Among the friends of Johannes Kepler (1571–1630), the name of David Fabricius (1564–1617) stands out for two reasons. First, as an observational astronomer he initiated the study of variable stars. Second, he was an ardent devotee of astrology. In keeping with the latter enterprise, he collected horoscopes. On 18 July 1602 Kepler sent his own horoscope to Fabricius. Dissatisfied with the lack of detail in what he had received, Fabricius pressed Kepler for additional information. In particular he wanted to know “on what day in the carnival season in the year [15]91 a fever attacked” Kepler.

Kepler's reply is preserved only in a copy prepared by a hired scribe. This copyist did not always understand what Kepler had written in his draft of the letter. In some cases the copyist made mistakes; in other cases he simply omitted what he could not read. Thus, where Kepler answered Fabricius's question about the fever in 1591, the surviving copy says: