The Moon Illusion

We are going to discuss two really interesting phenomena for anyone curious about the Moon within this blog article!             
It's obvious that you normally tend to take a look at the moon when you just walk outside in a beautiful night, especially if there's a full moon. While you enjoying this gorgeous view, have you ever noticed that, this same moon is appearing in different sizes in different days or different positions of the night sky?

If your answer is yes and you are still keen to know the real science behind this phenomena, then you are just about to find out in a simplified manner. We can break this problem in to two cases. One case is something really happening in the physical space and the other case is something happening inside your brain which can be understood with some hypothetical(simplified) explanations.

1. Super Moon

A super moon is a full moon or a new moon which is having a slightly lager disk than the usual apparent size of the full moon you see from the Earth, happening due to the nature of the orbit of the moon around the earth. This moon's orbit around the Earth is not a perfect cycle. It is an elliptical one having an average distance of 382,000km to the Earth. Scientists of NASA explain that the Earth, sun and all the other planets are exerting different gravitational pulling and pushing forces on the moon which give us the opportunity to have this elliptical orbit.

When the moon is at its Apogee position and it also at its full phase or a new moon, then this is called as a Micro-moon or a Mini-moon which may not be so familiar for you. In a similar but opposite manner, brightness will be reduced by 30 percent and disk size will be smaller than 14 percent than usual of a micro moon.

Because of the elliptical orbit of the moon, it shows an oscillating nature in the distance to the Earth as the above graph with the time. As you can see, full moons are in dots and the new moons are illustrated in x makers. In the year 2020, you can see that there are two full moons having the closest distance to the Earth. Those were two super moons on 9th of March and 7th of April which you may have already watched from your own eyes.    

 Is it the only reason?

In the above section we discussed about one reason for moon to be appeared in different size than usual. However, it is not the only reason and super moons are not really show a huge size increment for human eyes. Anyway, at the super moon, you cannot catch a considerable apparent size increment of the moon but, its brightness increment will be very noticeable and really beautiful. 

There is another reason which the image processing inside human brain makes us to believe that "Whoa! Moon is HUGE today!". This is called as the Moon Illusion and in this article we are going to discuss the reasons behind it. 

2. The Moon Illusion

This is an optical illusion which causes the moon to be appeared larger near the horizon than it appears in the higher sky(let's say in the zenith). Many of you might have experienced this thing around a full moon day. Unlike in the super moon situation, this can happen regardless the distance to the moon from Earth.

As in the above image, you see the moon larger near the horizon and it appears in usual size when it is away from horizon. That is the moon illusion and the real reason is still debated by scientists and several reasonable hypotheses are emerged to describe it fairly. Here we are going to discuss the two most reasonable explanations as follows.

* Apparent distance hypothesis

* Relative size hypothesis.  

2.1 Apparent Distance Hypothesis

This theory purposes that the horizon moon seems larger than the moon at the zenith (right above our head) because it looks farther away. Does that make no sense to you?? Let's say in your daily experience, an object overhead, like a bird or an aircraft flying by, appears closer and therefore larger than the same bird or aircraft near the horizon because it really is closer. Our brain built to think that objects near the horizon are (usually) more distant than those are above us because they appear to lie behind and beyond foreground objects. So the same image processing thing is happening inside our brain when we look at the moon.

The same distance and size relationship deduction process happening inside our brain, cannot be used to very far object to us such as moon, sun or other celestial bodies because even if they appear from the horizon or zenith, there won't be a considerable distance difference from us. 


However, our brain tries to apply the same processing to build the distance-size relationship when we look at the moon and it thinks horizon moon is farther away than the zenith moon and it try to compensate the size of the moon by itself. Therefore, our brain makes us to believe the actual size of the moon at the horizon is larger than zenith moon even if the retinal image is same size.

The background of the moon at the time we are looking at it also affect this size judging process inside our brain. Following image illustrates that effect. Most of us see the top Moon, seemingly located in the distance based upon the convergence of the railroad tracks, as larger than the bottom Moon. Yet they are identical. This is called as the Ponzo Illusion.

Now you see how marvelous is our brain. It can lie about what we see from our own eyes...Well that's why it's called as an illusion.

2.2 Relative Size Hypothesis

This is the next best explanation for the moon illusion, which says that, the perceived size of an object by the human brain is not only depending on the retinal image size, but also on the sizes of the surrounding objects which are in the immediate vicinity of our target object. So this effect is named as the Ebbinghaus illusion Do not get confused with the above definition. Let's take a look in to this in a simple manner.

This image is an illustration of the Ebbinghaus illusion. Which orange color circle is the bigger one from two flower kind of patterns? The correct answer is; they are same in size. But, with the sizes of surrounding blue circles, many of you may see the right side orange circle is bigger.    

This animation also shows the same effect more clearly with more steps. You may see the size of the orange color circle is decreasing when the surrounding circles are getting bigger. This means that, when the visual extent is smaller, a certain focused object may have perceived larger than the real size by our brain. 

When we apply this to our scenario which is the moon in the horizon (which represents the smaller visual extent), can be represented as the right side pattern with smaller surrounding circles. Then the moon in the zenith (a larger visual extent) can be represented as the left pattern with larger surrounding circles. Therefore, we see the moon in the horizon larger because there the surrounding vicinity is smaller than the wide sky. Now I think you have a clear idea on why the horizon moon appears bigger than the zenith moon.     

   

In this blog post we discussed two phenomena which cause the moon to be appeared in different sizes in the night sky. I hope you got a clear idea in both of them...Is that all????? Nope.

 How does the Moon Appear Big in Photographs? 

Now all the way I told that the big moon is just an illusion happening inside our brain. Then why the hell we see the moon very huge in some photographs just like the header image of this article and the following image?? 

This photo is a real one taken in Greece with a digital camera. Actually the moon can be captured with this kind of huge size in to photograph with a considerably good telephoto lens kit. But you need to move very far from the targeted objects. In above image you need to go far away from the building with the camera. Then zoom the building and the moon as the size you want with good details and take the shot.

That's all I have to share with you guys about the Moon Illusion. Feel free to ask any further details or something not clear to you regarding the content of the article.