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Saturn and its rings

Altissimum planetam tergeminum observavi”, “I saw the most distant planet [1] triple-bodied”, wrote Galileo in 1610 using a famous anagram to secure authorship of his discovery. (c) Istituto e Museo di Storia della Scienza di Firenze Before publishing his amazing observations in a book, he wrote to Belisario Vinta, Secretary of State at the Grand Duchy of Tuscany, sketching the extraordinary shape of Saturn, in order to spark curiosity — and generosity — among the Lords of Tuscany.

At present, Galileo’s depiction of Saturn may seem somewhat whimsical. We are so much accustomed to seeing highly detailed images of Solar System planets that we can hardly understand how one could see three bodies where we know there is planet surrounded by large rings!

The answer lies in an optical effect called diffraction. This phenomenon affects all luminous signals passing through a finite aperture. The main effect of diffraction when observing through a small telescope is to filter out some of the image content; the smaller the aperture, the bigger the effect.

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The image above shows Saturn as it would have been seen from Veneto, at the end of July 1610... through a telescope with an infinite aperture diameter, a 600X magnification factor, and under inconceivably stable atmospheric conditions!

See in the video below how the Saturn image is affected by diffraction through the small objective of Galileo’s telescope: the interference pattern is “destructive” around the central Saturn body and it makes the two ansae of the ring system appear as two separate bodies.

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But why does Saturn seem to move so frantically in the video? Our simulation includes the effect of atmospheric turbulence. The underlying Kolmogorov theory defines a characteristic scale below which turbulence stops. One can show that when this scale is comparable to the size of the telescope aperture, as here, the amount of jittering in the observed image is highest.

Saturne seen by Galileo ©Chiara Marmo - Photothèque Planétaire d’Orsay is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

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[1] Neptune and Uranus are not yet known in 1610.

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