The Moon’s Size is Relative

Some people wonder why the moon looks so small when it is higher in than sky as compared to when it is on the horizon. How does this happen is the moon physically changing size or moving farther from the earth? Or is it rather our perception of the moon’s size that is changing and why exactly do those changes occur. I can assure the moon does not change its size and the moon does not get closer and/or farther to the earth in just one night so the only answer to the change in its apparent size is our perception.

When you look at the moon in the middle of the sky there is nothing around it you are just seeing it as it is compared to nothing. On the other hand when you see the moon on the horizon you can see how big it is compared to a house, a tree, or even a skyscraper. Your perception takes these other relative objects into account the moon appears even larger in your mind when you see it behind other objects. With our knowledge of depth we think that the larger moon (on the horizon) has to be closer than the smaller moon amidst the stars.  (Goldstein, 2011) The following illustration is an example of an Ebbinghaus Illusion which is similar to the explanation above providing two identical circles that appear to have different sizes due to their different environments.  pysch


(supermoon, 2011)

This all appears to be bottom-up processing and I cannot find any top-down processing that could explain this misconception of perception.

Although the moon is the same size and distance in both positions in the sky when it appears different sizes we now know this is due to two things. First, is the Ebbinghaus illusion that different environments can cause the same object to appear different sizes. Adding to this is our understanding of depth and the relation of the size of other objects in front of the moon on the horizon making it appear huge.


Goldstein, E. (2011). Cognitive psychology (3rd edition). Belmont, CA: Wadsworth Cengage Learning.

The Supermoon Illusion. (2011, March 16). Retrieved September 13, 2015.

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