4. The diagrams
Fig. 3 illustrates the diagram for resulting from applying a linear ephemeris to the over the last 150 years-that is, the time interval with an almost uninterrupted sequence of maxima and minima. The shape of the curve is cyclic, not strictly periodic. The 123 maxima wander between 6 weeks early and about 6 weeks late, in accord with Maraldi's early findings and with Argelander's attempt to fit a cyclic term. The minima follow the maxima by days on average. The agreement between the two trends is very good, while short-term differences may be due to measurement errors and cycle-to-cycle jitter in the period.
Fig. 4 illustrates the diagram resulting from applying a linear ephemeris over the time span of three centuries. For the sake of illustration, we have highlighted datapoints with that follow about the same sampling distribution as the data belonging to the cycle interval 1-140.
Fig. 5 shows cycle lengths derived from 110 successive times of light maximum of Cygni for the last 150 years. The d 7, and the apparent standard deviation is 8:d9. Successive average period length is 408: maxima are connected by straight lines, the continuous curve is the running average (bin length 7 datapoints). The diagram shows sequences of intervals where the cycle length seems to be highly erratic ( d over 7 consecutive cycles: long cycles are typically followed by short ones) or much less fluctuating ( d over 7 consecutive cycles). The moving average also suggests that some real period variation is present.
© European Southern Observatory (ESO) 1999
Online publication: December 22, 1998