astrokit ephemeris

docs/optics.md

@@ -500,6 +500,32 @@ 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.
 
+## Where the Sun and Moon appear
+
+Code: `ephemeris.rs` (feature `astrokit`).
+
+With the `astrokit` feature, the Sun's and Moon's positions come from JPL's
+DE440s ephemeris, and Earth's orientation from the ITRF93 kernel, which
+includes the slow wobble of Earth's pole. These are geometric positions:
+where each body is at the epoch.
+
+A camera sees something slightly different, its apparent position:
+
+- **Light time.** Light from the Moon takes 1.3 s to reach Earth, and from
+  the Sun 8.3 minutes. The camera sees where each body was when the light
+  left it. For the Moon that is about 1.3 km of motion, a few microradians.
+- **Aberration.** A moving camera sees incoming light tilted toward its
+  direction of motion, as rain seems to slant toward a moving car. Earth's
+  orbital speed (30 km/s) tilts the Sun's apparent direction by about 20
+  arcseconds, 100 microradians. A spacecraft's own orbital velocity adds up
+  to about 5 arcseconds more.
+
+100 microradians is about one pixel of the example camera, and 18 pixels of
+the narrow 1 m focal length camera in the limb test. So the geometric
+positions are close enough for M1, but not for precise navigation against
+the Sun, Moon or stars. Rendering apparent positions needs the observer's
+velocity as an input, which the scene does not carry yet.
+
 ## Not modeled yet
 
 | Effect | Where |
@@ -517,3 +543,4 @@ the penumbra's width. Light reaching the target from Earth itself
 | Earth and Moon albedo maps | M2 |
 | Earth's atmosphere: scattering, the bright limb | M4 |
 | Motion blur, rolling shutter, stray light | M6 |
+| Apparent positions: light time and aberration | not yet planned |