Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

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. Booth 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?

share|improve this question
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. –  rfusca Jun 3 '11 at 2:37
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). –  logicbird Jun 3 '11 at 2:47
The illusion that the moon looks bigger near the horizon is called the en.wikipedia.org/wiki/Moon_illusion –  Gaurav Jun 26 '11 at 6:28
Same Astro.SE question: astronomy.stackexchange.com/q/170/476 –  Qmechanic Nov 23 '13 at 19:30

3 Answers 3

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.

share|improve this answer
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 –  ratchet freak May 7 '14 at 10:37
+1 especially for the crazy experiment in the second paragraph. That's fantastic: it works so well! –  WetSavannaAnimal aka Rod Vance Jun 10 at 0:10

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.

share|improve this answer
The term "harvest moon" is not a general term. It refers specifically to the full moon nearest the autumnal equinox. –  user11266 Jan 1 '13 at 6:27
@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. –  Larian LeQuella Jan 1 '13 at 15:23
Yes it did, as I thought it related to "that way." Thanks. –  user11266 Jan 2 '13 at 22:19

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.

share|improve this answer

protected by Qmechanic Mar 20 '13 at 10:37

Thank you for your interest in this question. Because it has attracted low-quality answers, posting an answer now requires 10 reputation on this site.

Would you like to answer one of these unanswered questions instead?