The Lorentz transformations in special relativity are derived from the invariance of \(x^ 2+y^ 2+z^ 2-c^ 2t^ 2\), which results from the principle of relativity and from the constancy of the speed of light. In the present paper the author investigates a mathematically similar situation in cosmology resulting from the invariance of a certain quantity \(x^ 2+y^ 2+z^ 2-a^ 2Z^ 2\), which is due to the cosmological principle and to Hubble's law. Here an observer finds (x,y,z,Z) for any event by the following method: (x,y,z) denote spatial coordinates such that \(x^ 2+y^ 2+z^ 2\) equals the distance between the event and the observer (obviously the author assumes tacitly a Robertson-Walker cosmos which is spatially flat) and Z is the redshift of the galaxy passing through the event seen by the observer. In the last part the method is generalized to spaces with constant (nonvanishing) curvature. - This short and rather elementary paper suffers from the fact, that the reader has to guess several things which the author does not say explicitly.