Sun Setting Illusion: Explaining Constant Angular Speed

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Discussion Overview

The discussion revolves around the perception of the Sun's motion during sunset, specifically addressing why it appears to accelerate as it approaches the horizon despite the Earth's constant angular rotation. Participants explore various optical effects and psychological perceptions related to this phenomenon.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes that the Sun appears to accelerate into the horizon, questioning how this can occur if the Earth rotates at a constant angular speed.
  • Another participant suggests that the perceived acceleration is an illusion caused by atmospheric refraction, which makes the Sun appear to slow down as it nears the horizon.
  • A different viewpoint agrees with the idea of an illusion, comparing it to the Moon's appearance near the horizon, which also looks larger than when it is high in the sky.
  • One participant provides a calculation regarding the angular diameter of the Sun and its apparent motion, indicating that it moves slower near the horizon.
  • Another participant points out that the Sun's motion is more noticeable near the horizon due to the presence of the horizon as a reference point, while it is less noticeable when the Sun is high in the sky.
  • A participant mentions various optical effects that occur during sunset, providing links to external resources for further exploration.

Areas of Agreement / Disagreement

Participants express differing views on the cause of the perceived acceleration of the Sun during sunset, with some attributing it to optical illusions and others discussing the effects of atmospheric conditions. No consensus is reached on the primary explanation.

Contextual Notes

Participants reference various assumptions about perception and optical effects without resolving the underlying complexities of these phenomena. The discussion includes calculations and comparisons that may depend on specific observational conditions.

Who May Find This Useful

This discussion may be of interest to those exploring topics in optics, perception psychology, and astronomy, particularly regarding celestial motion and visual phenomena.

hotcommodity
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So I was watching the Sun set the other night, and I noticed that the Sun appeared to accelerate into the horizon. If the Earth rotates at a constant angular speed, how could this be? I was thinking about swinging a ball on a string vertically, and how the ball would have different points of maximum acceleration along its path due to the different forces acting on it. But the Sun only has a single radial force that wouldn't vary thoughout its path (that is, if the Earth was taken as a stationary frame). Can anyone explain this?
 
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it must be an illusion because the sun actually slows down as it approaches the horizon due to atmospheric refraction. My guess would be that what you perceived is due to the sky being "bigger" lower down which causes people to overestimate distance from the horizon
 
I agree with Russ. It's probably the same basic mechanism that makes the moon look huge when it's on the horizon, and normal when it's high in the sky.
 
The angular diameter of the sun is 30 arcminutes ( +- seasonal things ) that is half a degree so if the sun takes about 12 hours to pass through 1800 then for half that would be 2 minutes. I mean the time it would take for the sun to pass if it "touched" the horizon till sunset.

As said above the sun "moves" slower near the horizon.
 
When the Sun is near to the horizon, it's easier to see its motion relative to the horizon. When it's high in the sky, there's no distant "static" object to compare it against, so the motion is less noticeable to the human eye.

Note that comparison against a nearby object like a tree or building doesn't work because nearby objects don't appear "stationary" (due to the parallax effect) unless you hold your head rigidly still.
 

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