Setting the baseline

Right, first order of business was how far I wanted to push the realism of my CG Earth. Here's a quick rundown:

1. Accurate proportions
2. Axial tilt
3. Terminator reddening
4. Atmospheric scattering
5. Volumetric clouds
6. Displacement mapped topography

Here's is a more detailed breakdown:

1. Accurate proportions - The Earth is not a perfect sphere but an oblate spheroid, slightly fatter around the equator and flatter at the poles, so there is no single value for the Earth's radius. The highest mountain ranges (Himalayas) are 8km above sea level. The lowest cloud layer starts around 2km above sea level and high clouds go up to 18,000km around the Tropical regions. Most of the air is contained within the Troposphere up to around 20km from sea level. To match the real thing more closely, the mountain, clouds and atmospheric elements my CG Earth would have to be scaled according to these proportions.

2. Axial tilt - The Earth's rotational axis is tilted at an angle to the orbital plane which varies between 22.1° and 24.5. According to Wikipedia, the current value is about 23.4° which is what I used. This axial tilt is the reason we have Seasons on Earth. Most of the CG Earth animations out there have their rotational axis perpendicular to the sun's rays, which only really happens during the Equinoxes as shown below:


 
3. Terminator reddening - As light passes through the Earth's atmosphere, it gets scattered by air particles and aerosols. Blue wavelengths of light get scattered more than red, resulting in the sky appearing blue. During sunrise and sunset, sunlight has to pass through more atmosphere which means much more scattering of blue light, resulting in the orange-red color of the Sun. From space, the terminator line along the Earth's shadow dividing night and day appears to redden due to such intense scattering. 

4. Atmospheric scattering - The atmosphere is a translucent layer of air that gives the sunlit side of the Earth a bluish glow when seen from space. The scattering causes the land below to fade due to atmospheric extinction, which is the reason why we can't see the stars during the day.

5. Volumetric clouds - Most CG Earth simply use a single shell mapped with a transparency map and possibly a bump map to create the cloud layer. I wanted mine to have actual thickness and cast volumetric shadows.

6. Displacement mapped topography - Again, most CG Earth I have seen on the internet uses a flat color texture and a bump map for the ground and sea. It should be quite straightforward to use the same elevation data with displacement mapping to make mountains actually rise up from the flat surface. Likewise, bathymetry data can be used to create the ocean depths. Of course, this would mean that rendering times will be much longer... :/

Naturally there are still more factors to consider, but these few areas form a good starting point to begin developing my CG Earth.