The Greenwich Photo-heliographic Results (1874 – 1976): Summary of the Observations, Applications, Datasets, Definitions and Errors

Abstract

The measurements of sunspot positions and areas that were published initially by the Royal Observatory, Greenwich, and subsequently by the Royal Greenwich Observatory (RGO), as the Greenwich Photo-heliographic Results (GPR), 1874-1976, exist in both printed and digital forms. These printed and digital sunspot datasets have been archived in various libraries and data centres. Unfortunately, however, typographic, systematic and isolated errors can be found in the various datasets. The purpose of the present paper is to begin the task of identifying and correcting these errors. In particular, the intention is to provide in one foundational paper all the necessary background information on the original solar observations, their various applications in scientific research, the format of the different digital datasets, the necessary definitions of the quantities measured, and the initial identification of errors in both the printed publications and the digital datasets. Two companion papers address the question of specific identifiable errors; namely, typographic errors in the printed publications, and both isolated and systematic errors in the digital datasets. The existence of two independently prepared digital datasets, which both contain information on sunspot positions and areas, makes it possible to outline a preliminary strategy for the development of an even more accurate digital dataset. Further work is in progress to generate an extremely reliable sunspot digital dataset, based on the programme of solar observations supported for more than a century by the Royal Observatory, Greenwich, and the Royal Greenwich Observatory. This improved dataset should be of value in many future scientific investigations.

Tests of Sunspot Number Sequences: 4. Discontinuities Around 1946 in Various Sunspot Number and Sunspot-Group-Number Reconstructions

“…The total sunspot area was computed (corrected for limb foreshortening) [A G ] from the RGO dataset (also called the Greenwich Photoheliographic Results: GPR) (Baumann and Solanki, 2005;Willis et al, 2013aWillis et al, , 2013b Any remaining data gaps were filled using photographs from many other solar observatories, including the Mount Wilson Observatory, the Harvard College Observatory, Melbourne Observatory, and the US Naval Observatory. The sunspot areas were measured from the photographs with the aid of a large position micrometer (see Willis et al, 2013aWillis et al, , 2013b and references therein).…”

“…The sunspot areas were measured from the photographs with the aid of a large position micrometer (see Willis et al, 2013aWillis et al, , 2013b and references therein). The A G -values are the total sunspot area (umbrae plus penumbrae) and have been corrected for the effect of foreshortening, which increases as sunspots are closer to the limb of the solar disc.…”

Tests of Sunspot Number Sequences: 1. Using Ionosonde Data

“…The fraction of days for which there are no contact prints is considerably higher before 1885 (Willis, Wild, and Warburton, 2016). The extant contact prints form part of the official RGO Archives, which are stored in the Cambridge University Library (Willis et al 2013a;2013b) Most of the information available before 1874 is in the form of sketches of the solar disk and/or tabulated sunspot and/or sunspot-group counts compiled by observers using a telescope. (However, we note that even after 1918 sunspot numbers were frequently compiled without the use of photographic images.)…”

Sunspot number recalibration: The ~1840–1920 anomaly in the observer normalization factors of the group sunspot number

“…The parameters for these observers are given in Appendix 1 of their paper, a portion of which is reproduced in Figure 2. Here we focus on the parameter in column 5, the so-called k 0 -factor 2 that is used to scale secondary observers to the primary or reference observer used by Hoyt and Schatten -the Royal Greenwich Observatory (RGO) (Willis et al 2013a(Willis et al , 2013bErwin et al 2013;Willis et al 2016aWillis et al , 2016b. In the Hoyt & Schatten (1998a, 1998b observer normalization scheme, k 0 -factors are formed by dividing the total number of sunspot groups observed by RGO by the corresponding total of the secondary observer, limiting the comparison to those days when both observers reported a non-zero group count.…”