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When Knowledge Conquered Fear is the third episode of Cosmos A Spacetime Odyssey. It first aired 23 March 2014 on Fox.


Neil deGrasse Tyson describes how pattern recognition manifested in early civilization as using astronomy and astrology to predict the passing of the seasons, including how the passage of a comet was often taken as an omen. He explains that the origin of comets only became known in the 20th century due to the work of Jan Oort and his hypothesis of the Oort cloud.

Tyson relates the collaboration between Edmond Halley and Isaac Newton in the last part of the 17th century in Cambridge. This resulted in the publication of Philosophiæ Naturalis Principia Mathematica, the first major work to describe the laws of physics in mathematical terms, despite objections and claims of plagiarism from Robert Hooke and financial difficulties of the Royal Society of London. Tyson explains how this work challenged the prevailing notion that God had planned out the heavens, but eventually influenced many factors of modern life, including space flight.

Tyson describes Halley's contributions based on Newton's work, including determining Earth's distance to the Sun, the motion of stars and predicting the orbit of then-unnamed Halley's Comet using Newton's laws. Tyson contrasts these scientific approaches to understanding the galaxy compared to what earlier civilizations had done, and considers this advancement as humankind's first steps into exploring the universe. An animation of the Milky Way and Andromeda galaxies' merging is based on the principles of Newton's laws.