Moon Illusion

[Tom OD]

Hi SGL,

Another attempt to demonstrate the Moon Illusion phenomenon. This pic is a background shoot of the marina with a 14mm lens. As the Moon rose in the East, I took a photo at 200mm when it was on the horizon, and another at 70mm, which I felt gave an approximate size of the Moon as seen by the eye. Every 8minutes I took another set of shots. This time at approx. 190mm and 70mm. I continued taking lesser zoomed images, and a standard 70mm image, I did this until the zoomed image was back at 70mm. The idea was to put the same set of images side by side to show how the eye sees the Moon as a larger object nearer the horizon, while a camera shows that the Moon size remains the same throughout it rising in the sky. The added bonus is the changing colour of the Moon due to the atmospheric refraction as it rises.
The 200mm zoom may be a little too large compared to how it appears, but I needed to use all the range of the zoom lens so that I would have enough photos to composite.

[tooth_dr]

Very cool Tom.  My dad actually asked me this last night, and I explained it doesnt change in size, it just looks bigger!   I almost though I had go it wrong after looking at your image (even though I have in fact supermposed my own images of the moon and verified it doesnt actually change in size!)

[Tom OD]

Thanks Tooth Dr

It still seems that its up for debate how it looks bigger. In this case the camera does not lie, given that the image size is the same for all altitudes of the Moon with the 70mm lens.

What you can do is look through the camera with one eye, then keep the other eye open outside the view of the view finder. You could adjust your zoom until both images seems the same size, and use that as the zoomed image.

Then keep going with the 70mm lens for the "Control" image.

This method is tricky, so I decided just to split up the 70-200mm lens into equal spacing distances, so the sizes probably do not match exactly.

Still a fun shot to do and a little tricky when layering. Clearly there is some issue putting the zoomed image onto the horizon of the 14mm image. But hey, its about the principle here

Tom

[spaceman_spiff]

The moon actually gets slightly bigger (about 20 arcsec if I remember correctly) at its zenith (due south) compared to when it is at the horizon because we have been rotated by the Earth towards it. A am ignoring and distortion cause by the atmosphere here.

Dan.

[Tom OD]

47 minutes ago, spaceman_spiff said:

The moon actually gets slightly bigger (about 20 arcsec if I remember correctly) at its zenith (due south) compared to when it is at the horizon because we have been rotated by the Earth towards it. A am ignoring and distortion cause by the atmosphere here.

Dan.

I was not aware of this. What is the effect that would do this, as in why would facing it directly make it larger?

Tom

[Paz]

I have read in the past that the moon looking bigger close to the horizon is psychological. The brain, for some reason, thinks the sky is a flattened dome rather than a circular hemisphere, so it thinks the moon is further away when on the horizon than when at the zenith. Because, for practical purposes, the moon actually appears to be the same size in both cases the brain assumes the object on the horizon must therefore be bigger (but the photos show that it is not in fact bigger).

[ollypenrice]

It's a shame that this has come up for discussion too late for our friend Beate to comment. She had a second home in nearby Laborel and was a good friend before her death last year. Her undergraduate thesis in psychology was on this very subject.

Olly

[clarkpm4242]

21 minutes ago, Tom OD said:

I was not aware of this. What is the effect that would do this, as in why would facing it directly make it larger?

Tom

In the extreme example of this the observer is nearer to the moon by approximately the radius of the earth as they rotate 'closer' to it (moonrise in the east, transits directly overhead and sets in the west).  Usually the movement 'nearer' is less owing to the arrangement of the earth, moon and observer.

[spaceman_spiff]

1 minute ago, Tom OD said:

I was not aware of this. What is the effect that would do this, as in why would facing it directly make it larger?

Tom

Hi Tom,

It's simple geometry.

As a simple example, if you are on Earth's equator and the Moon is located on the celestial equator (dec = 0 degrees) when the Moon is on the horizon, the line connecting you to the Moon is a tangent of Earth's curvature. Once the Moon is at its zenith, the line connecting you to the Moon is perpendicular to the Earth surface. You have actually travelled about 1 Earth radius (6000 Km) towards the moon (about 1/60th of the distance to the moon). It stands to reason that the Moon will appear about 1/60th larger (about 30 arcmin / 60 ~30 arcsec bigger). The effect will be most profound if you are on the equator and there is no effect at the poles.

I should add that this effect is too small to see visually but it is noticeable when imaging the Moon, I have to rescale my lunar panels slightly to get them to fit!

Dan.

[Tom OD]

3 minutes ago, spaceman_spiff said:

Hi Tom,

It's simple geometry.

As a simple example, if you are on Earth's equator and the Moon is located on the celestial equator (dec = 0 degrees) when the Moon is on the horizon, the line connecting you to the Moon is a tangent of Earth's curvature. Once the Moon is at its zenith, the line connecting you to the Moon is perpendicular to the Earth surface. You have actually travelled about 1 Earth radius (6000 Km) towards the moon (about 1/60th of the distance to the moon). It stands to reason that the Moon will appear about 1/60th larger (about 30 arcmin / 60 ~30 arcsec bigger). The effect will be most profound if you are on the equator and there is no effect at the poles.

I should add that this effect is too small to see visually but it is noticeable when imaging the Moon, I have to rescale my lunar panels slightly to get them to fit!

Dan.

Ah of course. I just never thought that we really get closer to any astronomical object as in general the distances are so vast. But in this case that would make sense. 

Thanks for the reply. 

Tom. 

[Stu]

Nice image Tom, it certainly creates the right sort of effect.

Tonight, as I pulled into the M3 heading home, I saw a very large orange disc low down on the horizon. It looked huge, and it took me a good few seconds to realise it was the moon! Lovely orange colour to it tonight.

[Scott]

My father-in-law was around today and knowing my interest in astronomy began telling me how the moon over the week-end was a special one as it was quite a bit larger than the norm. After stumbling over the right words to use I settled on mumbling something about the effects of the atmosphere closer to the horizon and quickly asked about his old 1964 ford anglia. Sometimes it's just not worth it

Nice pick Tom although I'm sure said father in law would see it as confirmation 

[Tom OD]

13 minutes ago, Scott said:

My father-in-law was around today and knowing my interest in astronomy began telling me how the moon over the week-end was a special one as it was quite a bit larger than the norm. After stumbling over the right words to use I settled on mumbling something about the effects of the atmosphere closer to the horizon and quickly asked about his old 1964 ford anglia. Sometimes it's just not worth it

Nice pick Tom although I'm sure said father in law would see it as confirmation 

Ha ha. Well I hope my pic can settle some of these debates. 

T. 

[Tom OD]

30 minutes ago, Stu said:

Nice image Tom, it certainly creates the right sort of effect.

Tonight, as I pulled into the M3 heading home, I saw a very large orange disc low down on the horizon. It looked huge, and it took me a good few seconds to realise it was the moon! Lovely orange colour to it tonight.

Thanks Stu

I think I lost a degree in the background due to some land mass. So it was probably. Lot redder a minute or two before the first snap. I love how it changes colour so quickly.