connection with the existing constructions of n-dimensional spaces and non-Euclidean geometry, so his philosophical model bears only little resemblance with spacetime physics, as it was later developed by Minkowski. Light constancy and the principle of relative motion In the second half of the 19th century, there were many attempts to develop a worldwide clock network synchronized by electrical signals. For that endeavor, the finite propagation speed of light had to be considered, because synchronization signals could travel no faster than the speed of light. In his paper The Measure of Time (1898), Henri Poincaré described some important consequences of this process and explained that astronomers, in determining the speed of light, simply assumed that light has a constant speed and that this speed is the same in all directions. Without this postulate, it would be impossible to infer the speed of light from astronomical observations, as Ole Rømer did based on observations of the moons of Jupiter. Poincaré also noted that the propagation speed of light can be (and in practice often is) used to define simultaneity between spatially separate events: The simultaneity of two events, or the order of their succession, the equality of two durations, are to be so defined that the enunciation of the natural laws may be as simple as possible. In other words, all these rules, all these definitions are only the fruit of an unconscious opportunism. In some other papers (1895, 1900b), Poincaré argued that experiments like that of Michelson and Morley show the impossibility of detecting the absolute motion of matter, i.e., the relative motion of matter in relation to the aether. He called this the "principle of relative motion". In the same year, he interpreted Lorentz's local time as the result of a synchronization procedure based on light signals. He assumed that two observers who are moving in the aether synchronize their clocks by optical signals. Since they believe themselves to be at rest, they consider only the transmission time of the signals and then cross-reference their observations to examine whether their clocks are synchronous. From the point of view of an observer at rest in the aether, the clocks are not synchronous and indicate the local time , but the moving observers fail to recognize this because they are unaware of their movement. So, contrary to Lorentz, Poincaré-defined local time can be measured and indicated by clocks. Therefore, in his recommendation