Re-examination of the Daily Number of Sunspot Groups for the Royal Observatory, Greenwich (1874-1885)

The daily number of sunspot groups on the solar disk, as recorded by the programme of sunspot observations performed under the aegis of the Royal Observatory, Greenwich, UK, and subsequently the Royal Greenwich Observatory (RGO), is re-examined for the interval 1874 - 1885. The motivation for this re-examination is the key role that the RGO number of sunspot groups plays in the calculation of Group Sunspot Numbers (Hoyt and Schatten in Solar Phys. 179, 189, 1998a; Solar Phys. 181, 491, 1998b). A new dataset has been derived for the RGO daily number of sunspot groups in the interval 1874 - 1885. This new dataset attempts to achieve complete consistency between the sunspot data presented in the three main sections of the RGO publications and also incorporates all known errata and additions. It is argued that days for which no RGO solar photograph was acquired originally should be regarded, without exception, as being days without meaningful sunspot data. The daily number of sunspot groups that Hoyt and Schatten assign to days without RGO photographs is frequently just a lower limit. Moreover, in the absence of a solar photograph, the daily number of sunspot groups is inevitably uncertain because of the known frequent occurrence of sunspot groups that exist for just a single day. The elimination of days without photographs changes the list of inter-comparison days on which both the primary RGO observer and a specified secondary comparison observer saw at least one sunspot group. The resulting changes in the personal correction factors of secondary observers then change the personal correction factors of overlapping tertiary observers, etc. In this way, numerical changes in the personal correction factors of secondary observers propagate away from the interval 1874 - 1885, thereby potentially changing the arithmetical calculation of Group Sunspot Numbers over an appreciably wider time interval.