2. The period of Cygni
To Gottfried Kirch, Cygni was a regular variable with a period of 404:d5 days, but in 1713 already Maraldi found a period of 405 days and pointed out that deviations of the cycle length of one month do occur, so that the time interval in between two maxima at some times would be 13 months and at other times 14 months. Indications that Cygni's period was lengthening, were also given by other observers (Le Gentil 1759, Pigott 1786, Koch 1802).
Olbers (1816) was the first who tried to mathematically describe the period lengthening (through his first-time application of the least-squares method; the procedure was communicated to him by Gauss in 1803). From a number of well-determined maxima from 1687 to 1815 Olbers (1841) also derived a quadratic ephemeris-though some cycles with deviations amounting to almost four weeks were not considered because Olbers regarded these deviations as irregularities of the period that could not be explained by his formula. The next to make a deep investigation of the period was Argelander (1869), who replaced the second-order term in Olbers' solution by a periodic term. Chandler (1894), then, combined Olbers' and Argelander's formulae by taking into account the quadratic as well as the cyclic term, but a satisfactory formal description for the period changes was never found.
Rosenberg (1906), in an extensive investigation of the star's light variations, confined himself to a representation with a quadratic term only, thus his elements turn out to be very close to those obtained by Olbers.
© European Southern Observatory (ESO) 1999
Online publication: December 22, 1998