[ 385 ] Geography and the World GED ® TEST SOCIAL STUDIES FLASH REVIEW _____________ • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • In 150 A.D. the Greek astronomer Ptolemy taught that the solar system was structured as shown in Figure 1. Much later, in the 16th century, the Polish astronomer Nicolai Copernicus proposed the structure shown in Figure 2. Saturn Fixed Stars Mars Jupiter Saturn Fixed Stars Sun Jupiter Venus Moon Mercury Mars Moon Earth Venus Mercury Earth Sun Figure 1. Ptolemy’s Solar System Figure 2. Copernicus’s Solar System The biggest difference between Ptolemy’s and Copernicus’s ideas is a. Ptolemy thought that Mars and Venus were comets, but Copernicus said they were planets. b. Ptolemy thought that Earth was at the center of the solar system, but Copernicus said that the sun was at the center. c. Ptolemy thought that Saturn was the most distant planet, but Copernicus said that there was another planet beyond Saturn. d. Ptolemy thought that the orbits of the planets were circular, but Copernicus said they were oval in shape.