Point of Divergence: 440-370 BC, i.e. Sometime during Democritus of Abdera's adulthood, after his travels through Egypt and Babylon, during his travels through Greece. The Ionian school of natural philosophy prevails against the more mystical Platonic philosophy. As a result, science and technology advance faster in ancient Greece; perhaps there is also more social progress and a closer political union of the Greek cities. This would also permit them to resist the Romans militarily and perhaps "conquer" them peacefully, with their way of life, philosophy and technology.
By the year 1980 AD, the first Interstellar spaceship is launched, as hypothesized in the episode "Journeys in Space and Time" of "Cosmos - A Personal Voyage".