Moon Shot - 400mm

joeymas · Dec 16, 2008

Have you ever noticed that the moon sometimes looks huge and other times it looks 1.5 times smaller?

The size of the moon nor it's distance changes.

In fact the moon only looks bigger at the horizon because you actually have other objects in your field of view that your brain uses to compare size.

When the moon is high in the sky, there is nothing else for your brain to use as a reference.

Seen here in this illusion... both of the center circles are the exact same size.

The bottom circle appears larger only because of the smaller objects around it.

Wanna test the theory for yourself?

Take 2 photos. 1 with the moon at the horizon and one with the moon up high.

You will notice that the camera can not be fooled as both moons are the same size, but when you looked at the real moon the horizon moon looked huge....

Neat

astrostu · Dec 16, 2008

Actually, the moon does change size. Its distance from Earth goes between 356,566 km and 406,601 km from Earth. It changes in size from 33'31" to 29'23", altering in size by a factor of 15%.

However, there is also the optical illusion that you spoke of when the moon is near the horizon.

EW1066 · Dec 16, 2008

I have heard over the years that the moon looks larger on the horizon because of a magnifying lens effect of the atmosphere. I have seen it look huge on the horzon in the middle of the ocean. Nothing to compare to for a size perspective.

Vince

ChrisOquist · Dec 17, 2008

Interesting theories - I'm intrigued.. when I have time tonight I'll definitely be looking this up. In any case, that photo is AWESOME!

I almost spend a few minutes staring at it to see if I could find the LEM or a flag. Great shot.

JaimeGibb · Dec 17, 2008

Interesting theories, and flippin amazing photo. I can only hope to add something like that to my portfolio one day!

joeymas · Dec 17, 2008

astrostu said:
Actually, the moon does change size. Its distance from Earth goes between 356,566 km and 406,601 km from Earth. It changes in size from 33'31" to 29'23", altering in size by a factor of 15%.

However, there is also the optical illusion that you spoke of when the moon is near the horizon.

I see what your saying but trust me, your eye can not detect a 50,000 km change in distance on an object over 350,000 km away.

Plus I don't think it travels that far (for everyone on the planet) in just a few hours. We spin.

astrostu · Dec 17, 2008

joeymas said:
I see what your saying but trust me, your eye can not detect a 50,000 km change in distance on an object over 350,000 km away. Plus I don't think it travels that far (for everyone on the planet) in just a few hours. We spin.

I was directly refuting your claim, "The size of the moon nor it's distance changes." This is not factually correct, plain and simple. Also, the eye can detect a change in 15% diameter fairly well. But, this change happens over the course of half a lunar month, the time between perigee and apogee. I was not attempting to relate this to the horizon illusion.

As for the horizon illusion, Phil Plait addresses it in his book, Bad Astronomy. The two posits from this thread - that the moon looks big because of comparison with near-field objects, or that the atmosphere acts like a lens - are not true.

The latter is not true because, while the atmosphere will refract light, it will do so to make the moon look vertically smaller. It has no effect on the horizontal dimension when the moon is on the horizon (also, taking a picture and comparing it to one when the moon is overhead will show they are almost exactly the same size, as can't be the case if this were true). The former is not the case, as the simple case that EW1066 referred to - it looks huge even where there's NOTHING on the horizon to compare it to.

Phil says claims that we still don't actually know why the horizon illusion happens. It's definitely in our head, with the way our brain interprets images, though. He posits that the two most likely factors are (1) how we judge the size of distant objects (even with nothing there for comparison), and (2) how we perceive the shape of the sky itself.

By the way, I never gave feedback on the photo. I think it's a little too long of an exposure - or you bumped up the highlights too much in processing. Try shortening the length of exposure by about 2-3 stops and see what happens.