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Name ______
EARTH’S ORBITAL SHAPE
Introduction – If you recall from earlier in the year, the shape of the Earth was very close to a perfect sphere. This shape was called an oblate spheroid. Similarly, the shape of Earth’s orbital path around the sun is very close to a circle. The orbital shape is called a slightly eccentric ellipse. An ellipse is like a “squished” circle. The degree to which an ellipse is “squished” is called eccentricity. Ellipses have two central points each called a focus (plural foci). Each planet revolves around the sun in an elliptical orbit; some orbits are more “squished” than others. For the Earth and the rest of the planets, the sun is located at a focus; the other focus is simply empty space. In this laboratory experiment, you will draw ellipses and compare Earth’s orbit with other planets.
Vocabulary
Ellipse –
Eccentricity –
Major Axis –
Focus –
Circle –
Materials
Cardboard
White paper
String
2 push pins
Ruler (cm)
Pencil
Procedure (horizontal)
1. READ and FOLLOW DIRECTIONS CAREFULLY!!!
2. Fold your paper in half twice.
3. Unfold your paper.
4. Place the paper on the cardboard, horizontally (landscape mode).
5. Make a dot in the center of the paper.
6. Draw a line going horizontally along the crease in the paper (this should be a LONG line)
7. Measure 2 cm to the left of the center dot and make a mark for foci 1.
8. Label this F1.
9. Measure 2cm to the right of the center dot and make a mark for another foci.
10. Label this F1. (yes, there should be two foci’s labeled F1).
11. Insert a push pin through each of the foci marks that you made. (2 pins)
12. Place the loop of string around the foci pins.
13. Place your pencil inside the loop and draw an ellipse (be sure to pull the string so it is tight)
14. Label this ellipse Ellipse1
15. Remove the pins and string.
16. Measure the distance between the foci and the length of major axis (to the nearest tenth). Record your answers in the data table.
17. Calculate the eccentricity of this ellipse to the nearest thousandth. Show all work in the data table.
18. Measure one cm outwards from both foci (F1). Label each of the new foci points F2.
19. Repeat steps 10-17. Label this ellipse Ellipse2.
20. Measure 1 cm outwards from both foci (F2). Label each of the new foci points F3.
21. Repeat your steps again to draw a third ellipse, Ellipse3.
22. Insert one push pin through the center dot and label it F4.
23. Draw Ellipse4 and label it. Calculate its eccentricity in the data table. THINK
Data Table
Distance between foci =
Round to the nearest tenth / Distance between foci =
Round to the nearest tenth
Length of Major Axis =
Round to the nearest tenth / Length of Major Axis =
Round to the nearest tenth
Calculated Eccentricity =
Round to the nearest thousandth / Calculated Eccentricity =
Round to the nearest thousandth
SHOW WORK HERE / SHOW WORK HERE
Ellipse #2 / Ellipse #4
Distance between foci =
Round to the nearest tenth / Distance between foci =
Round to the nearest tenth
Length of Major Axis =
Round to the nearest tenth / Length of Major Axis =
Round to the nearest tenth
Calculated Eccentricity =
Round to the nearest thousandth / Calculated Eccentricity =
Round to the nearest thousandth
SHOW WORK HERE / SHOW WORK HERE
Conclusion Questions – Answer in complete sentences.
1) State the relationship between distance between foci and eccentricity?
2) Which ellipse was the most elliptical?
3) How did you know that was the most elliptical?
4) Which ellipse had the lowest eccentricity? What was it?
5) On your ellipse page label the SUN at on foci.
6) On your ellipse page make an X on Ellipse2 to show where the fastest orbital velocity would occur.
7) Using the data in the ESRT page 15, which is more eccentric, Ellipse1 or Earth’s orbit? Explain.
8) Using the ESRT’s list the planets in order of increasing eccentricity.
Bonus Challenge: Using the data in the ESRT, calculate the distance between foci for Earth’s orbit.
Conclusion: Discuss the true shape of Earth’s orbit around the sun.