Hortensius, Martinus

also known as Ortensius, or Van den Hove, Maarten (b. Delft, Netherlands, 1605; d, Leiden, Netherlands, 7 August 1639)

astronomy.

Hortensius’ chief contributions were in the diffusion of Copernican astronomy and in his measurements of the angular size of the sun. The child of a man named Van Swaanswijk and a woman named Van den Hove, he studied mathematics with Beeckman and Snell and was a student at Leiden and Ghent from 1628 to 1630. During this period of studies, he most likely traveled to other countries, including Italy. He collaborated frequently with Philip van Lansberge in Middelburg, the Netherlands, and exchanged letters with Descartes, Mersenne, Gassendi, Huygens, and Galileo.

In 1634 Hortensius lectured on mathematics at the Amsterdam Atheneum, and in 1635 he became full professor there in the Copernican theory. He traveled often to Delft, Leiden, and The Hague, and later gave courses on nautical science, in which subject there was considerable interest. In 1638 he became a member of the commission that had to negotiate with Galileo on his method of longitude determination by observation of the satellites of Jupiter. In 1639 he was nominated professor at the Leiden university. He died shortly thereafter, leaving a natural son.

Hortensius was an autodidact in astronomy, first following Tycho, later giving serious consideration to the Copernican theory. He made observations on eclipses and on transits and endeavored to improve existing telescopes.

His findings concerning the angular diameter of the sun, mentioned in his preface to Lansberge’s Commentationes, were vehemently criticized by Kepler in 1631, but were eventually vindicated by Hortensius in 1634. By using one of the primitive telescopes of the time and studying solar eclipses, he found the solar angular diameter to be 36′ at perigee and 33′34″ at apogee, with a ratio of 1.072. Kepler, on the other hand, believed that a telescope distorted the image and preferred to employ a small hole at the end of a long tube; he found a mean value of 30′ and a ratio 1.033 (actual values: 32′04″ and 1.034). This question was of great importance, because it was directly connected to the matter of the eccentricity of the earth’s orbit, which Kepler had investigated by using observations of Mars. Kepler was vehement regarding this issue and used untenable arguments; Hortensius responded politely but suggested that Kepler might have altered the observational results in order to get agreement.

In the same preface to the Commentationes, Hortensius also criticized certain assertions of Tycho. Answers to his criticisms were given by Erasmus Bartholin and Longomontanus.

BIBLIOGRAPHY

I. Original Works. See Responsio ad additiunculum D. J. Kepleri praefixam Ephemeridi ejus in annum 1624 (Leiden, 1631), in which Kepler’s criticisms are reproduced in full; and Dissertatio de Mercurio in sole viso et Venere invisa (Leiden, 1633). Hortensius’ Pleiadographia sive Pleiadum descriptio, never published, was lost.

Hortensius also translated Philip van Lansberge, Commentationes in motum terrae diurnum et annuum (Middelburg, 1630), and Guil. Blaeu, Institutio astronomica de usu globorum et sphaerarum coelestium ac terrestrium (Amsterdam, 1634).

II. Secondary Literature. Biographical data can be found in P. C. Molhuysen and P. J. Blok, Nieuw Nederlandsch biograftsch Woordenboek, I (Leiden, 1911), cols. 1160–1164; and C. de Waard, Journal tenu par Isaac Beeckman, 4 vol. (The Hague, 1939–1953).

M. G. J. Minnaert