Star catalogues in the 16th century were all inspired by Ptolemy and contained discrepancies. These were due to mistakes made by printers when copying and republishing the catalogues, resulting in differences in the stars’ given positions.

There are variations between the globe published by Gemma Frisius (1537) and the one published by Mercator (1551), for example in the representation of Aires and Gemini. This suggests that Mercator drew on different sources to indicate stars’ positions. Since his globe was published later, he no doubt believed that these positions were more appropriate.