Earth Size

Earth Size
Relative Size of the Sun and Earth

Friday, February 7, 2014

Real Fakery

Schematic of motion across the sky

I'm finding, often, that trying to attain some imitation of realism in an illustration involves more and more clever fakery.  I was working on the above image for a chapter describing how we see objects move in the sky.  To make the stars stand out I figured I needed a nearly black background, but one subtle principle in mimicking outdoor scenes is that shadows ought to never be black.  The sky actually is an emitter of blue light.  Ok, usually then I'll turn on an "environment" background glow to give a nice subtle tint of blue to fill in the shadows, but then I'll lose the black background.  A simple solution is to make a blue-light-emitting plane and position it just out of view above the scene.  So far, so good.  The problem is that the sunlight does not actually emit from the "sun" in the image, as the real Sun's rays arrive from a very distant source and are about parallel when they illuminate things on Earth.  For example, if I put a tree just to the north side of the East axis, I don't want its shadow pointing in a different direction from a tree on the south side of the axis.  That would be weird.  So when I make the rays come from a true Sun-like distant source, the new problem is that the previously-cleverly-placed plane now casts a distinctly unrealistic shadow on the scene.

What I really need is a light emitting plane that does not cast shadows (!), and lo!  I discover there's a little checkbox on the plane's properties that turns off its shadow-casting properties.  Neat.  This image is the result.  Actually, I'll probably never have a static image that looks like that --- too busy.  The little stars are only visually meaningful in an animation, when it's clear they're following the "Celestial Sphere" around an axis going approximately through the North Star.  The nice thing is that once this is set up, I can use the same basic setup to illustrate the seasonal variation of the Sun in the sky as well as one view of Moon phases.