Bill Rooke, in Pasadena, wonders, "How far out in space does one have to go to be able to look back and see Earth as a sphere instead of a flat surface?" It depends on what you'll accept as "spherical." We get a hint of our almost-spherical Earth just watching ships sail over the horizon. High-altitude pilots and Concorde passengers say they can see the horizon's curvature clearly at about 60,000 feet. Astronauts might say it is whenever the planet appears smaller than their window.