Although Ptolemy was an astronomer and his system was a theory about the nature of the universe, he and his system are important to European history. That is because the overturning of the Ptolemaic System was one of the most important things that happened during the Renaissance.
Ptolemy's system was a geocentric theory of the universe. That is, Ptolemy argued that the Earth was the center of the universe. Around it, in concentric circles, moved the various heavenly bodies. The Earth was not exactly in the center of their orbits, he argued, but they do orbit around it. Ptolemy's system was based on mathematical calculations and observations, but it was, of course, flawed.
Ptolemy's system was taken as truth until Copernicus came up with a heliocentric theory of the universe in the 1500s. Copernicus's discovery was one of the triumphs of the new scientific method that came to be one of the bases of Renaissance thought. Therefore, the Ptolemaic System is important to the study of European History.