Chapter 5: Q5-88GP (page 109)
A science-fiction tale describes an artificial 鈥減lanet鈥 in the form of a band completely encircling a sun (Fig. 5鈥50). The inhabitants live on the inside surface (where it is always noon). Imagine that this sun is exactly like our own, that the distance to the band is the same as the Earth-Sun distance (to make the climate livable), and that the ring rotates quickly enough to produce an apparent gravity of gas on Earth. What will be the period of revolution, this planet鈥檚 year, in Earth days?

Short Answer
The period of revolution is 9 days.