# Why does the moon appear larger at the horizon?

1. Jun 22, 2013

### acesuv

I've heard two hypothesis:

1. The air causes the image of the moon to literally appear larger

2. The increased size of the moon comes purely from illusion of perspective

Which one is it?

2. Jun 22, 2013

### ModusPwnd

2

Try it by comparing it to an object for reference. (a ruler perhaps)

3. Jun 22, 2013

### acesuv

So #1 is definitely wrong? :uhh:

4. Jun 22, 2013

### D H

Staff Emeritus
That's why it's called the moon illusion. It is an incredibly compelling illusion, even for those who know that it's all in our heads.

5. Jun 22, 2013

### Staff: Mentor

Pick a camera, make one picture at the horizon and one when the moon is high in the sky (with the same camera settings of course), and compare.

The moon is nearly full now.

6. Jun 22, 2013

### technician

A good comparison object is a coin. I think you will find that a 2p coin held at arms length covers the moon wherever it is in the sky.(it also covers the sun but I would not recommend looking directly towards the sun)

7. Jun 22, 2013

### phinds

acesuv, just FYI, had you typed the subject line of this thread into Google you would have gotten the same information as give here. Try to remember that Google is your friend.

8. Jun 23, 2013

### Baluncore

Your brain's perceptual framework changes depending on the situation, but you do not recognise the vague area where the two methods of assessment blend and cross over. It is an illusion.

When seen near the horizon your estimate of the moon's size is assessed by comparing the angular diameter of the moon with the angular separation between the moon and the horizon.
size(low) = moon / gap.

When far from the horizon your estimate of the moon's size is based on the area of the moon compared with the area of the sky.
size(high) = moon^2 / sky^2.

Since: size(low)^2 = size(high), the dimensions of size are different and so cannot be honestly compared.

9. Jun 24, 2013

### Andrew Mason

No. 3. The increased size of the moon is real. Right now the moon appears largest because it is at its closest distance from to the earth. It is 14% larger and 30% brighter than at the apogee.

AM

10. Jun 24, 2013

### Staff: Mentor

Well, that is not related to its apparent position in the sky.

11. Jun 24, 2013

### bahamagreen

The best explanation I have seen is that you are mentally judging the moon's size by comparing its apparent size to its apparent distance - and your visual estimate of its distance is different when veiwed low to the horizon vs high in the sky.

We "stick" the moon onto the sky canopy, which appears relatively low above us, but extends way off in the distance to the horizon.

The way we actually "see" to sky is like a low flat barrier - as if the highest point of the sky was only a few miles above us and curves very slightly down to the horizon many miles far away. The problem is that we know and see a flat horizon as being about 7 miles away, and anything with elevation (like the rising/setting moon) is going to look like it is about 10 miles or so away.

The result is that the sky canopy itself looks extremely close above us and quite far away at the horizon. You notice this very strongly on an overcast day where you can see the choud patterns showing this. When the moon is up high in the sky, it looks like it is about three miles away, and we make the visual judgement that it must be pretty big to subtend that angle at that distance... when we see it low on the horizon, it looks like it is about 10 or so miles away, and we make the visual judgment that it must be larger to look that size at that distance.

12. Jun 24, 2013

### sophiecentaur

Human visual judgment is really 'poor' and subject to circumstances. Take a ladder. Lay it on the ground and look at it. Then lean it up against a wall and look at it. Then climb to the top and look down along it. You will perceive three very different lengths.

One big breakthrough, in Science, was when they decided to measure things, rather than just to look at them.

13. Jun 24, 2013

### Andrew Mason

I had just been out for a walk and remarked to my wife how much larger the full moon on the horizon appeared tonight than on previous occasions. So I just assumed the question as asking why the full moon appears larger at the horizon now as opposed to on other full moon occasions.

If the question is why does it look bigger on the horizon than higher in the sky I am not sure I agree with the premise. It looked pretty big higher up in the sky last night.

The moon should appear a bit bigger as the moon rises since the distance from the moon a point on the surface of the earth is a minimum when the point on the earth is on the line between the centre of the earth and the centre of the moon.

AM

14. Jun 24, 2013

### D H

Staff Emeritus
That is precisely the premise, Andrew. It's called the moon illusion. It's a very compelling illusion to a significant majority of viewers. I myself am susceptible even though I am fully aware that it's all in my head. A full moon on the horizon looks huge compared to the tiny circle the moon makes when it is high in the sky. There have been a number of competing hypotheses put forth to explain why this illusion is so compelling.

15. Jun 24, 2013

### Bandersnatch

I hear a good way to dispell the illusion is to bend down and put one's head between legs, so as to look at the Moon upside down.

Surgeon General's warning: Performing this experiment might damage your spine and/or social standing.

16. Jun 24, 2013

### Staff: Mentor

There is actually a real and quantifiable effect of size altering due to refraction, but "the moon illusion" myth gets it backwards: objects closer to the horizon are smaller, not larger than objects high up.

This is a byproduct of objects appearing higher than they really are, which is an effect requiring correction for accurate celestial navigation:
http://en.m.wikipedia.org/wiki/Atmospheric_refraction

Since the effect increases rapidly as you approach the horizon, objects appear noticeably flatter unless another atmospheric effect intervenes.

Last edited: Jun 24, 2013
17. Jun 25, 2013

### acesuv

Thanks, I'll definitely consider using google for simple questions like this.

18. Jun 26, 2013

### cabraham

A long time ago I observed the moon high in the sky and at the horizon comparing its image to a coin. I held the coin at arm's length and observed the relative size and coverage. To my surprise and in accordance with what I read about this subject, the size of the moon was unchanged. Blocking it with the same coin held at arm's length affirmed that the size of the moon's image is the same high in the sky and at the horizon.

Nothing at all to debate here. The image appearing larger at the horizon has already been correctly explained. We perceive the sky as a canopy with a low height, but the horizon seems far away. At the horizon the size is the same as high in the sky. But because we think the horizon is further away than the sky canopy, our mind falsely concludes that the horizon image is larger.

The coin affirms positively that the images are both the same size. Trust measurements more than your senses.

Claude

19. Jun 27, 2013

### Andrew Mason

Your measurements were not precise enough. The moon should be about 1.5 percent larger when directly overhead than when it is seen on the horizon.

20. Jun 27, 2013

### cjl

Yes, but that's irrelevant to the topic at hand, since a 1.5% change in size is not going to be visibly detectable, and thus could not account for the observed effect (also, the change is in the wrong direction, since the illusion is of a substantially larger moon at the horizon).

21. Jun 27, 2013

### sophiecentaur

Also, the length of your arm could effectively change as you raise it and lower it. That experiment is certainly near enough for jazz and you shouldn't really be dissin' it.

22. Jun 27, 2013

### Andrew Mason

I was just pointing out that the moon is not the same size high in the sky as it is at the horizon. And I don't know what "near enough for jazz" means but it doesn't sound like science.

AM

23. Jun 27, 2013

### sophiecentaur

Look it up on Wiki. It's in the same section as irony and humour.

24. Jun 27, 2013

### gerben

Moreover, the distance from (each of) your eye(s) to the tip of your finger is not the same for every orientation of your stretched arm.

25. Jun 28, 2013

### sophiecentaur

The whole experiment is pretty approximate but it's sufficient to show that the phenomenon in the OP is really just an illusion. Just how far do you go in any experiment? Time, money and inclination are limited resources.

It would be interesting to find out whether the fact that the Moon looks bigger when it's closer to the Earth is mainly because of the angle subtended or mainly the brightness difference. It certainly looks different on successive nights, when the surrounding light environment is different but the distance hasn't changed. There's another can of worms to discuss.