[填空题]
A science-fiction tale describes an artificial "planet" in tev.2 ;e2 o .f5ob+cs6t zxbspp+egrb.he form of a band cjg(d 7d:ex,5 j o+hvlfii(9xnompletely encircling a sun (Fig. 5-40). 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 isvo+:(idn5 hefj7l9(j dx,i xg the same as the Earth-Sun distance (to make the climate temperate), and that the ring rotates quickly enough to produce an apparent gravity of g as on Earth. What will be the period of revolution, this planet's year, in Earth days?
