Why the sun/moon looks bigger at the sunset/rise?

  • Context: High School 
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Discussion Overview

The discussion revolves around the phenomenon of why the sun and moon appear larger during sunset and sunrise compared to when they are higher in the sky. Participants explore various explanations, including optical illusions and perceptual factors, while seeking a scientific understanding of the phenomenon.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants suggest that the perceived size increase is an illusion, as explained by various teachers.
  • One participant shares personal experience, noting that objects appear larger when viewed from a lying position compared to standing.
  • Another participant proposes an experiment using a thumb to cover the moon, indicating that its size remains constant regardless of its position in the sky, supporting the illusion argument.
  • Contrarily, a participant argues against the idea that lack of reference points in the sky is the cause, referencing an article that states the moon appears larger on the horizon due to comparison with foreground objects.
  • Another participant mentions that the phenomenon may involve multiple illusions, particularly related to how humans perceive vertical angles and the perceived stretching of the sky dome at low altitudes.

Areas of Agreement / Disagreement

Participants express differing views on the cause of the phenomenon, with some supporting the illusion theory while others challenge it, indicating that multiple competing explanations remain unresolved.

Contextual Notes

Participants reference external articles and personal observations, but there is no consensus on a definitive explanation for the phenomenon, highlighting the complexity and subjective nature of visual perception.

pixel01
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May be this is an old question. Some teachers explained to me that is because of illusion. Anyway I feel not so sure about that. Can anyone explain this phenomenon scientifically.
 
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My own experience is that it is an illusion. If you lie on your back and look straight up, things (clouds, sun, moon) appear bigger than when you are standing up and looking up.
 
To convince yourself that it is an illusion is easy. When the moon is full, on the horizon and looks large, put your thumb out at arms length and you will note that your thumb covers the moon. Do it again when the moon is high.

You will note that thumb covers the moon the same no matter how big or small it looks. This means that it has to be an illusion because an experiment (your thumb) confirmed that the moon is the small size in both cases. Why that is so is another question which I don't know.
 
In the middle of the sky your eyes have no reference points for size.
 
Flying Penguin said:
In the middle of the sky your eyes have no reference points for size.

That is exactly what it is not. Read the first link in my earlier post.

Bad Astronomy: The Moon appears larger on the horizon than overhead because you are comparing it to foreground objects.
 
The link the BA provides is interesting: http://www.lhup.edu/~dsimanek/3d/moonillu.htm

It doesn't give a difinitive answer, but suggests it is partly due to a number of different illusions. The primary one, however, seems to be based on our inner ear's and eye's ability to distinguish vertical angles. Most people don't realize it because they don't know how high objects actually are, but people tend to overestimate the altitude of low objects in the sky. Because of this, we percieve the dome of the sky to be stretched at low altitude (the sky dome illusion).
 

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