The experimenters wanted to know if the Moon at the horizon looked bigger than the Moon at the...


The experimenters wanted to know if the Moon at the horizon looked bigger than the Moon at the zenith. They divided the size at the horizon by the size at the zenith. If the sizes were equal, this ratio should equal one. Then they collected a sample of people and calculated what the average ratio was for the sample. What is wrong with this logic?


Fallacy refers to the act of using invalid reasoning or incorrect procedures in the construction of an argument. It can be divided into two categories namely formal and informal. Formal fallacies are found in the structure of an argument, rendering the principal argument invalid and can be expressed as a standard system of logic whereas an informal fallacy occurs outside the structure of an argument as a reasoning error.

Answer and Explanation:

This case represents a logical fallacy. It is categorized as a false analogy, which is a type of informal fallacy.

The size of the moon is believed to be bigger at the horizon compared to when it is high in the sky due to an optical illusion that is generated in our mind. Size constancy is one of the phenomena that takes place in our minds. When we look at, take, for example, two people of the same size standing at different distances from us, our mind deduces that the person at a greater distance is bigger than the person who is standing at a shorter distance provided they both appear of the same size. The mind makes up for the increased distance to maintain size constancy.

Now, when we look at the moon at the horizon, we perceive it to be at a farther point in space than when it is high in the sky. And hence, even though the moon is at the same distance and of the same size, it appears to be bigger at the horizon. Now, our mind perceived moon to be farther at the horizon because the existence of various elements between the observer, us, and the object, provides us with a reference to perceive a difference. When the moon is high in the sky, there is no material but air between us. Looking above, the lack of a reference does not allow our mind to assume a distance.

When a sample of people is collected to find the average ratio of their sizes, the logic fails because it is a case of false analogy. What may be true for the moon won't apply in the same way for humans and other objects on the same parallel as us because unlike the moon, humans are actually observed at different distances that cause a change in their height. Obviously, the ratio will be equal to one as the size of people will be the same.

Learn more about this topic:

The Weak Analogy Fallacy: Definition & Examples

from Critical Thinking Study Guide

Chapter 6 / Lesson 17

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