The difficulty of measuring space and time
Notes
In antiquity, the positions of the stars could be measured, albeit crudely. Observations had shown that when one travelled north or south, the stars visible on any night of the year change. Further north, stars within a broad circle around the Pole Star are visible at all seasons, that is, they never set below the horizon. As you go south, this circle shrinks, but a greater number of equatorial stars become visible only sometimes and at some seasons. Contrastingly, when travelling east and west it was found that the same stars were visible at all locations along what could thus be defined as a line (really a circle) of latitude.
Similarly, measurements of the shadow cast by a gnonom – an upright stick in full sun – would give different results at different latitudes. By travelling up the River Nile past Aswan, one entered the tropical region, where the summer sun …