(b. Copenhagen, Denmark, 15 April 1718; d. Copenhagen, 19 September 1776)
Horrebow was the son of Peder Nielsen Horrebow, professor of astronomy, and Anne Margrethe Rossing. He was the fourth [?] child in a family of twenty. In 1732 he was sent to the University of Copenhagen, where in 1738 he obtained his M.S. degree. He became his father’s assistant at the Round Tower Observatory, working on the calendar and thus continuing a literary tradition begun by Wilhelm Lange.1 He computed the annual almanac from 1739 to 1770. In 1743 he became a designate professor, and from 1753 he was completely in charge of his father’s post at the observatory, which, in 1741, had been fully restored after the great fire of Copenhagen (1728). In 1764 Horrebow obtained the chair of astronomy. He was elected a member of the Royal Danish Academy of Sciences and Letters in 1747 and from 1769 he was a titular councillor of state. He married Anna Barbara Langhorn on 25 October 1754.
To a large extent Horrebow continued his father’s work in astronomy. Eustachio Manfredi had questioned2 the elder Horrebow’s alleged determination of stellar parallaxes from Römer’s observations of right ascensions of Sirius and Vega.3 To eliminate the doubt cast on his father’s arguments. Horrebow worked on parallax determination (1742–1743, 1746), confirming his father’s erroneous conclusion without realizing that the effect was due to the influence of temperature variations on clocks and instruments.4 Because Bradley’s theory of aberration surpassed the said Römer-Horrebow “proof” of the motion of the earth, Horrebow, in 1751, tried to develop a new micrometer method for determining stellar parallaxes, concentrating on some of the fainter stars believed to be close to the sun because of the small range of their vortices.
Horrebow’s observation of the transit of Venus (1761) has often been judged a failure, but only because of a misunderstanding concerning the correction of his clocks. On the other hand, his systematic observations of sunspots during his last fifteen years came to play a role in the later investigation of the period of sunspot activity.5 In theoretical astronomy he maintained a constant eccentricity of the earth’s orbit against Jacques Eugène Louville’s theory of the decrease of eccentricity with time.
In collaboration with his brother Peder, Horrebow also dealt with meteorological subjects. He showed that a theoretical table of barometer readings corresponding to different altitudes, prepared by his father in 1751, squared better with Juan’s and Ulloa’s determinations of mountain altitudes in America than did other tables accessible at the time.
Horrebow also prepared textbooks in the fields of astronomy and mathematics.
1. Wilhelm Lange, De annis Christi libri duo (Leiden, 1649).
2. Eustachio Manfredi, “De novissimis circa fixorum siderum errores observationibus. Ad... Antonium Leprottum... epistola,” in De Bononiensi scientiarum et artium instituto atque academia Commentarii, 1 (1731), pp. 612–618.
3. Peder Nielsen Horrebow, Copernicus Triumphans, sine de parallaxi orbis annui tractatus epistolaris (Copenhagen, 1727).
4. C. A. F. Peters, “Recherches sur la parallaxe des étoiles filxes,” in Mémoires de l’Académie impériale des sciences de St. -Pétersbourg, 6th ser., 5 (1853), 1–180.
5. T. N. Thiele, “De macularum solis antiquioribus quibusdam observationibus Hafniae institutis,” in Astronomische Nachrichten, 50 (1859), cols. 257–262.
I. Original Works. A full list of Horrebow’s printed writings is in Niels Nielsen, Matematiken i Danmark 1528–1800 (Copenhagen, 1912), pp. 97–99. Not included in this list is “Vindiciae aerae dionysianae, sive de annis Christi diascepsis,” in vol. II of Peder Nielsen Horrebow, Opera mathematico-physica (Copenhagen, 1741). On the annual parallax of the fixed stars, see De parallaxi fixarum annua ex rectascensionibus, qvam post Roemerum et parentem ex propriis observationibus demonstrat (Copenhagen, 1747). Most of his other writings are in Videnskabernes Selskabs Skrifter (1751–1770), as well as in his academic dissertations. The most interesting articles are “Afhandling om fixstiernernes distance fra Jorden” in Videnskabernes Selskabs Skrifter,6 (1751), 129–152; “Reflexioner anlangende veneris drabant,” ibid., 9 (1765), 396–403; and “Om soel-pletterne,” ibid., 10 (1770), 469–536. For reports on the sunspots, see Videnskabernes Selskabs historiske almanakker (1770–1775). His observations for the period 1767–1776 are published in Wolf’s Mittheilungen über Sonnenflecken, 19 (1865) and 33 (1873). Unpublished papers dealing with the meteorology and geography of Iceland and Greenland and with natural philosophy are extant at the Royal Library of Copenhagen.
II Secondary Works. For biographical information, see C. F. Bricka, Dansk Biografisk Leksikon, X (Copenhagen, 1936), 607–608; and Niels Nielsen’s article mentioned above. Horrebow’s determination of the transit of Venus is treated in Axel V. Nielsen, “Christian Horrebows observationer of venuspassagen i 1761,” in Nordisk astronomisk tidsskrift (1957), pp. 47–50.
Kr. Peder Moesgaard