sentences:  In 1906 he began researching problems bordering both astronomy and geophysics, starting with the warming of the Earth by the Sun.We owe mainly to Hopfner the sharp distinction he made between the daily and seasonal average irradiation.In 1927 he went into the subject in more detail, laying out his discoveries in his "Mathematical Foundations of an Astronomical Theory of Climatic Variation" ("Mathematische Grundlagen zu einer astronomischen Theorie der Klimaschwankungen"), which won him the "Seegenpreis".