At 8.45 on the morning of 15 April 136 BC, Babylon was plunged into darkness when the Moon passed in front of the Sun. An astrologer, who recorded the details in cuneiform characters on a clay tablet, wrote: "At 24 degrees after sunrise-a solar eclipse. When it began on the southwest side, Venus, Mercury and the normal stars were visible. Jupiter and Mars, which were in their period of disappearance, became visible. The Sun threw off the shadow from southwest to northeast." If present-day astronomers use a computer to run the movements of the Earth, Moon and Sun backwards...