Suppose you are on a strange planet and make the following observations: At location #1, you observe, at night, that the stars do not rise and set, but circle parallel to the horizon.
Next, you move in a constant direction for 9000 miles. As you move, the angle of the stars' motion gradually changes until at your new location on the planet, all stars rise straight up in the east and set straight down in the west, perpendicular to the horizon.
You continue moving in the same direction for another 9000 miles and then the stars are once again circling parallel to the horizon but in the opposite direction from location #1.

Based on these observations, what is the approximate circumference of the planet in miles?

a. 28,000 miles

b. 18,000 miles

c. 36,000 miles

d. 56,500 miles

e. 254 million miles