If you were an astronomer in the closing days of the Roman Empire (say,400 a.d.),your main reference book would probably have been:

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Quizplus · Accepted Answer

Ptolemy's Almagest was the most influential and comprehensive astronomical treatise of its time, and it continued to be the main reference for astronomers until the 16th century. Plato's Republic is a philosophical work and not specifically focused on astronomy. Copernicus's De Revolutionibus and Galileo's Dialogues Concerning the Two Chief World Systems were not written until the 16th and 17th centuries, respectively.

If You Were an Astronomer in the Closing Days of the Roman