Mysterium Cosmographicum lit.
       The Cosmographic Mystery,
alternately translated as                Cosmic Mystery,
The Secret of the World,
or some variation is an astronomy book
                                            by
                     German astronomer Johannes Kepler,
published at Tübingen in 1596
& in a second edition in 1621;
The full title being Forerunner of the Cosmological Essays,
Which Contains the Secret of the Universe;
on the Marvelous Proportion of the Celestial Spheres,
              & on the True & Particular
Causes of the Number,   Magnitude,
& Periodic Motions of the Heavens;
Established by Means of the Five Regular
                          Geometric Solids [Latin:
                                  Prodromus dissertationum cosmographicarum,
continens mysterium cosmographicum,
de admirabili                            proportione orbium coelestium,
de que causis coelorum numeri,                            magnitudinis,
motuumque periodicorum genuinis & proprijs,  demonstratum,
quinque regularia corpora geometrica].
Kepler proposed that the distance relationships
       between the six planets known at that time
       could be understood in terms
          of the five Platonic solids,
           enclosed within a sphere
that represented the orbit of Saturn