A 1970 science-fiction novel by Larry Niven set in his Known Space universe. The novel features the eponymous Ringworld, a megastructure resembling a ring around a star. It is one million miles (1.6 million km) wide with a diameter equal to that of the Earth's orbit. The inner surface facing the sun is habitable with a surface area equivalent to 3 million Earths. A smaller, inner ring consisting of a chain of connected squares is used to block the sun and provide a day/night cycle.
