When the moon is high in the sky, it appears small. When it is low near the horizon, it appears larger than usual.

This apparent change in size is an optical illusion that scientists call the moon illusion. While people have been aware of this illusion since ancient times, it's difficult to explain why it occurs.

As early as the 4th century BC, the Greek philosopher Aristo noticed that the moon appeared larger when it was closer to the horizon than further up in the sky, a phenomenon he attributed to the "reflection of light."

He hypothesized that the Earth's atmosphere acted like a watery lens, magnifying the low-hanging moon, sun, and stars. Greek astronomers Ptolemy and Cleomedes also proposed similar theories in the 2nd century AD.

This is the earliest recorded explanation of the moon illusion, although it is incorrect. Today, scientists have confirmed that the Earth's atmosphere has no role in the size of the Moon. The Earth's atmosphere only affects the color of the Moon that we see. So far, there is no unanimous explanation for the moon illusion, but there are several popular theories.

Ebbinghaus illusion

In 1890, German psychologist Hermann Ebbinghaus pointed out that our eyes perceive the size of objects by comparing them with the environment.

Surround two circles of the same size with circles of different sizes, the first circle is surrounded by a larger circle, and the second circle is surrounded by a smaller circle. At this point, the first circle looks smaller, even though they are the same size. So when the moon is on the horizon, the surrounding trees, houses, and mountains act as a reference, making the moon appear much larger than usual.

Similar to the Ebbinghaus illusion, the Ponzo illusion, named after Italian psychologist Mario Ponzo, suggests that the human brain determines the size of an object based on its context by drawing two identical straight lines. This illusion is shown to people by running through a pair of railroad-like lines that converge toward a point.

(Related knowledge: The moon phase is the shape of moonlight viewed from the Earth. The moon itself does not emit light. The part of the moon that is directly illuminated by the sun reflects sunlight and can be seen to shine. The shadow part is the dark side of the moon itself.)

Living in bustling cities, we pay less attention to the starry sky above. Although the Chang'e Project has sent a lunar rover to the lunar surface, we should not forget the simple wishes and curiosity of ancient times.