Science Challenge #23

Optical illusions, or atmospheric distortions?

Which of the following three visual effects are optical illusions, and which are due to distortions by the atmosphere?

1)		Why does the moon seem larger and yellow near the horizon?
		The author of the IFlScience document claims that it is an illusion. He claims that photographs of the moon at different positions in the sky show that it is always the same size.
2)		Why do stars twinkle?
		The author of this document says that the stars twinkle because of the atmospheric distortions.
3)		Why does the sun become distorted near the horizon?
		The sun seems larger and more yellow near the horizon, and it becomes distorted when it is partly below the horizon. This and this document has photos of the "Omega sun". The authors claim that it is due to atmospheric distortions. An even wider variety of amazing photos, (such as the photo above) are from Luis Argerich.

Why does the atmosphere cause stars to twinkle and the sun to be larger, distorted, and yellow, but it doesn't distort the image, size, or color of the moon?

If it is true that the Earth's atmosphere causes the stars to twinkle and the sun to appear larger and distorted near the horizon, then an astronaut on the moon would notice:

a)	Stars do not twinkle on the moon.
b)	The sun remains a circle of the same size and color regardless of whether it is overhead or near the horizon of the moon.

But what would the astronauts on the moon see when they look at the Earth? Would the Earth seem larger and more yellow when it was near the horizon of the moon? Or would it always be the same size and color no matter where it was in the sky?

If an optical illusion causes the moon to seem larger when it is near the horizon of the Earth, would that same optical illusion cause the Earth to appear larger to an astronaut on the moon when the Earth was near the horizon of the moon? Or does that optical illusion work only on the Earth? If it works only on the Earth, why does it not work on the moon?

The author of the IFlScience document claims that we can prove that the moon at the horizon is the same size as it is when it is overhead by taking photographs of the moon and measuring the size of the moon, but even if it is true that photographs of the moon always show it to be the same size, human eyes do not see exactly what cameras see. Consider what we see when we look at stars.

The light coming from the stars travels in a straight line until it hits the atmosphere. It then wobbles before it enters our eye. The light does not focus to a point on our retina. Instead, it creates a blurry and flickering object that is impossible for our eyes to focus on.

By comparison, when a camera takes a photograph of a star, it will create an image of just one moment in time, so there cannot be any flickering. Also, our cameras are not yet as good as our eyes at recording low levels of light, so the faint light from the stars will not be seen, as if those light rays disappeared after they entered the camera. The end result is that the camera will see a more precise point of light.

In the image above that shows the human eye, the person saw three photons of light (red dots), but the camera in the image above only saw the two photons that were close enough together to create enough light to be sensed by photographic film or image device, and the third photon had no effect.

The author of the document about the twinkling of stars (#2 above, this document) claims that the planets do not twinkle because they are closer to the Earth, "so their light is less distorted by the air."

If it is true that stars twinkle more than planets, then it is not because the light from stars is distorted more than the light from planets. The light that comes from planets is the light from stars. It's the same light, and it follows the same laws of physics, so it will be affected by the Earth's atmosphere in the same manner.

The reason it is possible that stars twinkle more than planets is because their light is traveling farther, and space is not completely empty. Space is essentially a very low density atmosphere, so the farther light has to travel through space, the more its path will wobble.

Another reason the distant stars will twinkle, or flicker, more than the nearby planets is because there is less light coming from them. I will leave it to you to explain that, or explain how I making a mistake in that assumption.

The author of this article claims that the sun does not twinkle because it "is closer to the Earth than the other stars. Therefore, it looks like a large shiny disc, not a dot. The beam of sunlight is big enough to pass through the atmosphere unaffected by refraction."

That remark is nonsense. The light from our sun wobbles through the atmosphere just like the light from a star. The sunlight follows the same physics as the light from stars, the moon, cities, airplanes, and the space station. The sunlight is affected by the atmosphere exactly like all other light. However, the sun is so large that almost all of the photons remain within the image of the sun, and only a small number pass outside of its image to give the sun a fuzzy edge.

By comparison, the stars are so tiny that even slight changes in the path of the photons will make the star appear to move and twinkle. In the image below, the view is towards the star or the sun. The red circles are the star and sun, and the dotted circles show photons that were shifted from position by the atmosphere.

Or am I the person who is making mistakes about these issues?

“I can solve that one!”