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A Transit of Venus Possibly Misinterpreted as an Unaided-Eye Sunspot Observation in China on 9 December 1874

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Pith's one-line read The paper argues that the 'black spot in the Sun' recorded in Chinese sources on 9 December 1874 was Venus transiting the solar disk, not a sunspot.

desk verdict A careful, honest case that the 1874 Chinese 'black spot' was Venus; the lost Harvard plate is a real caveat but not a fatal one. read the letter →

arxiv 1908.02452 v1 pith:5NJUOKJH submitted 2019-08-07 astro-ph.SR physics.hist-ph

Hisashi Hayakawa , Mitsuru Sôma , Kiyotaka Tanikawa , David M. Willis , Matthew N. Wild , Lee T. Macdonald , Shinsuke Imada , Kentaro Hattori
show 1 more author
F. Richard Stephenson
This is my paper · ORCID
classification astro-ph.SRphysics.hist-ph
keywords transitofVenusunaided-eyesunspotobservationsnaked-eyerecordsChinesegazetteersareathresholdsolarlimbvisibilityhistoricalastronomy
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

On 9 December 1874, Chinese sources recorded a "black spot in the Sun." This paper argues that the spot was not a sunspot but the silhouette of Venus crossing the solar disk during a transit. The paper reconstructs the transit's local timing and compares Venus's projected area, about 1048 millionths of the solar disk, with the day's photographed sunspot groups, whose largest whole-spot areas were only 80, 115, and 121 millionths, with essentially no umbra. Because the transit was visible across East Asia but not in the western hemisphere, the discrepancy between the Chinese sighting and Western photographic records disappears. If correct, the result restores the original date of a previously "negative" naked-eye sunspot record and calibrates what a dark object near the limb of the Sun must look like to be seen without a telescope.

What carries the argument

The argument turns on comparing projected areas on the solar disk. From the transit ephemeris, the paper obtains Venus's apparent diameter of 1.052 arcminutes and translates it into a projected area of about 1048 msd. It then sets that figure against the day's photographic sunspot measurements, three groups with whole-spot projected areas of 80, 115, and 121 msd and umbral areas at most 5 msd, and against published naked-eye visibility thresholds of roughly 452 msd for a whole spot and 61 msd for an umbra near disk center. The area comparison is supplemented by geometry: the paper computes the transit's visibility zone, contact times, and the "black spot's" position near the solar limb, showing East Asia lay in the visible region while the observatories that supplied the photographs did not. This combination of area, timing, and visibility zone is what identifies the recorded spot as Venus.

What would settle it

Re-examine the original solar photographs or glass plates for 9 December 1874, or seek independent contemporary sunspot drawings made in the eastern hemisphere, to confirm no large spot was present; alternatively, determine from the wording and timing in the Chinese sources whether the 'black spot' moved across the disk over roughly four hours, as Venus did, rather than remaining fixed as a sunspot would.

Watch

Extended reading notes

Core claim

The central claim is that the "black spot in the Sun" reported on 9 December 1874 in two Chinese local gazetteers and a contemporary diary was the disk of Venus transiting the Sun, not a sunspot. The paper computes Venus's apparent diameter during the transit as 1.052 arcminutes, corresponding to a projected area of about 1048 msd (millionths of the solar disk), and reconstructs the transit's path across the solar disk as seen from the Chinese observing sites, where it ran from about 09:50 to 14:34 local mean time at a solar altitude of 24 to 37 degrees. The photographic record for that day lists three small sunspot groups with whole-spot projected areas of 80, 115, and 121 msd and an aggregated umbral area of at most 5 msd, far below the nominal naked-eye thresholds of about 452 msd for whole-spot area and 61 msd for umbral area. The paper therefore concludes that the Chinese observers saw Venus, whose sharp, perfectly dark silhouette had an area roughly ten times the largest sunspot group and was located near the solar limb, around 54 to 58 degrees of heliographic latitude. This interpretation explains why the Chinese record had no counterpart in western photographic observations: the transit was invisible in the western hemisphere.

Load-bearing premise

The conclusion rests on the completeness and accuracy of the photographic sunspot record for 9 December 1874; if a large sunspot was missed or mis-measured that day, the black spot could still have been a sunspot rather than Venus.

Editorial extensions

If this is right

  • The 9 December 1874 Chinese record is reassigned from an unexplained naked-eye sunspot to a correctly dated observation of Venus, removing one of the apparent failures in the comparison between Chinese records and photographic sunspot data.
  • The same method can be applied to the remaining negative pairs, particularly 15 February 1900 and 30 January 1911, to test whether they too have non-sunspot explanations or genuinely require scribal-date corrections.
  • A dark silhouette with a projected area near 1000 msd is comfortably visible to the naked eye under suitable conditions, even close to the solar limb; this quantifies the detectability of large spots or planet silhouettes in historical records.
  • Local gazetteer 'black spot' entries should not be assumed to come from trained court astronomers; they may record any sufficiently dark obscuration of the Sun, including Venus.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • A natural next test is to screen every pre-telescopic Chinese 'black spot' record against known transit dates of Venus and Mercury; the 1874 case shows a planet silhouette can masquerade as a sunspot in local gazetteers.
  • If the cloud-veil mechanism is general, naked-eye sunspot records may preferentially survive from regions and seasons with thin cloud or haze, implying a weather-dependent selection effect in the historical archive.
  • The sharp-edged, fully dark Venus silhouette may be visible at smaller projected areas than a real sunspot, whose umbra is only 10 to 40 percent as bright as the photosphere; if so, the 1000-msd figure overstates the naked-eye threshold for genuine spots.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

1 major / 3 minor

Summary. The paper examines three Chinese historical records dated 9 December 1874 that report a 'black spot in the Sun', and proposes that these observations were not of a sunspot but of the transit of Venus, which was visible in East Asia on that date. The authors reconstruct the transit geometry using the JPL DE430 ephemeris, compute the apparent projected area of the Venus silhouette on the solar disk as about 1048 millionths of the solar disk (msd), and compare this with the visibility threshold for unaided-eye sunspot observations (approximately 61 msd umbral and 452 msd whole-spot). They then use the Greenwich Photoheliographic Results (GPR) Ledgers for 9 December 1874, derived from a Harvard College Observatory plate, to show that the three sunspot groups present had whole-spot areas of only 80, 115, and 121 msd and essentially no umbra (at most 5 msd). The paper concludes that the Chinese observers probably misinterpreted the dark silhouette of Venus as a sunspot, thereby resolving one of the 'negative pairs' identified by Willis, Davda, and Stephenson (1996).

Significance. If the interpretation is correct, the paper resolves a long-standing discrepancy between a Chinese unaided-eye solar observation and contemporary telescopic records, and it strengthens the case that dark silhouettes of roughly 1000 msd on the solar disk are visible to the unaided eye under suitable conditions. The analysis is transparent and reproducible: it uses a modern ephemeris, explicit unit conversions, and documented historical sources, with no fitted parameters or circular adjustments. The paper also appropriately distinguishes local gazetteer records from more reliable astronomical reports and sets a foundation for reconsidering other historical 'negative pairs'. The main significance lies in the historical method: it shows how careful astronomical reconstruction can resolve apparent contradictions in archival records, while also flagging the caution needed when using such records as solar-activity proxies.

major comments (1)
  1. [Section 6 and Section 9] The conclusion that no sunspot large enough for unaided-eye visibility existed on 9 December 1874 rests entirely on the Ledgers of the Greenwich Photoheliographic Results, which for that date are derived from a single Harvard College Observatory plate that the authors themselves state 'have probably been lost' (Section 9). Because the plate cannot be re-examined, one cannot fully exclude the possibility of a transcription error, incomplete disk coverage, or a missed large sunspot group. This is load-bearing for the central claim, because if a large spot (projected whole-spot area at least roughly 452 msd, or umbral area at least roughly 61 msd) had been present but unrecorded, the Chinese 'black spot' could equally be an ordinary sunspot and the Venus-transit interpretation would lose its necessity. The authors should either (a) search for independent contemporary telescopic observations of the solar disk on that date (for example, in the reports of the many transit-of-Venus expeditions in East Asia, or from other GPR contributing stations such as Melbourne, India, or Mauritius) and report the results, or (b) explicitly state that no such independent records have been located and temper the claim in Section 9 from 'it can be concluded that ... was not a sunspot' to a conditional formulation such as 'the most plausible interpretation is ...'.
minor comments (3)
  1. [Abstract and Section 9] The abstract uses cautious language ('possibly captured', 'probably misinterpreted') but Section 9 states 'it can be concluded that the black spot ... was not a sunspot but Venus transiting the solar disk.' These levels of certainty should be aligned, especially given the acknowledged loss of the Harvard heliogram and the absence of an independent verification of the sunspot areas.
  2. [Section 2 and Section 6] Section 2 states 'We then examine the contemporary sunspot observations using telescopes', but Section 6 for 9 December actually uses only the Greenwich Photoheliographic Results. If other contemporary observations (e.g., from observatories other than Harvard) were consulted and found to contain no data for that day, this should be stated; if they were not consulted, the sentence should be narrowed to avoid implying a broader survey.
  3. [Section 7] The statement that this case 'confirms the visibility of sunspots near the solar limb with the unaided eye' is stronger than what the evidence supports: the Chinese records do not specify the position of the spot on the disk, and the near-limb location is an inference from the computed Venus path. Recommending 'is consistent with' or 'provides evidence for' would be more accurate.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the Venus-transit reinterpretation is tested against independent ephemeris data, contemporary reports, and published sunspot-area measurements.

full rationale

The paper's central claim is that the Chinese 'black spot in the Sun' on 9 December 1874 was the silhouette of Venus transiting the solar disk, not a sunspot. This inference is assembled from three independent inputs, none of which is fitted to the Chinese record or defined in terms of the conclusion. (1) The transit itself is established by JPL-DE430 ephemeris computations and by contemporaneous Western observations in Beijing, Yokohama, and Nogeyama, with observed-minus-calculated contact times within roughly one minute (Section 5, Tables 1-3). (2) The projected area of Venus on the solar disk, 1048 msd, is computed from standard planetary and solar diameters and the Earth-Venus distance, independently of the historical record. (3) The alternative sunspot explanation is ruled out using the Greenwich Photoheliographic Results Ledgers, which list whole-spot projected areas of 80, 115, and 121 msd for the three groups on that date and an aggregated umbral area of at most 5 msd, all below the external unaided-eye thresholds of 61 msd (umbral) and 452 msd (whole-spot) taken from Keller and Friedli (1992). The conclusion is not equivalent to any one of these inputs by construction; it is a comparative inference. The paper cites prior work by co-author Willis (Willis, Davda, and Stephenson 1996) to set up the puzzle and to provide the sunspot-area ledger values, but the cited work is not used as a black-box proof of the conclusion. On the contrary, the paper argues against that prior work's preferred transcription-error interpretation and offers an independent astronomical explanation. The admission in Section 9 that the Harvard heliograms have probably been lost is a genuine data-completeness limitation, not circularity: it affects the strength of the negative sunspot premise but does not make the derivation feed on its own target. No fitted parameter is relabeled as a prediction, no uniqueness theorem is imported from the authors' prior work, and no known result is merely renamed. The central claim therefore has independent content and is not circular.

Assumptions & free parameters 0 free parameters · 4 assumptions · 0 invented entities

The paper introduces no new physical entities or free parameters. It relies on standard astronomical ephemerides, published sunspot area measurements, and historical assumptions about how the Chinese record was produced. The main unverified inputs are the completeness of the 1874 sunspot data and the behavior of the local observers.

assumptions (4)
  • domain assumption The Chinese date '同治十三年十一月朔日' corresponds to 9 December 1874 and the observation refers to that daytime.
    The lunar-to-Gregorian conversion is standard, and the diary by Guo Songdao gives an entry for every date, supporting the dating. This is used in Section 3.
  • domain assumption The naked-eye visibility thresholds (whole-spot 452 msd, umbral 61 msd) from Keller and Friedli (1992) are applicable to this historical case.
    The argument that sunspots were too small and Venus was visible depends on these thresholds. The paper acknowledges they are mathematical estimates. Section 6.
  • standard math The JPL DE430 ephemeris and Delta T = -3.2 s give accurate positions of Venus and the Sun for 9 December 1874.
    Used to compute the transit contact times, geometry, and the Venus silhouette area. Agreement with observed contact times to within 60 seconds supports this assumption.
  • domain assumption The Greenwich Photoheliographic Results Ledgers accurately record the sunspot groups present on 9 December 1874.
    The conclusion that no sunspot was large enough to see requires trusting the tabulated areas for Groups 138, 138*, and 140. This is the load-bearing data premise discussed in Section 6.

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Cite this review

Pith. "Pith review of A Transit of Venus Possibly Misinterpreted as an Unaided-Eye Sunspot Observation in China on 9 December 1874." pith.science (2026). https://pith.science/paper/5NJUOKJH

@misc{pith2026190802452,
  author       = {Pith},
  title        = {Pith review of: A Transit of Venus Possibly Misinterpreted as an Unaided-Eye Sunspot Observation in China on 9 December 1874},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/5NJUOKJH}},
  note         = {Machine review of arXiv:1908.02452}
}
read the original abstract

Large sunspots can be observed with the unaided eye under suitable atmospheric seeing conditions. Such observations are of particular value because the frequency of their appearance provides an approximate indication of the prevailing level of solar activity. Unaided-eye sunspot (UES) observations can be traced back well before the start of telescopic observations of the Sun, especially in the East Asian historical records. It is therefore important to compare more modern, UES observations with the results of telescopic sunspot observations, to gain a better understanding of the nature of the UES records. A previous comparison of Chinese UES records and Greenwich photo-heliographic results between 1874 and 1918 indicated that a few of the UES were apparently not supported by direct photographic evidence of at least one sunspot with a large area. This article reveals that one of the Chinese unaided-eye observations had possibly captured the transit of Venus on 9 December 1874. The Chinese sunspot records on this date are compared with Western sunspot observations on the same day. It is concluded that sunspots on the solar disk were quite small and the transit of Venus was probably misinterpreted as a sunspot by the Chinese local scholars. This case indicates that sunspots or comparable "obscuring" objects with an area as large as 1000 millionths of the solar disk could easily have been seen with the unaided eye under suitable seeing conditions. It also confirms the visibility of sunspots near the solar limb with the unaided eye. This study provides an explanation of the apparent discrepancy between the Chinese UES observation on 9 December 1874 and the Western sunspot observations using telescopes, as well as a basis for further discussion on the negative pairs in 1900 and 1911, apparently without sufficiently large area.

Figures

Figures reproduced from arXiv: 1908.02452 by the authors.

Figure 1
Figure 1. Drawing of the transit of Venus observed at Bĕijīng by James Craig Watson (PM: v.28, f.27a). Note that this drawing is not very precise. The minimum center-to-limb distance is slightly smaller than the calculated value [PITH_FULL_IMAGE:figures/full_fig_p021_1.png] view at source ↗
Figure 2
Figure 2. Photographs of Venus transit taken at Nogeyama by Covarrubias ( [PITH_FULL_IMAGE:figures/full_fig_p022_2.png] view at source ↗
Figure 3
Figure 3. Visible zone of the Venus transit in 1874 [PITH_FULL_IMAGE:figures/full_fig_p023_3.png] view at source ↗
Figures from the paper (1 more)
Figure 4
Figure 4. Figure 4: Reconstruction of the transit of Venus in 1874, as observed from Shíménxiàn [PITH_FULL_IMAGE:figures/full_fig_p024_4.png]

Discussion (0). Continue with ORCID to comment.

Reference graph

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Reviewed August 14, 2026 · model on record in the stance chip above.