Sunspots noticed accidentally – a preliminary survey

2024 August 9

John Simpson

A survey of British newspapers has uncovered 41 sightings of sunspots made by untrained people with no optical aid and without prompting. These truly accidental sightings mirror historical (pre-telescopic) western cases where there is no evidence of deliberate, intentional sunspot observation. The survey covered the period since 1874, when the Royal Observatory in Greenwich began compiling daily ledgers of sunspot sizes and positions from photoheliographic observations. This paper explains how candidate press reports were identified and then screened using objective criteria, and presents key results, in particular the circumstances under which the sightings were made, the sizes of the spots concerned, and indicators of solar activity (sunspot count, recurrence, geomagnetic storms) at the time. The survey found that 88 per cent of the accidental sightings were of sunspot groups whose whole spot projected areas exceeded 2,500 millionths of the visible solar disc, much greater than the 1,000 millionths usually quoted as a threshold for visual acuity. Furthermore, a spot ≥4,000 millionths in area was 100 times more likely to be noticed by accident than one with an area of 1,000–<2,000 millionths. Nearly all the accidental sightings were of recurrent groups and the associated mean sunspot number was >200. The possible implications of the results for the interpretation of historical sunspot reports are discussed.

Introduction

There are just 11 datable accounts of probable western sunspot sightings before the first telescopic observations by Galileo and Harriot, both in 1610.¹ None of these reports, the earliest being in 807, mention the use of any kind of light filter, such as smoked glass, and only one notes any optical aid (by Kepler in 1607, using a camera obscura).² Most notably, none result from an intentional search, in contrast to historical reports from East Asia (mainly China and Korea). Moreover, none suggest any familiarity with sunspots, so very probably all such western sightings were accidental, made by people casually noticing something puzzling, apparently either on, or in front of, the solar disc. Does an absence of careful observation devalue their reports? The opposite is argued in this paper – that because such accidental observations are associated with episodes of very large sunspots, they can reveal invaluable information about the level of solar activity at the time.

There are three measures of sunspot size in common use – (1) subtended angle (the visual diameter of a spot), (2) projected area in millionths of the visible disc (usually abbreviated to msd), and (3) the physical area, corrected for foreshortening, in millionths of the visible hemisphere (msh). The first two are directly related for circular spots, so that 1 arcmin is approximately 1,000 msd, the exact relationship varying slightly between the Earth’s perihelion and aphelion. For a spot located at the centre of the visible disc, 1,000 msd equates to 500 msh, assuming the Sun to be a perfect sphere, and this relationship varies with the spot’s distance from the centre. Any of these three measures may be used to express either the size of the umbra, or alternatively the sum of the umbra and penumbra, usually referred to as the ‘whole spot’ size.

A ‘whole spot’ area of 1,000 msd (or its equivalent) is usually quoted as the ‘canonical threshold’ for a sunspot, or sunspot group, to be visible to the unaided eye.^3,4,5 This is not the same as saying that such a spot would be noticed by accident. The major problem here is that it is difficult, if not impossible, to design an experiment where subjects with no experience must notice a sunspot accidentally, without prompting or any kind of optical protection or aid. Faced with this impasse, this paper explores a novel approach, where newspaper reports are critically examined for cases of truly accidental sightings.

An almost unbroken series of daily records of the projected and corrected areas (and positions) of sunspots and sunspot groups is available from 1874 Apr, when photoheliographic observations began at the Royal Observatory, Greenwich (renamed the Royal Greenwich Observatory, or RGO, from 1948).⁶ Compatible records from other observatories, for example at Dehra Dun in India and at the Cape of Good Hope in South Africa, were routinely sent to Greenwich, and were used to infill days when cloud or fog prevented observation there. Observations at Greenwich itself ceased in 1949 May, and the heliograph was moved to Herstmonceux Castle in Sussex, where observations continued, still under the RGO, until 1976 Dec, after which the Heliophysical Observatory at Debrecen in Hungary formally took over responsibility for continuing the RGO record. Daily observations have continued at several observatories worldwide and are collated at Debrecen – now available in an online archive.^7,8 In principle, it is possible to find the observed area of almost any spot seen with the unaided eye since 1874, to within several hours of that sighting.

All the group numbers in this paper are those assigned by the RGO, and all daily area values are those held in the Debrecen Archive. In the case of projected areas, the latter dataset differs very slightly (typically by <1 per cent) from those published in the original RGO annual ledgers. Mean corrected areas over the period during which a sunspot is visible are taken from the RGO ledgers, as these results do not feature in the Debrecen Archive.

A recalibration of sunspot areas, taking the period 1874–1976 as a baseline, was published by Mandal et al. (2020).^9,10 It followed a major research effort to obtain a homogenous, cross-calibrated daily dataset of spot group positions and whole spot areas (both projected and corrected) covering the whole period 1874–2019. This valuable dataset allows sunspot areas to be readily analysed statistically. For the baseline period 1874–1976, the areas match those in the Debrecen Archive, at least for the spot groups that feature in this paper.

Prior to 1874, there appear to be only two published compilations giving the areas of individual sunspot groups, which cover observations at Kew during 1862–1866.^11,12 These compilations have not been used here.

The paper begins with a brief resumé of previous studies of the limiting size of sunspots that can be seen with the unaided eye. A summary of possible cases of accidental sightings appearing in the scientific literature is then presented. It goes on to describe the survey of British newspaper reports, outlining the nature of the source material and the search method. The criteria that were used to distinguish genuine accidental sightings from others are set out, and the paper outlines how these sightings can be cross-referenced to observations by professional astronomers. A summary table of the reported sightings is provided, followed by a section where their scientific properties are presented. The main finding is that 88 per cent of the accidental sightings found in this survey of British newspapers are of sunspot groups larger than 2,500 msd. The implications of the results for interpreting historical sunspot reports are then discussed, followed by brief suggestions for further research.

The limiting size of sunspots that can be seen with the unaided eye

Useful summaries of this topic can be found in Willis et al. (1996) and Vaquero & Vázquez (2009).^13,14 Careful observations by Mossman (1989) and Wade (1994) show that spots as small as 22–26 arcsec in diameter (about 130–190 msd) may be visible without a telescope under good viewing conditions.^15,16 Other detailed studies, notably by Keller & Friedli (1992), Schaefer (1993), and Neuhäuser et al. (2020), have supported the view that sunspots considerably smaller than the canonical threshold can be observed with the unaided eye.^17,18,19

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