ACTIVITY 5 Working with Kepler's Laws
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B
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7. Examine Figure 5.3, which shows a planet, D, orbiting the Sun, A, in a counterclockwise
direction. Match the following terms with the letter that identifies the location in the figure:
• focus
aphelion
perihelion
• increasing speed
• decreasing speed
planet has fastest speed
planet has slowest speed
FIGURE 5.3
8. State Kepler's second law and give the physical reason why planets behave this way.
to
to
D
Not to scale
.
Step 4-Kepler's Third Law
Kepler's third law relates the time it takes a planet to go around the Sun (the period, P) to the
semimajor axis, A, of the orbit. If we measure Pin years and A in astronomical units (AU), the
simplified relationship (formula) is:
p2 = A3
9. According to Kepler's third law, all orbits with the same semimajor axis have the same period.
The two orbits in Figure 5.4 have the same value for A and thus must have the same P, but
how is this possible when the orbits have such different eccentricities? (Hint: With a circular
orbit, e = 0, does the distance of an object orbiting the Sun change?)
FIGURE 5.4