New facts on the nature of comets

The first astronomer since Kepler who was able to say something new about comets was François Arago (1786-1853). Having built a polarimeter, a small instrument capable of detecting the polarization of light, he observed comet C/1819 N1 Tralles in 1819 and discovered that the light from its coma and from the bright parts of its tail was polarized This indicated that it was sunlight scattered by the material of the comet, which did not emit light by itself. Arago confirmed this observation in 1835 on Halley’s comet. But since there was still no theory of light scattering at this time, he could not know if the scattering came from gas or from solid particles.

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Les différentes éjections par le noyau cométaire O sont repoussées par la pression de radiation de la lumière du Soleil et décrivent des paraboles, dont l’enveloppe est représentée. Comparer au dessin de droite de la tête de la comète de Halley. D’après Eddington (1910) Monthly Notices of the Royal Astronomical Society

Crédit : DR


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François Arago, par Charles Steuben, 1831. Coll. Observatoire de Paris, inv.I.63

Crédit : S. Pelly/Observatoire de Paris

Arago was of course not alone in trying to understand the physics of Halley’s comet. Many observations were made, especially in Germany by Heinrich Schwabe (1789-1875) and Friedrich Bessel (1784-1846), a great astronomer better known for having been the first to measure the distance of a star. They noted, in the front of the comet, one or more plumes whose orientation and shape varied from day to day. Bessel suggested that these are plumes of material ejected by the comet as a result of heating by the Sun, and that this matter was repelled by the Sun, describing parabolic trajectories then form the tail.

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Dessins de la tête de la comète de Halley, tiré de l’Astronomie populaire / Arago, 1854-1857. Remarquer les aigrettes qui sortent de la tête.

Crédit : Observatoire de Paris

This idea was developed much later by Arthur Eddington (1882-1944), who showed that the appearance of a comet’s head and tail could be explained perfectly well. But one still did not know, at the time of Bessel, the nature of the ejected material and the ejection mechanism. The answer was to come from spectroscopy.


Polarimètre d’Arago, vers 1810. Coll. Observatoire de Paris, inv.204

Crédit : S. Pelly/Observatoire de Paris