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Because of the lack of reliable sunspot observation, the quality of sunspotnumber series is poor in the late 18th century, leading to the abnormally longsolar cycle (1784--1799) before the Dalton minimum. Using the newly recoveredsolar drawings by the 18--19th century observers Staudacher and Hamilton, weconstruct the solar butterfly diagram, i.e. the latitudinal distribution ofsunspots in the 1790's. The sudden, systematic occurrence of sunspots at highsolar latitudes in 1793--1796 unambiguously shows that a new cycle started in1793, which was lost in traditional Wolf's sunspot series. This finallyconfirms the existence of the lost cycle that has been proposed earlier, thusresolving an old mystery. This letter brings the attention of the scientificcommunity to the need of revising the sunspot series in the 18th century. Thepresence of a new short, asymmetric cycle implies changes and constraints tosunspot cycle statistics, solar activity predictions, solar dynamo theories aswell as for solar-terrestrial relations.

A solar cycle lost in 1793--1800: Early sunspot observations resolve the old mystery