Q&A: How Big is That Moon on the Horizon?

Julia Jayes asks:

Why does the sun appear larger when it's near the horizon? ;)

My wife has frequently bothered me (I mean asked me) with this question too.  Recently the moon was at it's closest point to earth for the next few years and some discussion about how large the moon appears when close to the horizon popped up.  MSNBC.com ran an article on the larger than normal full moon.  Robert Roy Britt wrote: "At moonrise, the moon will appear even larger than it will later in the night when it's higher in the sky. This is an illusion that scientists can't fully explain. Some think it has to do with our perception of things on the horizon vs. stuff overhead.  Try this trick, though: Using a pencil eraser or similar object held at arm's length, gauge the size of the moon when it's near the horizon and again later when it's higher up and seems smaller. You'll see that when compared to a fixed object, the moon will be the same size in both cases."

I have my own theories about this phenomenon.  I haven't found any research yet that backs me up, but what the heck, that's never slowed me down before.  I believe that humans (and other animals like raptors, or birds of prey) do not have true telescopic vision, that is we are not able to manipulate the images that hit the back of our eyeballs and make them larger than they would normally appear.  If we could, that would consititue true telescopic vision.  But when we need to take a closer look at something far away our brain concentrates its efforts on the input from the light sensing neurons (called rods and cones) that are "seeing" light coming into the eye from that far away object.  That light would be concentrated in the center of the retina.  Perhaps the brain can even draw input from rods and cones that are only used or "switched on" when extra detail is needed.  When this happens, the perceived image has more resolution. 

If all of the above is true, then one of the byproducts is the perception that the object we are concentrating on appears larger.  Think about it.  When you try to observe the details of color or pattern on a bird that's flying nearby, do you actually "see" the nearby elements of your surroundings, like the low shrub that's growing just in front of you, or the person standing just to your right?  These are objects that are easily within the normal scope of your vision.  That is, when standing in the same place and looking in the same direction, but not concentrating on the bird, you would easily notice the shrub and the person.  I believe that the brain can, at its discretion, "ignore" the periphery of the field of view and concentrate on a smaller field when more resolution is called for.

Thus, when we look at the moon, especially when it's near the horizon, our brains concentrate on the center of our normal field of vision and consequently the moon appears larger.  Part of this perception is bolstered by the appearance of other features in view near the horizon.  For example, trees or mountains on the horizon tend to reinforce the perception that the moon looks larger than normal.  Although the effect is not as noticable when an object is viewed overhead, I believe it still is at work.  There's just nothing in the field of view to compare it to.

This is my theory.  If someone does the research and it turns out I'm right, I want the effect named "Steve's Theory of Perceived Telescopic Vision".  Yep.  That ought to do it.  

 

 

Comments

[Julia]

Heee!

Sounds good to me. If you do indeed get the theory named after you, can I get credit for being your inspiration?

[Steve]

Sure. Why not.