For centuries, an astronomy observatory in Greenwich, London -- the namesake of Greenwich Mean Time -- has been the reference point for lines of longitude, ships' navigation on the world's seas and the time zones used today. But Saudi Arabia wants to change that. It's building what it calls the largest clock in the world, atop the second-largest skyscraper in the world, in the Islamic holy city of Mecca -- in hopes of replacing GMT with "Mecca Time." The clock dwarfs London's Big Ben, with four huge faces each about 130 feet in diameter and inscribed with the Arabic words...