India, Aug. 3 -- One morning later this year, a new morning star will appear some 625 kilometres above us, heralding a weird retail business: "sunlight on demand". As someone severely deficient in vitamin-D, I might rush to place an order, perhaps through another rapid-delivery service dealing with celestial goods. Communities that live without sunlight for long periods might do the same, using it to generate solar energy. In a disaster-struck place, people could call for its service to continue rescue and relief work at night. When one is sold "day" at night, the imagination quickly races ahead, conjuring up countless possibilities built on the idea of this retail business. For a species that has nearly altered the planet's natural rhyth...