Updated Earth to be more realistically-colored

docs/optics.md

@@ -418,8 +418,16 @@ disk, which is the feature horizon-based navigation measures.
 
 ### Shading
 
-Earth and the Moon are Lambertian, with one albedo each, set by
-`Appearance`: 0.3 for Earth and 0.12 for the Moon, flat across wavelength.
+Earth and the Moon are Lambertian, each with one reflectance spectrum over
+the whole body, set by `Appearance`. The Moon's is a flat 0.12. Earth's
+default is a stand-in shaped like cloud-free ocean seen from space: about
+0.3 in the blue falling to 0.07 in the red, then back up in the near
+infrared. Most of that blue is sunlight scattered by the atmosphere above
+the ocean, the same scattering that makes the sky blue; the ocean itself is
+dark. In the example camera it gives channel albedos of 0.09, 0.15 and 0.26
+(red, green, blue), so Earth renders blue. Its overall brightness is below
+the real Earth's average of about 0.3, which includes bright clouds.
+
 Each point's radiance is rho cos(i) S_k / (pi d^2), with i the angle between
 the surface normal and the Sun. The night side has i past 90 degrees and is
 black. The Sun's distance d is taken from each body's center, since the