Carl Sagan shows 2 pictures. The first is the iconic photo of Earth taken by Apollo 17 astronauts. The earth appears as a blue marble. Africa is seen at the top (I wish it were North America) while Antarctica appears at the bottom. We see blue oceans and white cloud tops. In Sagan's second picture, the earth is a pale blue dot. It was taken by Voyager 1, 3.7 billion miles out. Our planet is no larger than a speck, and Sagan goes to great lengths to convince us how arrogant we are. In the picture known as the Pale Blue Dot, Earth is seen through a beam of sunlight reflecting off the spacecraft. Sagan remarks that this view of Earth would be seen from an alien spacecraft entering our solar system and its occupants would have no reason to think this point of light was of any significance.
Sagan presents us with a series of demotions. Not only is Earth not at the center of the solar system, but the sun is not at the center of the Milky Way and the Milky Way is merely one galaxy among billions. Finally, we are not at the center of the expanding universe because there is no center. An observer in any galaxy would perceive all the galaxies receding from him. Sagan's book was published in 1994, years before the Kepler spacecraft found exoplanets. Nonetheless, he foresaw it happening. He recognizes the Bible as a moral guide but understands that it reflects the rudimentary science of the time in which it was written. The earth is 4.6 billion years old, not 6 thousand years old as the Bible suggests. The universe itself has existed for 13.8 billion years. Sagan portrays the human species as a newcomer and in no way privileged. Although he stops short of saying religion is crap and there is no such thing as "God," he writes that "We seem compelled to project our own nature onto Nature." Sagan hints at a multiverse, infinitely old and unobservable. He strips us of all notions of being special. Sagan was a brilliant man who knew his stuff. He saw order and design as stemming from the laws of nature rather than from any kind of deity.
Carl Sagan has high praise for the Voyager 1 and Voyager 2 missions. Launched in 1977, they opened up the solar system over a 12 year period. Much was learned about the gas giants. All 4 were found to have rings, and volcanoes were discovered on Jupiter's moon Io. Thousands of new rings were discovered around Saturn as Saturn's moon, Titan, became an object of interest. With its methane-rich atmosphere, Sagan suggested that life may eventually rise. He did not live to see the Cassini spacecraft reach Titan.
Voyager 2 reached Uranus in 1986, and Neptune in 1989. Uranus' moon, Miranda, mystified astronomers with its bizarre terrain. Neptune's moon, Triton, showed an atmosphere similar to Titan's. Because it is so thin, we can see Triton's surface. Sagan concluded that the only life in our solar system is on Earth. He envisioned the Voyagers flying into the future, long after humans have become extinct or evolved into something else.
Sagan was a planet guy, and he wrote about Venus. The ancients thought the evening star and the morning star were different. They did not realize it was Venus circling the sun. Venus is permanently overcast. Its clouds reflect sunlight.
Sagan assumed Mars once had water. He wrote that things went wrong about 3.8 billion years ago, that the atmosphere thinned and the rivers dried up. His view is the predominant one today. Sagan wanted to explore Mars with robots, and such has been the case.
Although Sagan saw the end of the Cold War and increased cooperation between the United States and Russia, he died before 9/11. Muslim terrorists created a paranoia that did not exist in the 1990s. Only recently has public interest in Mars been revived. Sagan proposed that a manned mission to Mars might involve cooperation between the spacefaring nations.
Sagan devoted a chapter to Apollo 11. He was impressed by the moonwalks and noted that Apollo was more about politics than science. To control space was to control Earth!
Sagan wrote about the Search for Extraterrestrial Intelligence and how radio waves might be evidence of an alien civilization. The SETI Institute is in Mountain View, California.