What is the ideology that propels scientists to go to so much trouble? Think, for example, of the hazards involved in a voyage from Europe to our part of the world in the 18th century. Why would you go to all that effort just to observe the transit of Venus? This week, with the next transit just a few days away, we explore the philosophy of northern astronomy in the southern hemisphere with Simon Schaffer, professor of the history of science at the University of Cambridge.