Tails of a wondering Moon
September 27th 2007 05:58
Why does the Moon appear so large when it first rises?
Perhaps the most dramatic of all celestial illusions occurs when a seemingly giant Full Moon rises at dusk,( personally I look forward to seeing a full Moon.)
I am sure you must have wondered why it looks so large at these times and you may rationalize that the most likely cause is that the atmosphere somehow magnifies the Moon's image a common misconception that's been around for a couple of thousand years. The correct explanation, however, has eluded perceptual scientists for centuries and still remains uncertain.
This "Moon illusion" is believed to be caused by the foreground somehow influencing the way we perceive our nearest celestial neighbour. One compelling explanation involves how the eye and mind perceive rigid objects in motion. Think about standing by a road side and watching a car drive away. Visually, how do you know it's receding? Its image on the retina appears to shrink as the car approaches the horizon. As long as these two visual cues are in sync no problem. The moment you remove one visual cue then our perception of the object changes.
For example when you see a full moon rising and the surrounding trees and horizon are in view the Moon appears to be larger however if you then take a picture of the same scene and remove the horizon and trees the Moon appears as “normal” the Moon does not change in this particular instance its shape or size its just our perception of we consider “normal” (however we can see a change in the size if the Moon in certain instances and I will show you an example) It’s the same as seeing the “Man in the Moon”.
when we where children our parents told us to look at the at the Moon ,do you see a Man’s face? We then train our brains to perceive a image of an “Old Man’s face”. Some cultures see a rabbit it’s the way our eyes and brains perceive light and dark objects the brain then fills in the gaps to make the image meaningful in our mind’s eye.
There is another aspect of the Moon that has been a part of my research project for some years now that may be of interest .This also deals with perception this time it is what we don’t “see”, let me explain.
Every month the Moon continues on its merry way orbiting our Earth in the usual manner. The Moon’s orbit around the Earth is slightly elliptic and this causes the Moon’s distance from the Earth to fluctuate several thousands of kilometres either way a term referred to as Lunar Perigee and Apogee. One of the most spectacular phenomena in naked-eye astronomy escapes notice by the vast majority of people simply because the the eye and brain can't compare the size and brightness of objects observed on separate occasions, how is this possible?.
The combined effects of orbital eccentricity and the Sun's gavitational forces result in a substantial difference in the apparent size and brightness of the Moon at perigee and apogee. Extreme values for perigee and apogee distance occur when perigee or apogee passage occurs close to new or full Moon, and long-term extremes are in the months near to Earth's perihelion passage (closest approach to the Sun, when the Sun's gravitational forces are strongest) this occurs at certain times of the year.
I have prepaied a graphic example to show you, I have taken two images of a full Moon using the same cameral and lense on each occasion. The first image on the left was taken on July 31 2007 the Image on the right was taken on september 26 2007.
The Apogee ( greatest distance from Earth) was calculated at being around 404150 km on July 22 8:44conversly the Perigee (closest approach to Earth) was Sep 27 1:54 359419 km a variance of 44731 Km.
At the time these pictures were taken the size of the Moon appears to be the same in both images, in fact there not the same as the image on the right is slightly larger because it was closer to Earth at the time you can measure both images using special software available on the net the Moon image on the right is around 6 pixels bigger. When I took these pictures I did not notice any difference and there was an actual difference it is only when these images are compared that we find there is even though the eyes tricked us.
Photos: Carl
Perhaps the most dramatic of all celestial illusions occurs when a seemingly giant Full Moon rises at dusk,( personally I look forward to seeing a full Moon.)
I am sure you must have wondered why it looks so large at these times and you may rationalize that the most likely cause is that the atmosphere somehow magnifies the Moon's image a common misconception that's been around for a couple of thousand years. The correct explanation, however, has eluded perceptual scientists for centuries and still remains uncertain.
This "Moon illusion" is believed to be caused by the foreground somehow influencing the way we perceive our nearest celestial neighbour. One compelling explanation involves how the eye and mind perceive rigid objects in motion. Think about standing by a road side and watching a car drive away. Visually, how do you know it's receding? Its image on the retina appears to shrink as the car approaches the horizon. As long as these two visual cues are in sync no problem. The moment you remove one visual cue then our perception of the object changes.
For example when you see a full moon rising and the surrounding trees and horizon are in view the Moon appears to be larger however if you then take a picture of the same scene and remove the horizon and trees the Moon appears as “normal” the Moon does not change in this particular instance its shape or size its just our perception of we consider “normal” (however we can see a change in the size if the Moon in certain instances and I will show you an example) It’s the same as seeing the “Man in the Moon”.
when we where children our parents told us to look at the at the Moon ,do you see a Man’s face? We then train our brains to perceive a image of an “Old Man’s face”. Some cultures see a rabbit it’s the way our eyes and brains perceive light and dark objects the brain then fills in the gaps to make the image meaningful in our mind’s eye.
There is another aspect of the Moon that has been a part of my research project for some years now that may be of interest .This also deals with perception this time it is what we don’t “see”, let me explain.
Every month the Moon continues on its merry way orbiting our Earth in the usual manner. The Moon’s orbit around the Earth is slightly elliptic and this causes the Moon’s distance from the Earth to fluctuate several thousands of kilometres either way a term referred to as Lunar Perigee and Apogee. One of the most spectacular phenomena in naked-eye astronomy escapes notice by the vast majority of people simply because the the eye and brain can't compare the size and brightness of objects observed on separate occasions, how is this possible?.
The combined effects of orbital eccentricity and the Sun's gavitational forces result in a substantial difference in the apparent size and brightness of the Moon at perigee and apogee. Extreme values for perigee and apogee distance occur when perigee or apogee passage occurs close to new or full Moon, and long-term extremes are in the months near to Earth's perihelion passage (closest approach to the Sun, when the Sun's gravitational forces are strongest) this occurs at certain times of the year.
I have prepaied a graphic example to show you, I have taken two images of a full Moon using the same cameral and lense on each occasion. The first image on the left was taken on July 31 2007 the Image on the right was taken on september 26 2007.
The Apogee ( greatest distance from Earth) was calculated at being around 404150 km on July 22 8:44conversly the Perigee (closest approach to Earth) was Sep 27 1:54 359419 km a variance of 44731 Km.
At the time these pictures were taken the size of the Moon appears to be the same in both images, in fact there not the same as the image on the right is slightly larger because it was closer to Earth at the time you can measure both images using special software available on the net the Moon image on the right is around 6 pixels bigger. When I took these pictures I did not notice any difference and there was an actual difference it is only when these images are compared that we find there is even though the eyes tricked us.
Photos: Carl
The image on the right is slightly bigger
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Comment by Kleonaptra
Kalikapsychosis
I love the moon, and I love learning about it. This is great!