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arxiv: 1906.10884 · v1 · pith:AIVREKKMnew · submitted 2019-06-26 · 🌌 astro-ph.SR

Christian Horrebow's Sunspot Observations -- I. Life and Published Writings

Carsten S{\o}nderskov J{\o}rgensen , Christoffer Karoff , V. Senthamizh Pavai , Rainer Arlt This is my paper

Pith reviewed 2026-05-25 15:34 UTC · model grok-4.3

classification 🌌 astro-ph.SR
keywords sunspotssolar cycleChristian Horrebowhistorical observationsCopenhagenDansk Historisk Almanaksunspot records
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The pith

Christian Horrebow stated in 1775 that the Sun's spot number and size repeat after a certain number of years based on his observations from 1761 onward.

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

The paper reviews the life and writings of Christian Horrebow, who recorded sunspots daily from a tower in Copenhagen over fifteen years. In one 1775 publication he described the Sun's appearance as repeating with respect to spots after a fixed interval. This places an explicit suggestion of solar periodicity several decades before the later accepted discovery. The authors supply a full English translation of the relevant almanac entries along with tables of the daily counts and an assessment of how complete those records are. The work establishes Horrebow as an early observer capable of noticing long-term patterns in solar activity.

Core claim

Through sustained daily counts of sunspots from 1761 to 1776, Horrebow wrote in 1775 that the appearance of the Sun repeats itself after a certain number of years in both the number and size of spots, thereby advancing the notion of a cyclic Sun well before the period was measured.

What carries the argument

The 1775 statement in the Dansk Historisk Almanak that interprets the pattern of sunspot observations as repeating after a fixed interval.

Load-bearing premise

The 1775 almanac passage is read as a deliberate hypothesis of periodicity rather than an offhand remark, which depends on the translation and the surrounding sentences being accurate.

What would settle it

A new translation or fuller context from the original Danish almanac that shows the 1775 sentence does not claim the Sun repeats its spot pattern would remove the basis for the earlier hypothesis.

Figures

Figures reproduced from arXiv: 1906.10884 by Carsten S{\o}nderskov J{\o}rgensen, Christoffer Karoff, Rainer Arlt, V. Senthamizh Pavai.

Figure 1
Figure 1. Figure 1: Example of a drawing based on observations with Rota Meridiana. For the remainder of Christian Horrebow’s time as head of the observatory, Machina Æquatorea was the only instrument used to obtain coordinates for the observed sunspots. After Horrebow’s death in 1776, Rasmus Leivog (under Thomas Bugge’s leadership) continued to make observations with the method developed by Christian Horrebow until the end o… view at source ↗
Figure 2
Figure 2. Figure 2: Example of how sunspot observations are undertaken with Machina Æquatora. but his naming of the ‘preceding’, ‘lower’ and ‘upper’ limbs of the Sun fits this orientation. In general, we have very little information of the physical dimensions of the telescopes, such as aperture sizes and focal lengths. Such information can be found for earlier instruments constructed by Ole Rømer in Horrebow (1735) and for la… view at source ↗
Figure 3
Figure 3. Figure 3: Illustration of the relative orientation between Machina Æquatora (panel a) and the Gregorian Telescope (panel b). It is clear that (b) is rotated by 180 degrees with respect to (a). included on the chart of spots.” The figure is reproduced in [PITH_FULL_IMAGE:figures/full_fig_p011_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Reproduction of the figure in the 1776 article in Dansk Historisk Almanak. The drawing in the middle is from 23 October 1769, which is the day when Christian Horrebow saw the largest number of spots on the Sun. The fact that Christian Horrebow suggested that the Sun repeats itself with respect to the number and size of the spots, 69 years before Heinrich Schwabe discovered the solar cycle and estimated its… view at source ↗
Figure 5
Figure 5. Figure 5: The drawing from 23 October 1769, but this time reproduced from the notebook. See Paper II for a detailed description of the notebooks. 5.1. Summary During his time as director of Rundet˚arn, Christian Horrebow made systematic sunspot observations. Regular observations span the time from 1761 to 1776. Machina Æquatorea was inaugurated in 1767 and used as the primary instru￾ment for the remaining sunspot ob… view at source ↗
read the original abstract

Between 1761 and 1776, Christian Horrebow made regular observations of sunspots from Rundetaarn in Copenhagen. Based on these observations he writes in 1775 that "it appears that after the course of a certain number of years, the appearance of the Sun repeats itself with respect to the number and size of the spots". Thus, Horrebow hypothesized the idea of a cyclic Sun several decades before Heinrich Schwabe discovered the solar cycle and estimated its period. This proves the ability of Horrebow as a sunspot observer. In this article, we present a general overview of the work of Christian Horrebow, including a brief biography and a complete bibliography. We also present a translation from Danish to English of his writings on sunspots in the Dansk Historisk Almanak. These writings include tables of daily sunspot measurements of which we discuss the completeness.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 2 minor

Summary. The paper provides a biography of Christian Horrebow, a complete bibliography of his published writings, and an English translation of his sunspot-related entries in the Dansk Historisk Almanak (1775), including daily observation tables whose completeness is discussed. The central historical claim is that a 1775 passage shows Horrebow hypothesizing that the Sun's spot appearance 'repeats itself' after a certain number of years, thereby anticipating the solar cycle concept decades before Schwabe.

Significance. If the 1775 passage is correctly interpreted as an explicit periodicity hypothesis, the work would be significant for solar-history studies by documenting an earlier articulation of cyclic solar behavior based on systematic observations. The manuscript's strengths include its use of primary-source translations and explicit discussion of table completeness, which supports the archival claim without reliance on post-hoc data selection or quantitative fitting.

major comments (1)
  1. [1775 writings / translation section] The section presenting the 1775 writings: the claim that Horrebow 'hypothesized the idea of a cyclic Sun' rests entirely on the English translation of the passage stating that the Sun's appearance 'repeats itself' after a certain number of years. The manuscript supplies only the English version and does not reproduce the original Danish text or extended surrounding context from the almanac, leaving open the possibility that the Danish wording is more observational or tentative than the translation suggests. This directly affects the load-bearing anticipation claim.
minor comments (2)
  1. [Abstract] The abstract and introduction could more explicitly state the scope (Part I focuses on life and published writings) to avoid any implication of data re-analysis.
  2. [Tables section] Figure or table captions for the daily sunspot tables should note the source almanac year and page for each entry to facilitate independent verification.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful reading of the manuscript and for the constructive comment on the 1775 writings section. We address the point below and will incorporate the suggested changes in the revised version.

read point-by-point responses

  1. Referee: [1775 writings / translation section] The section presenting the 1775 writings: the claim that Horrebow 'hypothesized the idea of a cyclic Sun' rests entirely on the English translation of the passage stating that the Sun's appearance 'repeats itself' after a certain number of years. The manuscript supplies only the English version and does not reproduce the original Danish text or extended surrounding context from the almanac, leaving open the possibility that the Danish wording is more observational or tentative than the translation suggests. This directly affects the load-bearing anticipation claim.

    Authors: We agree that reproducing the original Danish text is necessary for full transparency and to allow independent verification of the translation. In the revised manuscript we will add the original Danish passage from the 1775 Dansk Historisk Almanak immediately adjacent to the English translation. We will also include additional surrounding context from the almanac entry to demonstrate that the wording is consistent with a hypothesis of periodicity rather than a purely descriptive statement. These additions will be placed in the relevant section without changing the paper's conclusions or interpretations. revision: yes

Circularity Check

0 steps flagged

No circularity: archival historical paper with no derivations or fitted parameters

full rationale

The paper is a biographical and archival study presenting Horrebow's sunspot observations, a translation of 1775 Danish text, and a bibliography. It contains no equations, no parameter fitting, no predictions derived from models, and no self-referential derivations. The central historical claim rests on direct quotation and translation of an external 18th-century source (Dansk Historisk Almanak), which is independent of the present paper. No load-bearing step reduces to a self-citation, ansatz, or renaming of the paper's own inputs. This is a standard non-circular archival work.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

This is a historical archival paper with no mathematical modeling, so the ledger contains only standard domain assumptions about textual interpretation.

axioms (1)
  • domain assumption 18th-century Danish astronomical writings can be translated into modern English without loss of intended meaning about periodicity.
    Invoked when the paper presents the 1775 quote as evidence of a cyclic-Sun hypothesis.

pith-pipeline@v0.9.0 · 5705 in / 1116 out tokens · 22584 ms · 2026-05-25T15:34:37.750566+00:00 · methodology

discussion (0)

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Reference graph

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