This famous photo was taken on Christmas Eve 1968 by the crew of Apollo 8 during the first manned orbit of the moon. The image had a profound impact, becoming the first of Life magazine's "100 images that changed the world." At that time, Vietnam, the cold war, civil rights tensions and the growing threat of overpopulation were prominent public concerns, and this image became a powerful reminder that we all share the same fragile and finite planet. Looking back from a barren moon to see our cozy home hanging in the black abyss of space also reminded us that we are true cosmic citizens - our "space-ship Earth" is a part of the Universe no more or less than any other planet, star or galaxy.
Now look at the lunar surface in the foreground. It was Galileo who first pointed a telescope at the moon in 1609 and was startled to see a cratered and pock marked surface. This was quite contrary to the firmly established Aristotelian view that saw all celestial objects as perfect and unblemished and therefore different from the imperfect terrestrial realm. In fact, it seemed to Galileo that the moon was just another world, basically like the Earth, with mountains and oceans. This and other observations (we'll meet in a moment), led Galileo to support the Copernican "Revolution", and write extensively against the Greek geocentric cosmological model, and for the newly emerging heliocentric cosmological model.