Leaving aside the archaic use of 'runneth' for 'runs', we have here a 'running' metaphor. The Sun is often said to 'run its daily course' especially by poets. This sounds far more interesting to most people than the utilitarian 'the Earth and the Sun revolve at what may be considered a fixed rate about the centre of mass of the two-body system'.
There are different definitions of 'month'. Calendar months are arbitrarily defined and variable. Lunar months are approximately equal to the four weeks of another definition, and far more nearly constant. However, even the term lunar month has been ascribed to different phenomenon-related intervals, of from about 27.3 to about 29.5 days. 28 days is some sort of average.