Mayer is known for lunar observation, particularly in relation to navigation; he also drew superb lunar maps. Mayer wrote to Euler (who had already heard of the young man) about astronomical refraction and lunar theory; they corresponded for four years, and Euler helped him to a professorship. Lacaille and Mayer wrote to each other particularly about the 1761 Passage of Venus, each arguing that the other’s instrument was less accurate than his own (Mayer was right, though Lacaille wasn’t to know it). Lambert, interested like Mayer in colour perception, visited him in Göttingen, and extended his work.