Metallographic examination of the brass in an astrolabe made in Nuremberg in 1537 shows it to be an alloy of copper and 19% zinc containing about 2% of iron, lead, and sulfur impurities. The outer ring of the astrolabe is a casting but the other parts were cut from hammered sheet that was fully annealed before use. The brass is of remarkably good quality except for surface defects in the sheet. The lead and sulfur impurities are present as inclusions visible in the microstructure. The lead particles are small and are uniformly dispersed. The sulfur is present in complex copper-zinc sulfides found in clusters or bands in the microstructure. These inclusions were probably formed during the cementation process by which the brass was made.