Two hundred light years from Earth, the Ringworld is an artificial habitat of enormous size. It is a ring of solid matter that circles a Sun much like Earth's, at a distance roughly the same as the distance from the Earth to the Sun. The ring is about 1,000,000 miles wide (as wide as its Sun) and almost 300,000,000 miles long. The ring spins at a speed that produces an acceleration of one gravity on the inner surface. Along the edges of the ring there are huge walls rising 1000 miles upward toward the center. These keep the atmosphere from floating away.
Without special provisions, it would always by high noon everywhere on the Ring. Day and night are simulated on Ringworld by use of the Shadow Squares. These are twenty rectangular shapes that orbit the Sun much closer than the Ringworld. They are face on to the Sun and just as wide. As the Ringworld and Shadow Squares turn, the shadows they cast on the surface simulate night and day. The Shadow Squares are joined by extremely strong threads, called Shadow Square Wire so that they form a sort of bracelette around the Sun. The wire keeps the squares face on to the sun and allows them to spin around the sun faster than the normal orbital period for their distance. The Shadow Squares absorb a huge amount of sunlight energy, which is beamed to the Ringworld as it's primary source of power.