Name________________________________Date_______Period_____
The Earth’s Shape
Introduction: Pictures of the earth taken from space show that the earth appears to be
perfectly round and smooth. However, to us, the earth appears to have a highly irregular
surface. In addition, accurate measurements of the earth’s shape show that the equatorial
diameter is slightly different than the polar diameter.
Objective: After you complete this lab, you will better understand the true roundness and
smoothness of the earth.
Vocabulary:
Relief:
Model:
Oblate spheroid:
Sphere:
Procedures:
A. Roundness
The ratio of the polar diameter to the equatorial diameter of a sphere is a measure of its
roundness. We call this a roundness ratio. In a perfectly round sphere, dividing the
polar diameter by the equatorial diameter would give a value of one since both
diameters of a perfect sphere are equal. The farther from one (1) the actual computed
ratio is, the less spherical is the globe.
1. Use the values given for the equatorial and polar diameters of the earth in the
data chart on the report sheet to calculate the roundness-ratio of the earth.
Record this value on the chart provided.
2. Measure the equatorial and polar diameters of the diagram in your lab. Record
these measurements on the chart provided.
3. Calculate the roundness-ratio for the diagram using the data from Procedure 2.
Record this value on the chart provided.
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Report Sheet
A. Roundness
Earth
Polar
Diameter
Equatorial
Diameter
12,714 km
12,756 km
Roundness
Ratio
Globe
diagram
B. Smoothness
A relief globe shows the relative height of its surface features such as mountains. It is a
scale model of the earth. The following procedures will show you whether or not these
features are constructed to scale on the enclosed diagram. To do this you must use the
following proportion:
Actual Height of the surface feature (km)
Earth’s Diameter (km)
=
Relief Globe Height of surface feature (cm)
Relief Globe Diameter (cm)
When you substitute values into the equation, you will have one unknown – the relief
globe diameter. Use the actual height of Mt. Everest divided by the diameter of the earth
which is given. Measure the height of Mt. Everest on the globe using your cm ruler.
*Note: The following chart information is to be recorded on the data chart
under ‘smoothness’ on your report sheet. *
1. Measure the relief globe height of Mt. Everest in cm using the diagram of the
earth.
2. The actual height of Mt. Everest, the highest point in the world, is 8.8 km.
3. Calculate the average diameter of the Earth. Find the average of the polar
diameter (12,714km) and the equatorial diameter (12,756 km).
4. Calculate the average diameter of the Relief Globe. Measure the equatorial
diameter and the polar diameter, and then find the average. (Use the diagram of
the Earth).
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5. Using the values obtained in procedures 2 through 4 and the above equation,
solve for the relief globe height of the surface feature (Mt. Everest) to correct
scale for the globe. Draw Mt. Everest to scale on the Relief Globe to the best of
your ability.
6. Determine the percentage error between the height of Mt. Everest on the diagram
and the height it should have been if drawn to the correct scale.
B. Smoothness
1. Height of Mt. Everest on the diagram
cm
2. Actual Height of Mt. Everest
km
3. Average diameter of the Earth
km
4. Average diameter of the Diagram
cm
5. Correct scale height of Mt. Everest on Globe Diagram
cm
6. Percent Error (comparing the correct scale height to
measured height of Mt. Everest)
%
Show all work for the calculations above.
5.
6. Percent Deviation = Difference from Accepted value
Accepted Value
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*100 (%)
21
The diagram below represents an average classroom relief globe.
Mt. Everest
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Discussion questions: (ANSWER IN COMPLETE SENTENCES!!!!)
1. Using the roundness ratio you calculated, which is a more perfect sphere, the globe
diagram or the earth?
2. How does the earth’s polar diameter compare with its equatorial diameter?
3. Is the earth a perfect sphere? How does your data confirm your answer?
4. Determine the roundness-ratio for the following celestial objects,
Celestial Object
Polar Diameter
(km)
Equatorial Diameter
(km)
Mars
6,752
6,792
Jupiter
133,709
143,884
Saturn
108,728
120,536
Neptune
48,682
49,528
Moon
3,471.94
3,476.28
Roundness-Ratio
(thousandths)
List the objects above in order of least to most spherical. Include the Earth.
A. _________________________
B. _________________________
C. _________________________
D. _________________________
E. _________________________
F. _________________________
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5. Using your calculations under Procedure B (smoothness), explain why or why not you
think that the earth is, or is not smoother than the average classroom relief globe.
6. A 0.1cm deep scratch was made in the surface of a classroom globe with a diameter of
20 cm. Calculate the actual depth of the surface feature (represented by the scratch) on
the real earth. Use the information you gained from the previous calculations. (SHOW
ALL WORK!!!)
7. In terms of roundness and smoothness, name an object which would be a good model
of the earth. Explain your choice.
Conclusion:
Describe how this lab helped you to understand that the earth is both round and smooth.
Include specific information.
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