The Greenwich Photo-heliographic Results (1874 – 1885): Observing Telescopes, Photographic Processes, and Solar Images

D. M. Willis,G. M. Appleby,M. N. Wild,L. T. Macdonald

doi:10.1007/s11207-016-0894-1

Abstract

Potential sources of inhomogeneity in the sunspot measurements published by the Royal Observatory, Greenwich, during the early interval 1874 – 1885 are examined critically. Particular attention is paid to inhomogeneities that might arise because the sunspot measurements were derived from solar photographs taken at various contributing solar observatories, which used different telescopes, experienced different seeing conditions, and employed different photographic processes. The procedures employed in the Solar Department at the Royal Greenwich Observatory (RGO), Herstmonceux, during the final phase of sunspot observations provide a modern benchmark for interpreting the early sunspot measurements. The different observing telescopes used at the contributing solar observatories during the interval 1874 – 1885 are discussed in detail, using information gleaned from the official RGO publications and other relevant historical documents. Likewise, the different photographic processes employed at the different solar observatories are reviewed carefully. The procedures used by RGO staff to measure the positions and areas of sunspot groups on photographs of the Sun having a nominal radius of either four or eight inches are described. It is argued that the learning curve for the use of the Kew photoheliograph at the Royal Observatory, Greenwich, actually commenced in 1858, not 1874. The RGO daily number of sunspot groups is plotted graphically and analysed statistically. Similarly, the changes of metadata at each solar observatory are shown on the graphical plots and analysed statistically. It is concluded that neither the interleaving of data from the different solar observatories nor the changes in metadata invalidates the RGO count of the number of sunspot groups, which behaves as a quasi-homogeneous time series. Furthermore, it is emphasised that the correct treatment of days without photographs is quite crucial to the correct calculation of Group Sunspot Numbers.

Highlights

Characterising the varying levels of solar activity is complicated by the fact that there are currently two main sunspot-number indices: i) the International Sunspot Number and ii) the Group Sunspot Number (Cliver, Clette, and Svalgaard, 2013; Clette et al, 2014; Cliver et al, 2015)
In view of the serious nature of these criticisms of the early sunspot observations conducted under the aegis of the Royal Observatory, Greenwich, the results presented in Figure 3 of the article by Cliver et al (2015) have been reinvestigated
The procedures employed in the Solar Department of the Royal Greenwich Observatory (RGO) during the final phase of the programme of sunspot observations (1960s – 1976) provide a modern benchmark for interpreting the early sunspot measurements (Section 2)

Summary

Introduction

Characterising the varying levels of solar activity is complicated by the fact that there are currently two main sunspot-number indices: i) the International Sunspot Number and ii) the Group Sunspot Number (Cliver, Clette, and Svalgaard, 2013; Clette et al, 2014; Cliver et al, 2015). As discussed in detail in the companion article by Willis, Wild, and Warburton (2016), the programme of sunspot observations conducted under the aegis of the Royal Observatory, Greenwich, which later became the Royal Greenwich Observatory (RGO), plays a key role in the derivation of Group Sunspot Number for two main reasons (Hoyt and Schatten 1998a, 1998b). Willis, Wild, and Warburton (2016) derived a new dataset for the RGO count of the number of sunspot groups for the interval 1874 – 1885, which differs from that published by Hoyt and Schatten (1998a, 1998b)

Objectives

Discussion

Conclusion