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What Spinoza and Huygens Would Have Seen that Summer Night

Telescoping with Spinoza and Christiaan Huygens

The night sky July 13th 1665 near Voorburg

The night sky, 11:31 p.m. July 13th 1665 near Voorburg

 …with which they have been able to observe the eclipses of Jupiter caused by the interposition of satellites, and also a kind of shadow on Saturn as if made by a ring. – Spinoza to Oldenburg, May 1665

It is tempting to imagine that having seemingly met and discussed with some enthusiasm issues of astronomy and microscopy in late April of 1665, Spinoza may have visited often with Huygens at his country estate which was some 10 minutes walk from Spinoza’s rooms on Kerkstraat. Aside from issues of social standing, Spinoza as a maker of telescopes and microscopes, surely would have wanted to talk with Huygens on the state of the art of the day, and further, Christiaanin esteem and sharing may have wanted to share the facts of his experiences of discovery with his most compelling neighbor. But we must ask, what could Spinoza and Huygens have seen, if they had looked through a telescope together?

It is probably without doubt that Huygens had set up one of his long telescopes permanently on the estate, for though he had not made astronomical discoveries for nearly decade, issues of significance were happening in the sky. In the Winter of 1664-5 a brilliant comet had showed itself, and then another in March 27thof ’65, the last being visible to the naked eye for a month. These were occasions not only for religious fervor, and signs of the end of Times, but also windows into the structure of the universe, events to observe closely so to feed the growing theories on the nature of cosmic bodies and their travel. Plague and war was rife, and yet and imperative of knowledge was blooming. As a general note, everyone’s eyes were on the sky, and Huygens’s telescope most surely was trained there. 

from Lubinetski’s 1667 treatise Theatrum Cometicum

from Lubinetski’s 1667 treatise Theatrum Cometicum

Aside from the striking sky pyrotechnics of comets, there is further in evidence that the sky was still much on Huygens’ mind in the summer of 1665. As recently as 1660-1661 Huygens was busy defending the power and accuracy of his telescope to the accusations of fraud come from the famed Italian telescopist Divini. (Huygens had controversallydiscovered the rings of Saturn in 1656, lead to them by his discovery the moon of Saturn later to be named Titan, in 1655, which he regarded as “my moon”.) Withthe existence of the Saturn’s rings still in dispute, evidence for them resting solely on the strength of his telescope, the prestigious Prince Leopold of Tuscany had twice proposed a paragone, a face-off field test between Huygens’s telescope and Divini’s, before persons of high social status, an offer that Huygens each time refused. In this vien of concerns, Roger Hahn in “Huygens and France” suggests as quite likely that Huygens continued interest in the telescope in April of 1663 lead him to the house of Adrien Auzout in Paris, and to a group that included Pierre Petit who were working on a 80 to 100 ft. telescope under the promise of seeing Huygens’ rings of Saturn more clearly. Then, what must have been a great relief to Huygens, in April of 1664 the rising Italian telescopist Campani himself faced the arrogant and well-connected Divini in a paragone, and definitively defeated him, soon after publishing a confirmation of Huygens’s Saturn findings through the report of the shadow of the questionable rings (look closely at the wording of Spinoza’s letter 26, where this shadow is mentioned). Following this history of observation and dispute, Spinoza writes of his early meeting of Huygens in May of 1665, and their talk of issues of astronomy. He mentions in his letter their discussion of the rings of Saturn, as well as the eclipses of Jupiter. With Saturn, comets and Huygens’s telescope in the forefront of the last years of European astronomy, and fresh to their friendship, one can easily imagine Spinoza having walked the ten minutes to the Huygens estate (pictured below), as the sun was lowering into the late evening of a long summer day, in order to look through the long-contested and now vindicated device. The sky would not have completely lost the sun’s light until just after 11:30.

If we imagine the night to be something like that of July 13th, there would be no moon. The canal’s lapping could be heard perhaps from the upper story, and somehow too the breadth of the property, the rush of the breeze across the rows of orchard and bush, so symmetically laid forth. Dark shadow-lines set out in geometry, ringing faintly as if strings. Here, would not Christiaan Huygens have trained his telescope on Saturn, the home of his distantly reached sight of rings, and a moon he had discovered? How many times had he looked at it? Saturn happened to be at its zenith on this night, due South, low on the horizon as the sky blackened. Christiaan had carved into the lens withwhich he had seen Saturn’s moon and rings with a line from Ovid: Admovere oculis distantia sidera nostris : They carried distant stars to ours eyes. This would have been a remarkable moment for Spinoza as he contemplated the Infinite.

East on the ecliptic there was Jupiter. Would they not have focused then on that great planet, having discussed the discovery of its eclipses only a few months earilier? Would not the glass telescope have brought to two great, but quite distinct minds into intersecting conversation. Neighbors of such diversity, such disjunction, living a short walk from each other, stretched thin across the solar system by means of a glass and metal? Would Huygens have mentioned, tipping that lens to its precise point, that he believes that light moves in spherical waves?

The Huygens Estate at Voorburg

The Huygens Estate in Voorburg