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From Galileo to Rosetta

For some people, the recent images of the Rosetta space program have been slightly disappointing. We expected to see the nucleus of the Churyumov-Gerasimenko comet as a brilliantly shining body. Instead, images from Rosetta are dominated by the extremely dark aspect of the comet nucleus – as black as an enormous lump of coal – contrary to the idea of a bright celestial body, with the phantasmagorical appearance of its tail.

The surface of the Churyumov-Gerasimenko comet
Fig 1. The surface of the Churyumov-Gerasimenko comet photographed by the Rosetta mission. Credit: Lander search, 13 December 2014. Public domain via ESA.

Galileo Galilei would be among those who would not share this sense of disappointment. He knew full well that, under certain conditions, an intrinsically dark body can be perceived as an intensely brilliant object (and vice versa). His particular aptitude for seeing beyond the immediacy of sensory perceptions emerged in the course of a harsh dispute with the German Jesuit, Christoph Scheiner. This dispute exploded in 1611, shortly after Galileo’s first telescopic observations, and concerned the nature of sunspots, the apparently dark feature of the sun’s surface. Their existence seemed to demolish the ancient dogmas defining the perfection and immutability of celestial bodies.

Scheiner considered it unlikely that there would be such spots on a body as luminous as the sun, but that, if there were, they would be far darker than any ever observed on the Moon. For Galileo on the other hand, sun spots will not only be less dark than the dark spots on the Moon, but will be no less bright than the most luminous parts of the Moon. In his opinion, the Jesuit was wrong because he compared the two visual objects under very different conditions: the spots against the brilliant disk of the Sun, and the Moon against the dark background of the nocturnal sky. The error stemmed from the mechanisms of vision that cause the perceived brightness of a visual target to depend upon the intensity of the region surrounding it. An example of this can be seen in Fig. 2, where the grey square appears to vary in shade in the presence of a black or white background.

a simple contrast effect
Fig.2. a simple contrast effect of modern psychophysics, whereby the same grey square appears brighter or darker as a consequence of the background surrounding it. Credit: Marco Piccolino

Galileo was well aware that in order to establish a “psychophysically” correct comparison, visual objects should be observed against backgrounds of similar physical intensity. There were, however, obvious difficulties in comparing the body of the Moon and the spots of the Sun under similar conditions. Indeed, when the Moon comes near to the Sun it becomes invisible  for astronomical reasons, because it will receive the sun light on the surface not looking to the Earth (and thus  becomes a new Moon). As a solution, Galileo conducted a thought experiment. He first considered Venus, the brightest planet in the night sky, and more brilliant than the Moon. In twilight, Venus only becomes visible when it is positioned far from the sun. This happens because the field surrounding the sun is no less brilliant than Venus. In a similar way, Galileo argued, the full Moon would become invisible if one could put the moon nearer the Sun, because it would be positioned in a field no less shining and clear than its own face.

Having established through Venus a comparison between the Moon and the brilliant area around the Sun, Galileo compared the brightness of this area with the sunspots. He referred to telescopic observations indicating that the black spots were not darker than the area surrounding the Sun. He argued that, if this was true, and if the full Moon becomes invisible in brightness of the same ambiance, then by necessary consequence, the sunspots cannot be less bright than the brightest parts of the Moon.

The essence of Galileo’s reasoning can be made clear by comparing two telescopic images of the Sun; the first one aimed at visualizing the brilliant zone surrounding the sun’s disk; the second one designated to visualize the sun spots.

The sun's corona
Fig. 3. On the left, a visualisation of the sun’s corona obtained during a total sun eclipse, compared to the image on the right, aimed at visualising the sunspots. Both pictures were obtained with dark filters, but the filtering was stronger with the image at the right. A comparison between the two images shows that sunspots are more brilliant than the field surrounding the solar disk, although this is still much more luminous than the sky in daytime. Credits: left image: Total Solar Eclipse  by Luc Viatour CC-BY-SA-3.0 o, via Wikimedia Commons. Right image: Huge sunspot group by SOHO (ESA & NASA). Public Domain via SOHO.

The physical brilliancy of the dark sunspots is only one example of Galileo’s ability to see beyond immediate appearances. Another important discovery concerned the interpretation of the dim light that can be perceived on the dark body of the Moon. Galileo correctly attributed this light to the sunlight reflected from the Earth to the Moon, a subject worthy of its own blog. It is with these and other observations that Galileo initiated a long series of discoveries, which led, a few months ago, to a small robotic lander leaving its mother aircraft and eventually grasping (with some hesitation) the surface of the Churyumov-Gerasimenko comet.

Featured image: Lander search, 13 December 2014, by ESA/Rosetta/MPS for OSIRIS Team, public domain via ESA.

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