The moon illusion is a visual deception when people perceive the angular diameter of the Moon/Sun near the horizon larger than that of the one higher in the sky. Some theories have been proposed to explain this illusion, but not any is generally accepted. Although several psychophysical experiments have been performed to study different aspects of the moon illusion, their results have sometimes contradicted each other. Artists frequently display(ed) the Moon/Sun in their paintings. If the Moon/Sun appears near the horizon, its painted disc is often exaggeratedly large. How great is the magnitude of moon illusion of painters? How different are the size enlargements of depicted lunar/solar discs? To answer these questions, we measured these magnitudes on 100 paintings collected from the period of 1534–2017. In psychophysical experiments, we also investigated the moon illusion of 10 test persons who had to estimate the size of the lunar/solar disc on 100 paintings and 100 landscape photographs from which the Moon/Sun was retouched. Compared to the lunar/solar disc calculated from reference distances estimated by test persons in paintings, painters overestimated the Moon's size on average
Q
= 2.1 ± 1.6 times, while the Sun was painted
Q
= 1.8 ± 1.2 times larger than the real one, where
Q
=
r
painted
/
r
real
is the ratio of the radii of painted (
r
painted
) and real (
r
real
) Moons/Suns. In landscape photos, test persons overestimated the Moon's size
Q
= 1.6 ± 0.4 times and the Sun was assumed
Q
= 1.7 ± 0.5 times larger than in reality, where
Q
=
r
test
/
r
real
is the ratio of the radius
r
test
estimated by the test persons and the real radius
r
real
of Moons/Suns. The majority of the magnitude of moon illusion
Q
= 1.6, 1.7, 1.8, 2.1, 2.8, 2.9 measured by us are larger than the
Q
-values 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.8 obtained in previous psychophysical experiments due to methodological differences.
