Moon Near Horizon
Watch this video which provides summary of this post.
Overview
On a full Moon Day, in the evening, we often enjoy the breathtaking beauty of the rising full Moon in the eastern sky. As it rises from the horizon, it appears so beautiful and big. However, after a while, as it comes up high in the sky, it looks normal in size.
Have you ever wondered why the Moon appears bigger when it is near the horizon while rising or setting? What makes it bigger near the horizon? Let us investigate.
Illusion
First, look at the picture below; particularly compare the red line at the bottom and the black line at the top. Which one is bigger? Obviously, the black line looks bigger. But actually, both the lines are of the same length. What is going on? Why does the black line appear bigger?
The simple answer is it is all in our brain. No physical phenomenon is responsible for this. It is our brain which tricks us and makes us think that the black line is bigger than the red.
Our brain makes decisions about size based on the perception of distance and the size of things around it. The sizes of objects in the foreground give clues to the brain in perceiving the distance.
Moon Illusion
The same thing happens when we look at the Moon near the horizon. The objects in the foreground give clues to the brain for deciding the distance of the Moon. It creates an illusion that the Moon is much closer and consequently much bigger, though the Moon’s size and its distance from us remain the same, on a given day, regardless of whether it is near the horizon or at the zenith.
The point is, no physical phenomenon is responsible for this Moon illusion. It is our brain which makes us visualize that the Moon is bigger near the horizon.
This is the best explanation science can provide now, though the exact reason for the Moon illusion is not completely understood.
The bottom line is, we do not completely understand why we perceive the Moon to be bigger near the horizon.
Yellow or Orange Hue
You might have also noticed the Moon appearing yellow or orange when it is near the horizon. This is because light, from the Moon, has to travel a longer distance in the atmosphere to reach our eyes. The dust particles in the atmosphere scatter away all the colors at the blue end (with high frequency) of the visible spectrum. What reach our eyes are the colors at the red end of the spectrum. That is the reason we see the Moon, or the Sun, near the horizon in yellow or orange hue.
If you would like to read more on the scattering of light, its effect, and why the sky is blue, you can read here.
References
- The Cambridge Guide to the Solar System by Kenneth R. Lang, Cambridge University Press.