Why does the moon appear bigger near the horizon?

Cecil's Columns/Staff Reports
1

I just read one of your old columns, Why does the moon appear bigger near the horizon? Even though the effect may be an optical illusion, the “prove it to yourself” test you gave isn’t entirely accurate. You suggested measuring the moon’s diameter by holding a caliper (such as a bent paper clip) at arm’s length, and I’ve seen that test suggested elsewhere as well.

The difficulty is that raising your arm brings the caliper closer to your eye. I just did some informal measurements and found that the caliper is 24-1/2 inches from my eye if I sight along my arm when it’s horizontal, 20 inches when it’s at about 45 degrees, and 18-1/2 inches when it’s straight up. Wouldn’t that be enough to change the calibration of this ad-hoc “test instrument”?

2

That would be enough to make a difference: The apparent size of the calipers is inversely proportional to the distance, so at straight up when it’s only 3/4 the distance away, the calipers would appear to be 4/3 times larger. If you want to be more rigorous about it, you’d use a cross-staff, a caliper-like device on a long arm, the end of which you rest on your face right next to your eye, so you can keep the distance uniform.

3

In this age of digital cameras, it’s even easier to test it yourself. Take a photo of the Moon near the zenith and take another one near the horizon when it looks big, using the same camera and zoom setting. The Moon images will be exactly the same size…

4

So how do they get pictures of the moon looking big in the sky? (I’m sure there will be better examples, but I’ve not found them yet.)

5

Well, that one’s definitely manipulated: There’s no way it’s possible for the Moon to look bigger than the Sun like that. But if you’re talking about things like two lovers silhouetted in front of the rising Moon, the trick is that the Moon looks the same size as it always does; it’s the two lovers who appear smaller. If you take a picture right next to the lovers or from a thousand feet away, the Moon is still going to be a half a degree; a thousand feet doesn’t make any difference compared to the distance from the Earth to the Moon. But the lovers are going to appear much smaller from the great distance, and from a great enough distance, you can get them to less than half a degree in the field of view. Then, you just zoom in really close so they (and the Moon behind them) fill the field of view.

6

See the Wikipedia article on Dolly Zoom.

7

catalert - you’re absolutely right that your hand (and thus the caliper) are closer to your eye overhead than outstretched in front of you.

Unless you lie down to do it when the moon is overhead. Then your arm is outstretched both at the horizon (when you’re standing), and overhead.

8

That’s a computer generated image of an alien sky.

9

It’s unlikely you’d get a picture like that under real circumstances. The geometry is such that when a planet is that close to the sun-observer line, the crescent is actually a lot narrower. And a sky that bright would hide the crescent.

10

Cecil is 100% correct, actually, there is a little more that goes with it. It has to do with the brain (of which the eye is a direct outgrowth from) overestimates horizontal distances compared to vertical. It’s too much to show you the math, just take a picture of the moon with a digital/phone camera at the horizon when you think it looks big, then take a picture when it is directly above you. If you measure them, you will find they are exactly the same size…it is an optical illusion. I can give you a few more. eye doctor/ bachelor of Science in visual optics… 3paws

11

Since there’s a current thread on this subject, I’ll close this old one and direct further comment to the more recent one here

bibliophage
moderator CCC