Monday, December 28, 2015

Recreating the Earth's Atmosphere In a Computer

The Earth as sunset approaches
(computer generated, my own work)
There is a common expression that says something is more of an art than a science. This usually means that there is no precise formula or set up steps (what we call in computer science an "algorithm") to produce the desired result. To get results you must use a large amount of intuition and previous experience. Scientific approaches are repeatable if the same steps are followed; that is part of the definition of "scientific." 

Computer generated art is very much a mix of science and art. We take the scientific principles of computer science and make something that must be interpreted from an intuitive or emotional approach. Taking the quantitative ones and zeros and making something that is described as pretty.

In creating a model of the Earth I have used a lot of scientific data. The relative diameters of the Earth and Moon for example. The altitude of clouds above the surface of the earth. Images taken of the Earth from satellites in orbit.

Mountains majestically fading out to purple
(actual photo, not my work)
The atmosphere of our planet appears transparent on the short distances we usually pay attention to, about under a mile. Fog and smoke can change this, but usually it is pretty clear. But look at the mountains in the distance and you will notice that they are not their natural color. This effect is immortalized in the song "America the Beautiful" in the line describing the beauty of the United States referring to "purple mountain majesty." This effect is caused by the atmosphere scattering and absorbing different colors of light by different amounts, referred to as Rayleigh Scattering after the man who discovered the scientific principles behind it. This is the same effect that causes the sky to be blue.

When creating a model of the Earth as viewed from space the atmosphere is the most important part. While the surface and clouds are important, most of the blue coloring of the Earth comes not from the water but from the atmosphere. In reality you are seeing the sky from the backside.

To recreate the effect properly the science is very important. For a more technical explanation of the science you can read about Rayleigh Scattering. In a nutshell the atmosphere scatters blue light in random directions more than other colors. This means that the sky looks blue during the day because that is the color most redirected to our eyes. During sunrise or sunset most of the blue has been scattered out, leaving reds and yellows. This is why sunsets and sunrises are reddish, especially near the horizon.

While I was creating my model of the Earth, I used actual scientific data on how the atmosphere scatters light to get the colors right. I simulated in the computer the scattering effect of the air. Below is a view of the atmosphere at sunset, seen from about 90 miles above the surface of the Earth, created using my model of the Earth. You can see the red color of the lower atmosphere which fades to blue at higher and thinner altitudes, finally fading to black at the extremely thin portions. The effect of scattering is dependent on the density of the air, and the density is dependent on the altitude.
Atmosphere at sunset as seen about 150 km (about 90 miles) above the surface of the earth.
(computer generated, my own work)
This image was created using a mathematical model. All of those ones and zeros being converted into something that looks like something real, something this artist thinks is beautiful. By applying the math in just the right way I am able to create an image that both looks true to nature and is pleasing to the eye
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