Saturday, June 25, 2005

The Large Moon

I was just reading an article referenced on Slashdot that supposedly explains why the moon appears much larger on the horizon than when it is high in the sky.

The nuts and bolts of it is that it is just an optical illusion, and that the moon is, in fact, the same size. Many of the comments about the article go to expound on the thoughts in the original article.

Excuse me, an optical illusion? Am I just a dupe? In my experience, the moon appears WAY bigger on the horizon than high in the sky, and the claim that it really isn't flies in the face of my repeated observations since I was a kid!

Now, I'm not foolish enough to assume I'm 100% right, but I had just assumed that the reason was due to a "lens" effect of the atmosphere. When the moon is on the horizon, there is more of earth's atmosphere in the way, and it is thicker, being closer to the earth, that acts as giant lens that magnifies the moon somewhat. This also explains why it is darker, too--the same amount of light is dispersed over a larger area. It also happens with the sun, which appears much larger just at sunrise and sunset than at high noon.

The article, and several of the commenters, say to take a coin or rolled-up paper and hold it up for comparison and the optical illusion will be evident. I think I'll do something even a little more scientific. The next full moon is on July 21. I am going to use my digital camera to take high-res photos of the full moon high in the sky and on the horizon and figure this thing out once and for all. If I'm really motivated, maybe I'll even throw together some ray-tracing diagrams to show what results would make sense.

Whatever happens, I'm sure there will be more to come on this subject. Also, whatever happens, nothing will take away from the coolness factor of a huge moon on the horizon.

Edit: After re-reading this post, I hope my meaning came across the way intended it. I know, of course, that the moon isn't physically bigger. (Duh.) What I meant was that the moon appears bigger. That is, the size of the disc as it appears in the sky, not the actual size of the sphere in space. Hope no one thought I was being too stupid!


At 7:54 PM, Anonymous weaverhouse said...

I'll bet money you can;t take a picture of the moon without a special filter.

At 5:03 PM, Anonymous Anonymous said...

Why can't you take a picture of the moon without a special filter? And, what special filter would you need?

At certain times of the year the effect seems to be more substantial. Anyone else notice this?


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