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I've read various ideas about why the moon looks larger on the horizon. The most reasonable one in my opinion is that it is due to how our brain calculates (perceives) distance, with objects high above the horizon being generally further away than objects closer to the horizon.

But every once in a while, the moon looks absolutely huge and has a orange red color to it. Both the size and color diminish as it moves further above the horizon. This does not seem to fit in with the regular perceived size changes that I already mentioned.

So what is the name of this giant orange red effect and what causes it?

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  • $\begingroup$ It sounds like you understand why its large (it is a perceptual change and not an optical) - if thats the case, you may want to rephrase it to be clear that you're just asking about the color. $\endgroup$ – rfusca Jun 3 '11 at 2:37
  • $\begingroup$ But I don't understand what makes it even larger than normal during the event I described. Sometimes it's not just bigger near the horizon but really really bigger. Thats the part I am asking about (as well as the accompanying color change). $\endgroup$ – logicbird Jun 3 '11 at 2:47
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    $\begingroup$ The illusion that the moon looks bigger near the horizon is called the en.wikipedia.org/wiki/Moon_illusion $\endgroup$ – Gaurav Jun 26 '11 at 6:28
  • $\begingroup$ Same Astro.SE question: astronomy.stackexchange.com/q/170/476 $\endgroup$ – Qmechanic Nov 23 '13 at 19:30
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Harvest Moon (Source, Wikipedia Commons)

The moon is generally called a "Harvest Moon" when it appears that way (i.e. large and red) in autumn, amongst a few other names. There are other names that are associated with specific timeframes as well. The colour is due to atmospheric scattering (Also known as Rayleigh scattering):

may have noticed that they always occur when the Sun or Moon is close to the horizon. If you think about it, sunlight or moonlight must travel through the maximum amount of atmosphere to get to your eyes when the Sun or Moon is on the horizon (remember that that atmosphere is a sphere around the Earth). So, you expect more blue light to be scattered from Sunlight or Moonlight when the Sun or Moon is on the horizon than when it is, say, overhead; this makes the object look redder.

As to the size, that is commonly referred to as the "Moon Illusion", which may be a combination of many factors. The most common explanation is that the frame of reference just tricks our brains. Also, if you look straight up, the perceived distance is much smaller to our brains than the distance to the horizon. We don't perceive the sky to be a hemispherical bowl over us, but rather a much more shallow bowl. Just ask anyone to point to the halfway point between the horizon and zenith, and you will see that the angle tends to be closer to 30 degrees as opposed to the 45 it should be.

University of Wisconsin discussion on the Moon Illusion.

NASA discussion on moon illusion.

A graphical representation of this:

Optical Illusion illustrated

Dr. Phil Plait discusses the illusion in detail.

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  • $\begingroup$ The term "harvest moon" is not a general term. It refers specifically to the full moon nearest the autumnal equinox. $\endgroup$ – user11266 Jan 1 '13 at 6:27
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    $\begingroup$ @JoeH that is correct. As I said in the answer: The moon is generally called a "Harvest Moon" when it appears that way (i.e. large and red) in autumn Maybe the parenthetical caused a miscommunication. $\endgroup$ – Larian LeQuella Jan 1 '13 at 15:23
  • $\begingroup$ Yes it did, as I thought it related to "that way." Thanks. $\endgroup$ – user11266 Jan 2 '13 at 22:19
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The two effects are not related.

The size appearing larger is a matter of some speculation to this day, but it is purely a psychological effect. If you want to prove this, take a look a the moon while standing up and looking between your legs. It won't look nearly as large.

The red/orange color is related to the sunset being red. In fact, it's the same thing exactly. The blue and green light has already been scattered, leaving only the red/orange light. This can be exacerbated by any of the things which cause sunsets to be more vivid, including pollution, clouds, dust, volcanic activity, etc.

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  • $\begingroup$ or look at the moon at both points with a consistent reference (like a scale at a fixed distance from your eyes) at the horizon and when up high $\endgroup$ – ratchet freak May 7 '14 at 10:37
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    $\begingroup$ +1 especially for the crazy experiment in the second paragraph. That's fantastic: it works so well! $\endgroup$ – WetSavannaAnimal Jun 10 '15 at 0:10
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It is an optical illusion. It only looks bigger near the horizon because it can more easily be compared to familiar objects on the ground. If you hold up a coin in front of your line of sight while looking at the moon and then compare your arm extension for a low moon and a high moon you see that they are the same. IOW, the diameters are the same.

Don't have a link because I learned this from the late night PBS astronomy show Jack Horkheimer: Star Gazer years ago and I don't think Jack would lie to us ;o)

Edit to answer the color question. At a low angle your line of sight is cutting through more atmosphere, so the "color saturation" goes up depending on what gases are in the atmosphere at that particular time and area.

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figure

As shown in the figure, the line A on the left represents the retinal plane. Line B represents the convex lens plane and M represents the moon.

When approaching the ground, due to the influence of the objects on the ground, the eyes will feel the moon is far away, so the eyes will use a longer focal length to image the moon (such as oa for its focal length). At the zenith, because there are no other objects, the eyes will feel the moon is closer, so the eyes will use a shorter focal length for the moon imaging (as shown in Figure sa for its focal length). The result is that the imaging ad of the former is larger than the imaging ab of the latter. So the observer will feel that the moon near the ground looks larger than the moon on the zenith.

So why is the moon on the ground as big as the moon on the zenith when photographed with a camera? Because the camera uses the same focal length to shoot the moon.

Is there any evidence to support my explanation?

Please put your thumb close to your eyes, and let your eyes, thumbs, and the moon align roughly at three points. Then focus your eyes on your thumb and watch the moon in the sky behind your thumb. Mother, you'll find that the moon is terribly small! Why? Because when you focus on your thumb, the focus of your eyes is very short. According to the theory introduced earlier, if you look at the moon with a short focal length, it will appear smaller. So conversely, when the focal length is longer, the moon looks larger.

This thumb experiment can be done by anyone, and it also confirms our theory.

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protected by Qmechanic Mar 20 '13 at 10:37

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