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Lunar Phases

The phases of the Moon are caused by Earth's shadow; they have names.

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The Moon Illusion

The Moon Illusion

Credit: Ingrid Taylar
Source: http://www.flickr.com/photos/49503118795@N01/6848581047
License: CC BY-NC 3.0

When the moon is on the horizon it look huge! When it’s high in the sky, it looks much smaller. This phenomenon is called the moon illusion. Everyone sees it, but no one quite knows what causes it.

Why Does the Full Moon So Much Bigger on the Horizon?

  • Ptolemy suggested the reason was the way light refracts in the atmosphere.
  • The Ebbinghaus Illusion: this is when an object appears to be a different size due to the size of the surrounding objects.
  • Credit: AlexWorth91
    Source: http://en.wikipedia.org/wiki/File:Ebbinghaus_Illusion.svg
    License: CC BY-NC 3.0

    The two middle circles are the same size - The Ebbinghaus illusion [Figure2]

  • The Ponzo Illusion: this is how artists use size to depict perspective. The viewer perceives objects that are near the horizon as larger than they are.
  • Convergence microposia: the convergence of the eyes cue the brain as to the distance of an object. If an object is at the bridge of your nose, your eyes nearly cross. When you look at something far away, each eye focuses straight ahead. But when you look up to the moon in the night sky, there is no reference point. Your eyes default to their resting focus. This is just a few meters away. So your brain thinks the moon is closer than it is.

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With the link below, learn more about the moon illusion. Then answer the following questions.

  1. How can you test if the moon is a different size at different locations in the sky?
  2. How does the Ebbinghaus Illusion explain this phenomenon?
  3. What evidence is there that the Ebbinghaus Illusion does not explain the phenomenon?
  4. Use the explanation of convergence micropsia. Is the moon most likely the size it appears to be on the horizon or the size it appears to be high in the sky? Why?

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Image Attributions

  1. [1]^ Credit: Ingrid Taylar; Source: http://www.flickr.com/photos/49503118795@N01/6848581047; License: CC BY-NC 3.0
  2. [2]^ Credit: AlexWorth91; Source: http://en.wikipedia.org/wiki/File:Ebbinghaus_Illusion.svg; License: CC BY-NC 3.0

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