The Sun/Moon Illusion in a Medieval Irish Astronomical Tract

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.