What is the theme in Pale Blue Dot: A Vision of the Human Future in Space by Carl Sagan?

The title of the book, "Pale Blue Dot" indicates the book's primary theme. Sagan is particularly impressed with how small the Earth is compared to the universe as a whole. He was one of those who had one of the Voyager satellites turn to face the earth from millions of miles away and take a picture of our "Pale Blue Dot," and he draws a moral and theological lesson from the photo. It is this: we are small, insignificant and incredibly petty. Humanity should be united in hope and joy in the face of colonizing the universe and its potential for cosmic discovery. Instead we let nationalism, ideology and religion get in our way.