The Sun/Moon Illusion in a Medieval Irish Astronomical Tract.

Helen E Ross

doi:10.3390/vision3030039

Abstract

The Irish Astronomical Tract is a 14th–15th century Gaelic document, based mainly on a Latin translation of the eighth-century Jewish astronomer Messahala. It contains a passage about the sun illusion—the apparent enlargement of celestial bodies when near the horizon compared to higher in the sky. This passage occurs in a chapter concerned with proving that the Earth is a globe rather than flat. Here the author denies that the change in size is caused by a change in the sun’s distance, and instead ascribes it (incorrectly) to magnification by atmospheric vapours, likening it to the bending of light when looking from air to water or through glass spectacles. This section does not occur in the Latin version of Messahala. The Irish author may have based the vapour account on Aristotle, Ptolemy or Cleomedes, or on later authors that relied on them. He seems to have been unaware of alternative perceptual explanations. The refraction explanation persists today in folk science.

Highlights

Latin translation of the eighth-century Jewish astronomer Messahala
The sun/moon illusion is the apparent enlargement of the celestial bodies when near the horizon compared with their appearance when higher in the sky
Similar statements were made by Posidonius and Seneca

Summary

Early Explanations of the Sun or Moon Illusion

The sun/moon illusion is the apparent enlargement of the celestial bodies when near the horizon compared with their appearance when higher in the sky. The importance of apparent distance was elaborated by Ibn al-Haytham in much more detail He described the flattened-dome account in his Optics 1039), and it was repeated by the thirteenth-century writers Roger Bacon, John Pecham and Witelo On this theory the dome of the sky appears flattened because intervening objects expand the apparent distance towards the horizon, and the apparent size of the celestial bodies expands with the apparent distance. John of Sacrobosco repeated a version of this based on the ninth-century Arab astronomer al-Farghani, but in this version the flattened dome had a physical reality, and the midday sun was closer than at sunrise or sunset This should produce the opposite effect to the sun illusion. The popular press today continues to report the refraction account [4], or sometimes the apparent distance account

The Irish Astronomical Tract

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