Moon optical illusion

[Brian Gaff (Sofa]

A fiend of mine was very excited yesterday evening when he was driving home. He said the moon was three times its usual size and reddy yellow colour.

Obviously I cannot comment myself, but I do recall back in the 80s, sitting on the Breakwater a resort on Tenerife, and in one direction the sun was setting and that was dim but you could see it reflected in the see, and in the other was this apparently huge mooned rising. My thought then as now were that its an optical illusion or some artefact due to the low angle through more of the atmosphere at that angle.

Brian

[Martin Brown]

It is a purely optical illusion.

The moon when it is rising is fully one Earth radius further away from an observer and slightly vertically compressed for good measure.

Chapter and verse here together with some nice photos.

You brain finds it convenient to make things on the horizon look bigger than things that are high in the sky. Perhaps because humans have never experienced predation by huge raptors (unlike smaller creatures).

[Andy Burns]

The moon does appear much larger when it's near the horizon, but it's a illusion. There is a small variation as its distance varies throughout each month, and also over the course of about 200 days (and no doubt other longer cycles too) because its orbit is elliptical, if this occurs when the moon is full, it's called a supermoon.

[Dave Plowman (News]

I've only seen it once. The first time I went abroad on holiday - to the Algarve. In the days when you were only allowed to take 50 quid out of the country.

I first thought it was a hot air baloon or something. Quite spectacular.

[newshound]

Never heard that theory before, very interesting!

[Jim Stewart ...]

The Isla Craig can look like that coming oner the hill to the coast at Trump Turnberry

[Tim Lamb]

A book I read many times as a youth had a sequence of photos demonstrating that the Moon size does not change as it rises above the horizon.

Rather left me wondering if there is a difference between a camera lens and the human eye.

[Robin]

cameras don't have monkey brains

[Max Demian]

It's supposed to be about how the brain determines the absolute size of things when we don't know how far away they are. When the moon is near the horizon, we assume it is at the horizon. When it's high in the sky, we assume it's much nearer. (I think I've got that the right way round.)

[Tim Streater]

I think our mental perception of the sky above is not as a hemisphere, but rather a flattened one.

[newshound]

But given that the brain has, in principle, the hardware to calculate the angle subtended, how does comparison come into it? I'd be much more inclined to go with Martin's argument that the eye "zooms in" on objects on the horizon because they are much more likely to be threats.

[Peter Able]

IIRC - Tell your pal to watch the phenomena through a smoked piece of glass.

PA

[Brian Gaff (Sofa]

Hmm, well I don't actually buy that, since at the time I remember trying how much I need to reach out to cover it with my thumb, and its less far than when it is full and high up. I am willing to agree it may be some kind of atmospheric lensing effect, but if its the brain I cannot square the old perspective test. Unfortunately as I say, being blind now I cannot really run any tests on it and its kind of the last thing you are thinking off when on your holidays. Brian

[Brian Gaff (Sofa]

Well assuming no zoom there should not be a lot of difference, but then if we are talking what effect the brain uses, then it could be a bit like the software zoom some cameras use.It does not seem or did not seem, when I was able to see, that the sun and the planets in the same position had the effect. But of course you are not supposed to look directly at the sun,I'd ban the Sun as a health and safety risk, so its a bit hard to tell.

Brian

[Brian Gaff (Sofa]

No its the other way around to work as observed, surely. The difference seems to be the proximity to something known though. This can of course change if you happen to be on top of a hill, since the horizon is further away. I'm still not convinced by the pure illusion theory, and feel there may be some kind of process in the mix somewhere. Brian

[Dave Plowman (News]

When I saw it, was in town. Appeared above the end of the street, so had buildings to get a reference from.

It's a long time ago, but guess it looked about 4 times the size of normal. And of course red rather than white/yellow.

[newshound]

That's exactly what the brain does. The distance between excited receptors is a proxy.

No, the brain either does a calculation for range if it thinks it can assume the size, or a calculation for size if it can estimate range.

Balls. If you are in a dip in the ground, your horizon may only be fifty metres away. In tiger or brown bear country, be very very afraid.

[Andy Burns]

But your brain was probably convinced it wasn't far beyond the end of the street, rather that a quarter of a million miles away

[Peter Able]

The other way around? Re-read my post. I've not made any comment, relative or absolute, on observations reported - just suggested that your pal might acquire more experimental data.

Unless you mean smoking your pal and telling the glass to look through him !!!

PA

[Dave Plowman (News]

Quite. I'd guess we'd not have been enjoying an evening stroll if the moon had shifted that much closer. ;-)