Moon Illusion: why the Moon looks larger when it’s near the horizon?

If you go out on a clear night when it’s full moon, you may notice how gigantic the Earth’s satellite looks when it’s near the horizon. But, in fact, that moon is the exact same size as every other time as you’ve ever seen it in the sky. You can test this by holding your thumbnail at arm’s length and comparing it to the size of the Moon when it is near the horizon and high in the sky, and you’ll see it doesn’t change size. Photographs of the Moon at different elevations also show that its size remains the same. In fact, it plays a trick on your brain which called the “Moon Illusion”. This illusion has been known since the ancient times, and an explanation of this optical phenomenon is still debated.

The moon illusion is subtle for some and dramatic for others (including me). A small fraction of people even don’t see it.

The Moon Illusion - a harvest moon
A Harvest Moon. The Moon illusion is an optical illusion which causes the Moon to appear larger near the horizon than it does higher up in the sky. This image was taken with a Canon EOS 5D Mark III on February 14, 2017 with 600.0mm, f/8.0s, 1/500s, ISO 200. Image source: pxhere

First of all, the eccentricity of the Moon’s orbit isn’t the cause. The Moon follows an elliptic orbit around Earth and sometimes it gets closer to the Earth than the other times. When it’s also full moon at its closest point to the Earth, it’s called Supermoon. As a result, the moon appears larger and brighter than usual in the sky. But a supermoon is not that “super”, it only looks around 14% in apparent diameter or 30% in apparent area than the Micromoon, its counterpart. But the moon in the horizon looks way bigger than that. And, to make matters worse, the moon is actually farther away when it’s on the horizon than it’s high in the sky: an entire radius of Earth farther, which is about 6,371 km or 3,959 mi. See the image below:

The distance of the moon
The moon is actually farther away when it’s on the horizon. Image source: the video titled “Why does the Moon look so big?” by The Science Asylum
on Youtube

Possible explanations of the Moon Illusion

Why does our brain process the size of the moon differently when it is in different places in the sky? Here are some possible explanations of the Moon Illusion:

  • The main theory suggests that when the moon is near the horizon, there are many visual reference points: trees, hills, buildings, mountains… with which it’s size can be compared. But when the moon resides high in the sky there is nothing around it to compare, and it seems smaller against the vastness of the night sky. It’s similar to an effect known as the Ebbinghaus illusion: in the image below, the two orange circles are exactly the same size; however, the one on the right appears larger. The small circles are comparable to the horizon moon with smaller objects in sight, while the large circles comparable to the high moon surrounded by the vast emptiness of the night sky.Notes 1

    Ebbinghaus illusion
    The Ebbinghaus illusion or Titchener circles is an optical illusion of relative size perception. Named for its discoverer, the German psychologist Hermann Ebbinghaus (1850–1909), who pioneered the experimental study of memory, the illusion was popularized in the English-speaking world by the British psychologist Edward B. Titchener (11 January 1867 – 3 August 1927) in a 1901 textbook of experimental psychology, hence its alternative “Titchener circles”. In the best-known version of the illusion, two circles of identical size are placed near to each other, and one is surrounded by large circles while the other is surrounded by small circles. As a result of the juxtaposition of circles, the central circle surrounded by large circles appears smaller than the central circle surrounded by small circles. Image and text: Wikipedia
  • Another possible explanation is the “Ponzo illusion”, a geometrical-optical illusion that was first demonstrated in 1911 by the Italian psychologist Mario Ponzo (1882–1960). Ponzo suggested that the human mind judges an object’s size based on its background. He showed this by drawing two identical lines across a pair of converging lines, similar to railway tracks (see the image below). The upper line looks longer because we interpret the converging sides according to linear perspective as parallel lines receding into the distance. In this context, we interpret the upper line as though it were farther away, so we see it as longer – a farther object would have to be longer than a nearer one for both to produce retinal images of the same size. The Ponzo illusion is one possible explanation of the Moon illusion: if something looks like it’s getting farther away (like the moon in the horizon), but doesn’t change size, then it must be growing. Also known as the apparent distance theory, this explanation evidently was first clearly described by the ancient Greek astronomer Cleomedes around 200 A.D.
    Moon Size Illusion and Ponzo Illusion
    Top: a diagram of the moon seen against a cloud of the same size, at different heights in the sky. When the moon is high, the clouds it is against are closer to the viewer and appear larger. When the moon is low in the sky, the same clouds are further away and appear smaller, giving the illusion of a larger moon. Bottom: An example of the Ponzo illusion. Both of the horizontal yellow lines are the same length.

  • The Sky-dome model: unlike the Ponzo illusion, which assumes a flat sky, the sky-dome model assumes a curved sky above a flat ground. It suggests that maybe our brains map all the really far stuff (like stars and the moon) onto an imaginary dome above our heads. We connect the sky to the ground at the horizon, and as a result, underestimate the distance to sky objects. When the moon is high in the sky, our brain thinks it is way closer than when it is on the horizon, hence it must be smaller (or vice versa).

Here in the video below, physicist Nick Lucid talks about the “moon illusion” and lists several possible explanations of why the moon looks larger when it’s near the horizon. If you haven’t subscribed to his channel yet, I would strongly suggest that you do now. He publishes great videos covering some interesting science topics.

Here’s another video by the ASAPScience channel, written and created by Mitchell Moffit (twitter @mitchellmoffit) and Gregory Brown (twitter @whalewatchmeplz):

Notes

  1. Maybe Ebbinghaus illusion is not the cause. In the image below, I deleted the big circles on the left, so it resembles the moon in the sky. Now both orange circles look the same size (at least, to me).
    Ebbinghaus illusion-2

Sources

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