To improve my results, I implemented a realistic distribution of ozone. Average data for the Earth's atmosphere was difficult to find, and the exact distribution of ozone is pretty variable anyway, so I read data from the NASA graph below, and made a few tweaks based on other information I had found (namely, lowering the concentration in the troposphere, and weighting the data up to make the total concentration closer to 0.6 ppm). I might want to tweak or replace the data in the future, but it's already much more accurate than the constant value I was using before.
|Ozone concentration in the atmosphere. Image from NASA.|
Below is a new render, and some old ones for comparison. In the new render, compared to the old ozone render, the sky around the zenith is bluer, and the the color and brightness near the horizon is more accurate.
|Ozone with realistic vertical concentration profile.|
|Ozone with uniform concentration throughout the atmosphere.|
Here's another new render, this one taken during twilight in the direction of the sun, with the sun −3° below the horizon:
|Twilight + ozone.|
By the way, ozone absorption should make the color of the Earth shadow at twilight more accurate (darker and bluer), so I'm looking forward to rendering some shots that illustrate that.