Renderer setup, plus some actual early speed numbers

docs/optics.md

@@ -494,7 +494,8 @@ satellites (Montenbruck and Gill), the same as astrokit's. A test checks it
 against tracing lines from the target to thousands of points spread over
 the solar disk.
 
-The fraction is computed once per target, at its center. The whole target
+The renderer counts both Earth and the Moon as occulters. The fraction is
+computed once per target, at its center. The whole target
 dims together, which is accurate unless the target is a sizable fraction of
 the penumbra's width. Light reaching the target from Earth itself
 (Earthshine), which matters most when the target is in shadow, comes in M2.