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We all exhibit a powerful tendency to see what we want to see. And it often takes a huge effort to effect a radical shift in our perceptions. When Galileo trained his improved ‘spyglass’ on the moon in the winter of 1509, he became convinced that ‘sane reasoning’ can reach no other conclusion than that the moon ‘is full of prominences and cavities similar, but much larger, to the mountains and valleys spread over the Earth’s surface’. But he had reckoned without the powerful grip of those philosopher-theologians who wished to sustain their perception of the heavens as ‘immaculate’.

For Galileo the argument seemed as clear as the nose on one’s face - or as plain as the Apennines that provide the spine of central Italy. Looking at the changing borderline between the lit and shaded hemispheres, he observed features that behaved like the shining and shaded ridges of mountains flanking valleys on earth. Bright points were seen to sprout and grow within the shadowy border, like earthly mountains progressively receiving more the rays of the ‘rising’ sun. Working out the optical geometry of the most prominent spots in terms of the passage of light across the surface of a rough sphere, Galileo  computed the most prominent mountains as no less than 4 miles high.

Not everyone saw the same thing however. Thomas Harriott in England in England had already viewed and drawn only a ‘strange spottednesse’ with his low-grade instrument, while Christopher Clavius and his fellow ‘mathematicians’ in Galileo’s Rome reported that it was ‘more probable that the surface is not uneven, but rather that the lunar body is not of uniform density and has its dense and rarer parts’. Roman Jesuits were committed to the translucent immaculacy of the moon, in accordance the Song of Songs in the Bible where the ‘Virgin’s’ pure beauty is identified with the moon - pulchra ut luna. Accordingly they condemned the ‘filth of opacity’ implied by Galileo’s characterisation.

Why did Galileo see and represent so clearly? The quality of his telescopes was clearly important, as was his conceptual freedom from dogmatic theology. But an essential ingredient was his ability to articulate his acts of scrutiny and graphic record through a sophisticated understanding of mobile light on bodies with uneven surfaces. As an accomplished draftsman and aficionado of perspective, he was fully acquainted the science of cast shadows outlined in such texts books as Albrecht Dürer’s Instruction in Measurement (1528) Daniele Barbaro’s The Practice of Perspective (1569).

A member of the artist’s Academia del Disegno in Florence and friend of the leading artist, Ludovico Cigoli, Galileo was also aware of the tradition inaugurated by Leonardo da Vinci of minutely systematic observation and depiction of light in landscape according to the angles and direction of the sun relative to the observer. Cigoli’s own treatise on perspective directly quoted from Leonardo’s unpublished writings. When Cigoli came in 1610-12 to portray the ‘Virgin of the Immaculate Conception’ in the dome of the Capella Paolina in S. Maria Maggiore in Rome, he depicted Mary standing on a pitted and maculate moon rather than the smooth crescent of convention. In his defence, the painter could have pointed to an earlier tradition which justified the placement of the moon below Mary’s feet on the grounds that its patchy and inconstant appearance testified to its lowly status in the heavens.

Yet for all his hard looking and knowing draftsmanship, the details of Galileo’s cratered moons are difficult to align precisely with actual features. With a field of view of less than half the moon’s diameter, and the inevitable difficulties of charting details within no existing framework, Galileo has in effect shown what kind of thing is happening as sunlight skates over the lunar landscapes rather than constructing a precise map. His purpose was to show the moon as earth-like. He is able to see what he wants to see and persuades us to do the same.

Text taken from ‘Maculate Moons’ by Martin Kemp