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The Moon Illusion

Moon Illusion

 Automatic translation  Automatic translation Updated June 01, 2013

You have all seen a huge full moon rising on the horizon just above the houses of our cities. The observation is surprising because it seems that the moon is huge, this famous optical illusion has no straightforward explanation. If we take a ruler and you measure the apparent size of the moon when near the horizon, and when it is high in the sky, we obtain the same extent, there is no difference.
Why our brain recreates a different picture of reality?
One explanation (still unclear) is that when we look at buildings or trees on the horizon, our brain interprets a distance not so far away.
When the moon is on the same horizon, the brain assumes that the Moon is also on this same distance scale, and amplifies its size.
Conversely, when the moon is high in the sky, the brain compensates for this by making distance seem smaller.


However the Moon moves in an elliptical orbit like all cosmic objects and its distance from Earth varies.
At its apogee, the farthest point from Earth, is at 405 696 km, while at its perigee, it is 356 410 km.
Of course when it is at its perigee, it appears larger, more exactly 14%, 14% is the difference between the two apsis (points nearest and farthest) the orbit of the Moon relative to Earth. But the illusion of great Moon occurs as well for the apogee than the perigee.

NB: We feel that our Moon is very far from Earth, but if you wanted to travel this distance by car on a highway at 130 km/h, we would have only 115 days.

Image: Difference in the apparent size of the moon between apogee and perigee. At perigee (left), the Moon is 14% greater than apogee (right).

 Moon, apparent size

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