Humans have been closely observing the sky for millennia. This exhibit from the Library of Congress, entitled “Understanding the Cosmos: Changing Models of the Solar System and the Universe” includes primary sources from the ancient Greeks to the Medieval Arabic astronomers, and beyond. All files are available by HTML and for free PDF download. Begin with the astonishing 15th century Arabic manuscript by Abd Al Raḥman Ibn Umar. Look at original drawings of solar and lunar eclipses by Galileo and Copernicus. Educators will find the Teacher’s Guide, a short compendium to the exhibit, especially useful.