• Why is the moon big?

    Alexey Ganzenko
    Alexey Ganzenko
    February 25, 2013
    Why is the moon big?

    The lunar illusion (illusion of the moon) is an optical illusion that creates the impression that the perceived size of the moon increases several times when the Earth’s satellite is above the horizon and decreases when the moon soars high into the sky. The projection of the heavenly body on the retina of our eye remains unchanged in both cases. The interest of mankind to the mysterious phenomenon has been known since ancient times, which is reflected in many evidences of antiquity. In the end, every curious child to the question of where the big moon, with confidence will answer: the earth! So why all the same, dropping to the horizon, the moon has become more?


    A popular misconception explains the increase in the size of celestial bodies near the horizon by the so-called increase effect, the cause of which supposedly lies in the atmosphere of the Earth. In fact, the factor of astronomical refraction near the horizon, on the contrary, somewhat reduces the observed dimensions, slightly flattening the moon along its vertical axis.The angular size of a celestial object depends solely on the distance between it and the observer. Small changes in this distance are in no way connected with the optical sensation caused by a perception error when the Moon at the horizon appears to be magnified many times over. Measurements show that the angular dimensions of our satellite vary by no more than 0.5 °. The size of the projection on the retina is 0.15 mm.


    The simplest way to prove the illusory nature of an increasing moon is to compare its size with the size of a coin stretched out in a hand, covering one eye. Comparing the size of the coin when the “biggest” Moon is visible above the horizon, and repeating the experience when the night light rises to the zenith, it is easy to make sure that its size does not change. The size of the body can be determined either by setting its physical parameters, or through the angular size of the object. The differences between these two concepts are due to the peculiarity of our vision. For example, if two identical objects are placed one in five and the other in ten meters from the observer, then their real angular dimensions will differ twice. The observer will not confirm this.Conversely, if the angular size of a distant object is equal to the angular size of a closer one, the observer will claim that the approximate object is two times larger than the distant one.


    Why is the moon big? To date, there is no consensus on the nature of the known optical effect. The discussion is conducted around the question: Does the moon at the horizon increase due to the fact that its perceived angular size or physical size seems to be increased? In other words: do we see the Moon as close to us or increased in size? The final explanation is still waiting for its researcher.

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