Johannes Hevelius. Historical engraving of Johannes Hevelius (1611-1687), the Polish-German astronomer, in the 1660s with his quadrant. The quadrant was used for measuring star angles. At this time it was known that accurate timekeeping was essential in astronomical observation as shown here by the three clocks (top right and far left). Hevelius opened his own observatory and published several major works on observational astronomy. These included Selenographia (Pictures of the moon) published in 1647, Cometographia (1668), a list of previously observed comets and an accurate star catalogue, Prodomus Astronomiae, despite refusing to use a telescope to study the stars.