Most people who went to Hay-on-Wye to the 2015 HowTheLightGetsIn festival will have used a road map to get there. It’s an accurate and useful tool for navigation, but it’s full of mis-representations. The map is accurately ‘to scale’ – so it’s possible, for example, to estimate the distance to Hereford. But there are anomalies – the roads on the map are far too wide, and the representative symbols are of course absent on the ground. This commonplace map is helpful and accurate in many useful ways, but we imposed onto the map our vision of the structure of the landscape and its contents that are simply not there, which is not to say that they are not useful.

Maps of the universe share similar qualities. When William and Caroline Herschel made their first maps of the Milky Way they were unaware of the presence of obscuring dust in the galaxy’s plane. They based their map on the rough proposition that the bright stars all have the same intrinsic brightness so when they found fewer bright stars in the direction of the Milky Way, and even fewer in the direction of the galactic centre, they concluded that it extended less far in those directions – the opposite of what we now believe. However the Herschel’s map correctly revealed a highly flattened figure for the Milky Way.