To do: Print out pdf document for Lab B Exercise

Materials needed: pencil, eraser, textbook, and calculator

Part I. Earth-Sun Relationships

Based on the materials presented in the textbook and supplementary lecture notes, complete parts A and B on page B-1.

Questions C and D pertain to Figure B-2 which shows earth at various positions in its orbit. The only positions you need to concern yourself with are those numbered 1-4 - they correspond to the four seasons. Your job it to figure out which is which.

Remember: The hemisphere tilted toward the sun is the one experiencing summer since the sun's rays are more direct. You can use the position of the polar axis to help you see this. Refer back to your online lecture materials for additional information.

Question D is asking you to determine at what latitude the sun's rays would strike the earth at a right angle. In other words the sun would be directly overhead at solar noon at this latitude on the particular date. This is what we called the sub-solar point or solar declination.

To do: Complete all parts.


Part II. Daylight and Darkness

Part B. How many hours of darkness would be experienced on Dec. 21st at? Hint: Figure out hours of darkness for Point Barrow, Alaska and South Pole. Compare your answers from Parts A and B for these two locations. Do you see a pattern forming? If so, you'll be able to quickly answer the other four locations. Also, the Equator should be easy to do.

To do: Complete all parts.


Part III. Calculating Solar Altitude (Elevation Angle or a)

a We are calculating the noon solar altitude, a, which is the angle the sun forms with the horizon at solar noon (time when sun is the highest in the sky for that date). The calculation is simple and requires you to know two things: the latitude of the observer and the solar declination for the date. We use the formula:

a = 90° - (latitude +/- solar declination)

To decide whether to add or subtract, we use the following rule:

- if solar declination and latitude are in the same hemisphere, subtract (larger number minus smaller)
if solar declination and latitude are in different hemispheres, add

*Also, if a comes out negative, then the sun is below the horizon (it's dark outside). Please record your answer as "BH" or "Below Horizon" - don't give a negative number for an answer.

*If you follow the equation precisely and don't use "larger number minus smaller" when the sun and latitude are in the same hemisphere, it is possible for your answer to come out greater than 90°. Since maximum solar altitude is exactly 90°, you will have to take 180° - 'your answer' to get the correct answer.

Example: Calculate the noon solar altitude for Minneapolis, MN on June 21.
We know that the declination is 23.5°N from our notes or Table B-2 in the lab manual. We look up the latitude for Minneapolis and find it to be 45°N. Therefore, we plug these values into our formula:

a = 90° - (45°N +/- 23.5°N).

Since both the latitude and declination are in the northern hemisphere, we will subtract.

a = 90° - (45° - 23.5°) = 90° - 21.5° = 68.5°

To do: Using Table B-2, complete all calculations on page B-5. For question H-2 on page B-5, estimate the declination as accurately as possible. Remember, just use the degree symbol without an "N" or "S". On your quiz, don't use the degree symbol at all. See instructions on the quiz.

Part IV. Radiation Intensity

The questions for this portion of the lab are linked to Part III. The locations are the same so you have latitudes and solar altitudes for these places.

To do: Complete all questions.

**If you want more practice with unite conversion problems, click the link:

This takes you to my main (non-online) website page for practice problems. Select "noon solar altitude / zenith angle practice". Ignore the questions that ask you to calculate the zenith angle. These are not to be submitted for grading and are not worth any points, they just give you more practice if you would like it before you take your quiz.

WHEN YOUR LAB IS COMPLETE...You will check your answers against mine at our lab meeting.


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Date Last Modified: September 12, 2007.
Copyright © 2007 Lynn Newman