  Rieger et al (1984) reported observations of a 154 day periodicity in flares
during solar cycle 21. This paper discusses the observations in the light of a
simple empirical planetary model of sunspot emergence. The planetary model
predicts sunspot emergence when Mercury and Earth approach conjunction and
Mercury approaches the Sun. We show that the reported times of flare activity
are coherent with the planetary model. While the base period of the model is
170 days, the average model period, over the interval of flare recordings, is
157 days due to a 180 degree phase change in the planetary forcing near the
middle of the record interval. We conclude that the periodicity at 154 days
arises when the phase change in planetary forcing and the resulting progressive
phase change in total sunspot area emergence and flare occurrence shifts the
major peak in the flare spectrum from the planetary forcing period, 170 days,
to 154 days.
