The astronomer Ole Rømer emphasized the mechanical nature of the practice of astronomy and this paper attempts to unravel what Rømer meant by the close association between mechanics and astronomy. The point of departure is Rømer's work with Tycho Brahe's observations and his stay at the Royal Academy of the Sciences in Paris. Analyses of Rømer's letters and treatises show that he not only focused on direct presentations of observations and instruments, but demanded an independence of his results that went beyond the rhetoric of first-hand knowledge. Rømer wanted his observations and instruments to demonstrate the physical theories themselves, rather than asking for theories to be accepted on his authority. His invention of the meridian circle with its facility for cross-checking measurements is seen in connection with this drive to make what is observed ‘deserve credit in its own right rather than because of the observer.’