https://tinyletter.com/scottsieke
and enter an email and hit "subscribe." I'll send out a quick notification when I post something, and that's it. They don't share you email with anyone other than me.
On the the good stuff:
To start, I just read a great book by astronomer and science evangelist Phil Plait called Bad Astronomy. In it, he dispels many misconceptions that nearly everybody has about astronomical concepts, myself included. I'm going to go over my favorite here, but if you want the much more thorough version, along with much more, I highly recommend picking up a copy of his book.
Moving on. Nearly everyone is aware that the moon appears larger on the horizon, and nearly everyone, including myself, thought they knew why. There are several theories floating around, but the most common, and the one I believed, is that the moon looks larger near the horizon because there are trees and buildings and all that to compare it against. When the moon is high in the sky, it is all alone, and so looks smaller. Even if this is not precisely what you thought, it sounds true enough, and is therefore a quite pernicious falsehood.
The best way to test an idea is not to try to prove the idea right, but rather to try to prove it wrong. In our case, how could we prove this moon hypothesis wrong? If we think trees and buildings are the cause of the illusion, let's find an area with no trees or buildings, such as a beach or cruise liner. As it happens, the illusion holds true even on the "scientifically idealized" horizon of the ocean, where there is nothing to compare the moon against. Now for the other counter-example: when in the center of a large city or a "non-idealized horizon," buildings (and light, but that's beside the point) block your view of much of the sky. However, when the moon is located within the same reference frame as the building, and you can see both high up in the sky, the moon still looks small, even with the building to compare it against.
It turns out the answer is indeed an interesting illusion, but not that exact illusion.
When people depict the sky, we draw it as if it is about the same distance away in all directions:
... or so it seems. In illustrations such as this, we draw the sky to be equidistant in all directions, but as it turns out, in actuality we perceive the horizons to be farther away than the zenith.
I'll get back to the moon in a minute, but this is the crux of the illusion, so I'm going to flesh it out a bit. Rather than the celestial sphere looking like a hemisphere as in the above drawings, we perceive it as more of a shallow or flattened bowl, with the perceived zenith much closer to the observer. A great way to test this is to gather up: 1) Some friends you don't mind pestering and 2) a protractor. Head outside, and ask your friend(s) to point up so their arm is at a 45 degree angle to the ground. Measure the angle and record the results. What you will get is a measurement that is almost certainly between 30 and 40 degrees, and certainly not 45 (unless you have clever friends). Using the protractor, point your own arm up to 45 degrees and it will almost certainly surprise you just how far up 45 degrees in the sky is. This is because, for whatever reason (feel free to speculate wildly here) the horizon just seems much farther away than the zenith.
On to the moon! The moon is always roughly the same size in the sky. It varies by about 4%, but that's pretty imperceptible to us.
If the whole sky takes up 360 degrees all the way around, the moon takes up about half a degree, or less than your pinkie nail at arms length (try it).
The moon looks bigger near the horizon, because the horizon seems farther away. An object the same size on a background that seems farther away will look larger than one on a "closer" background.
and enter an email and hit "subscribe." I'll send out a quick notification when I post something, and that's it. They don't share you email with anyone other than me.
On the the good stuff:
To start, I just read a great book by astronomer and science evangelist Phil Plait called Bad Astronomy. In it, he dispels many misconceptions that nearly everybody has about astronomical concepts, myself included. I'm going to go over my favorite here, but if you want the much more thorough version, along with much more, I highly recommend picking up a copy of his book.
 |
| If you were wondering, this is what the first page looks like. (Results may vary) |
Moving on. Nearly everyone is aware that the moon appears larger on the horizon, and nearly everyone, including myself, thought they knew why. There are several theories floating around, but the most common, and the one I believed, is that the moon looks larger near the horizon because there are trees and buildings and all that to compare it against. When the moon is high in the sky, it is all alone, and so looks smaller. Even if this is not precisely what you thought, it sounds true enough, and is therefore a quite pernicious falsehood.
 |
| This excellent photo was taken by Shay Stephens, and was featured here on APOD. |
The best way to test an idea is not to try to prove the idea right, but rather to try to prove it wrong. In our case, how could we prove this moon hypothesis wrong? If we think trees and buildings are the cause of the illusion, let's find an area with no trees or buildings, such as a beach or cruise liner. As it happens, the illusion holds true even on the "scientifically idealized" horizon of the ocean, where there is nothing to compare the moon against. Now for the other counter-example: when in the center of a large city or a "non-idealized horizon," buildings (and light, but that's beside the point) block your view of much of the sky. However, when the moon is located within the same reference frame as the building, and you can see both high up in the sky, the moon still looks small, even with the building to compare it against.
It turns out the answer is indeed an interesting illusion, but not that exact illusion.
When people depict the sky, we draw it as if it is about the same distance away in all directions:
 |
| Image: TWCarlson |
... or so it seems. In illustrations such as this, we draw the sky to be equidistant in all directions, but as it turns out, in actuality we perceive the horizons to be farther away than the zenith.
I'll get back to the moon in a minute, but this is the crux of the illusion, so I'm going to flesh it out a bit. Rather than the celestial sphere looking like a hemisphere as in the above drawings, we perceive it as more of a shallow or flattened bowl, with the perceived zenith much closer to the observer. A great way to test this is to gather up: 1) Some friends you don't mind pestering and 2) a protractor. Head outside, and ask your friend(s) to point up so their arm is at a 45 degree angle to the ground. Measure the angle and record the results. What you will get is a measurement that is almost certainly between 30 and 40 degrees, and certainly not 45 (unless you have clever friends). Using the protractor, point your own arm up to 45 degrees and it will almost certainly surprise you just how far up 45 degrees in the sky is. This is because, for whatever reason (feel free to speculate wildly here) the horizon just seems much farther away than the zenith.
On to the moon! The moon is always roughly the same size in the sky. It varies by about 4%, but that's pretty imperceptible to us.
 |
Image: Marco Langbroek
|
If the whole sky takes up 360 degrees all the way around, the moon takes up about half a degree, or less than your pinkie nail at arms length (try it).
The moon looks bigger near the horizon, because the horizon seems farther away. An object the same size on a background that seems farther away will look larger than one on a "closer" background.
The lower line is analogous to the moon at zenith (closer) and the upper line is analogous to the moon near the horizon (further).
Here is a great graphic that pulls everything together in one image:
 |
| Image: Lloyd Kaufman and James H. Kaufman |
Cheers,
- Scott
LINKSTORM:
Here is a great demo of the scale of the solar system
This is a great way to look at the equation E=mc2
This is weirdly satisfying
Awesome new science about the history of Mars
This topo map may help (Warning: this image is monstrous)
Possibly the most time sink-y page on Wikipedia, but also an important one
So this monkey rides around on a Border Collie
No comments:
Post a Comment