Centaurus 1967/11 [antikvár]

The Moon's Orbit in Kepler's Somnium by EDWARD ROSEN* Johannes Kepler (1571-1630) wrote the first serious scientific study of the moon in his posthumous Somnium , a little work consisting of two parts, a brief text and numerous footnotes. The "text mentioned in passing that the duration of lunar days and nights fluctuated slightly", according to a recent book2. Let us look at this passing mention: All over Levania [as the moon is designated in Kepler's Somnium] the days are almost exactly equal to the nights. The discrepancy is that each day is uniformly shorter than its night for the Privolvans [Kepler's designation for the lunar hemisphere perpetually turned away from the earth], and for the Subvolvans [the lunar hemisphere perpetually turned toward the earth] each day is uniformly longer than its night. But what varies in a period of eight years will be discussed later on3. Before going on to Kepler's later discussion, let us recognize that what we have already read is in itself far too important to merit being described, in the words of our recent author, as having been "mentioned in passing". Now let us move ahead to Kepler's later discussion, where he deals with the apparent rotation of the entire heavens as viewed from the moon: That daily motion is not uniform even at the same hour of each day. On the contrary, sometimes it is slower, not only for the sun but also for all the fixed stars, and faster at the like hour of the day in the opposite season of the year. Moreover, that retardation shifts throughout the days of the year, so that at one time it occurs on a summer day, and at another time on a winter day, which in a different year had experienced the acceleration. One such cycle is completed in a period of a little less than nine years. Therefore the day sometimes becomes longer (through a natural retardation, not as * City University of New York.