Background:

	Vocabulary Global Energy Balance Inverse Square LawSolar Constant AphelionPerihelion For additional background information see Solar Irradiance Brief   Equations So = E(sun) x (R(sun) / r)2 So = Solar Constant E = 6.29 x 107 W/m2 = Surface Irradiance of the sun R = 6.96 x 105 km = Radius of the sun r = 1.5 x 108 km =Average Sun Earth Distance

Procedure: Worksheet Your job in this activity is to calculate the solar constant. The solar constant is the amount of energy received on a surface oriented perpendicular to the suns rays (at the mean distance of the earth from the sun). When you calculate the solar constant you will be calculating the amount of energy received at the top of the earths’ atmosphere. As you work you will be faced with problems that scientists encounter as they calculate the amount of energy reaching the earth.

Step 1: **PROBLEM #1: Like the light from a flashlight, electromagnetic energy from the sun
spreads out as it moves farther from the suns’ surface. The law that describes
the change in intensity with distance from the source is called the inverse square law. Study the diagram below, visit the inverse square law web site & then define the inverse square law on your worksheet.




Your Estimate: Solar Surface Intensity = 6.29 x 10⁷ W / m². Estimate the solar constant. How much of that energy will be received by the earth in W/m²?

Step 2: Calculate the solar constant. (See Background for Equation)

Step 3: Calculate the percentage of solar radiation not reaching the earth due to the inverse square
law.

	% of Solar Surface Irradiance not reaching earth =

Step 4: PROBLEM #2: Because the orbit of the earth around the sun is not a perfect circle, the
earth is not always the same distance from the sun. Orbit Diagram At the aphelion position sun-earth distance = 1.53 x 10⁸km. At the perihelion position the sun-earth
distance = 1.47 x 10⁸ km. Recalculate the Solar Constant for the aphelion distance and for the perihelion distance.

Step 5: Calculate the percentage change in the solar irradiance due to the earths’ orbit around the sun.

Step 6: A common conclusion that a person may draw from the previous calculation would be that
this change in distance creates seasons. This conclusion is entirely incorrect!!
See this site for the real reason. WHY DO SEASONS OCCUR? If you live in the
Northern Hemisphere, you experience your summer season while the earth is farthest from
the sun (Aphelion below), and winter while the earth is closest to the sun (Perihelion).
Examine the diagram below and the link above and then answer the question on your
worksheet.
**You will investigate seasons in more detail in the next investigation.

Coding:

Maryland Core Learning Goals (Science): ES 2:3:1 ES 2:5:1
National Standards (Science): 9-12 A:3 D:1 E:2
National Standards (Geography): 7:1,2,3
National Standards (Mathematics): 9-12 1:2,4 2:3

All investigation calculations and questions appear on the student worksheet. It is suggested that the investigator print a copy of the worksheet and complete it as they go through the activity.


Credits:

	Gregg A. Stickler NASA Teacher Ambassador GESSEP Program Lee Kyle NASA Goddard Space Flight Center Distributive Active Archive Center Greenbelt, MD