Tidbit of the Week

Ever seen an illusion like this? It's called the Ponzo illusion. Which line looks bigger? They are actually both the same, but the top one looks larger. This is because the human brain interprets the two slanted lines as parallel, like railway tracks. Railroad ties are all the same size, but the further away ones appear smaller because of perspective. But if the further tie appears exactly the same size as the closer one, the brain says it's bigger.

If you thought the moon actually appears bigger in when it's low on the horizon, go out with a ruler and measure. It only appears to appear bigger. It's an illusion.

There is a little known fact that people perceive the sky as bowl-shaped. In reality the sky is thought of as a dome shape, like a hemisphere, but when asked to point ou the midpoint between horizon and zenith, most people make a 30 degree angle, when the true midpoint is at 45 degrees. About a thousand years ago some Arab guy said the sky is probably subconciously compared to the ground in people's minds. The nearest point on the ground is directly beneath you, and the furthest point on the ground is way out at the horizon. So perhaps in subcouncious parts of the brain the zenith of the sky is compared to the point beneath your shoes, and that point on the horizon is waaaaaay far away.

So if the moon is really the same size on the horizon as at its zenith, the brain says "Oh, same size but further away. It's bigger!" Just like the Ponzo effect. But instead of two slanted lines that look like parallel railroad tracks, it's a domed sky that looks rather flattish.

I haven't actually tried this myself, but I think I will. I always thought that the moon looked bigger because the atmosphere of the Earth has more of a lens effect at that angle, but I guess I was wrong. This illusion idea is something I learned in part from the book "Bad Astronomy" by Philip Plait.

Be it known, however, that nobody knows for absolute certain why the moon appears bigger low in the sky. This is just an educated guess.


Chip said...

Cool. I have always wondered about that. I really enjoy your blog.
As regards the space probe to Mars, there is always gravitational swing.
Have you ever seen the movie Apollo 13? It is good, and really interesting. There is some language.


DTH Rocket said...

Yes I have seen it, but not recently. I should see it again. I'd probably get more out of it nowadays. :)