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Rieger et al (1984) reported observations of a 154 day periodicity in flaresduring solar cycle 21. This paper discusses the observations in the light of asimple empirical planetary model of sunspot emergence. The planetary modelpredicts sunspot emergence when Mercury and Earth approach conjunction andMercury approaches the Sun. We show that the reported times of flare activityare coherent with the planetary model. While the base period of the model is170 days, the average model period, over the interval of flare recordings, is157 days due to a 180 degree phase change in the planetary forcing near themiddle of the record interval. We conclude that the periodicity at 154 daysarises when the phase change in planetary forcing and the resulting progressivephase change in total sunspot area emergence and flare occurrence shifts themajor peak in the flare spectrum from the planetary forcing period, 170 days,to 154 days.

Revisiting 154-day periodicity in the occurrence of hard flares. A planetary influence?