We can here leave out of account the fact that Roemer’s reasoning is based on the assumption that the Copernican conception of the relative movements of the members of our solar system is the valid conception, an assumption which, as later considerations will show, cannot be upheld in a science which strives for a truly dynamic understanding of the world. For the change of aspect which becomes necessary in this way does not invalidate Roemer’s observation as such; it rules out only the customary interpretation of it. Freed from all hypothetical by-thought, Roemer’s observation tells us, first, that the time taken by a flash of light travelling from a cosmic light-source to reach the earth varies to a measurable extent, and, secondly, that this difference is bound up with the yearly changes of the earth’s position in relation to the sun and the relevant planetary body.