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A solitary observer watches a huge, cratered full Moon rise above a city of rooftops at dusk
Illustration

Thought experimentScience & mathematics10 min

Why does the Moon look bigger near the horizon?

Test your own perception, then meet the 11th-century scholar who argued that seeing is a kind of reasoning — and who insisted on checking claims by controlled testing.

Step 01
17%

Step 01

The giant Moon

You've probably seen it: a huge Moon rising over the rooftops. Hours later, high in the sky, it looks smaller.

Measure it with a camera and the Moon is essentially the same size in both places.

Question

Why does the horizon Moon look bigger?

§Scholarly interpretation

Still not fully solved

The Moon illusion has been discussed for over two thousand years. Modern researchers agree it is psychological, not physical, but disagree about the precise mechanism.

1 source

Sources in this experience

  • The Optics of Ibn al-Haytham, Books I–III: On Direct Vision — A. I. Sabra (trans.), 1989, Warburg Institute
  • Theories of Vision from al-Kindi to Kepler — David C. Lindberg, 1976, University of Chicago Press View source
  • Alhacen's Theory of Visual Perception — A. Mark Smith, 2001, American Philosophical Society View source
  • The Mysterious Moon Illusion — Helen Ross & Cornelis Plug, 2002, Oxford University Press
  • Opticae thesaurus — Friedrich Risner (ed.), 1572, Basel
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