Sun is highest in the sky at noon, the Sun's rays are more direct and therefore more intense. You expect it to be hotter than in the morning or evening when the rays are indirect and lower in intensity. When the northern hemisphere is tilted toward the Sun, the light striking that area is more intense than in the southern hemisphere. The northern hemisphere experiences summer. When the northern hemisphere is tilted away, the less intense light will produce winter conditions.

*Here's a teaser for you. Why is it usually warmer between 2 and 3 PM than at noon?

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Summer Science Institute 2007 Prince George’s Community College Barbara Gage 1