What the charts were for
The Apollo 11 note gave the crew a frame every 20 to 40 seconds of the twelve-minute powered descent: the view through the two front windows (170°, with the landing point designator scale) beside the view up through the docking window (100°, with its scribe). The report’s Figure 5 is a pair like these. The crew flew the first part of the descent with the windows facing the surface, then yawed to face up about four minutes in.
How it is recomputed
- When. The note’s planned ignition, 102:35:40, plus the frame’s time into the burn.
- Where. The LM on the note’s planned descent profile: surface range to the landing site and altitude against time, read from its Figure 6.2.0-1(b), along the great circle from the Mission Report’s ignition point to the landing point.
- Which way it points. Fitted from the labelled stars in each frame alone: 3 to 8 stars, 0.04° to 0.15° RMS. Those residuals show the quality of the fit; the Sun, the planets and the horizon are predicted from it.
- The window. The docking-window outline and scribe are traced from the first frame and reused; the plot is fixed to the LM.
How close it comes
Tests set before measuring:
| Frame | Sun (limit 2°) | Lunar horizon (limit 1.0° RMS) |
|---|---|---|
| (a) begin burn | 0.43° | 0.72° RMS but max 4.2° — missed |
| (e) 2:06 | 0.26° | 0.49° RMS, max 1.0° — pass |
| (j) 5:26 | 0.60° | 4.15° RMS, max 8.6° — missed |
| (p) 8:06 | 0.42° | (no horizon in view) |
Venus and Saturn land within 1°. The Sun’s agreement shows these charts were drawn for the real date, unlike Figure 1. Early in the burn the drawn horizon lies close to the geometric one; after the yaw to windows-up it does not. On Figures 1, 2 and 3b the program’s horizon also parts from the geometry, and the rule it used is still unknown.
The first run of this test put the LM on the landing site’s radius (the datum its altitudes are measured from) but drew the horizon on the Moon’s mean radius, 2 km larger. The final review caught the mismatch; with one datum, frame (e) passes. Both sets of numbers are in the ledger.
Limits
- The front-window views (170°, with the LPD scale and the crater models built from photographs of the landing site) are shown as scans only; recomputing them waits on the horizon rule and the LM attitude history.
- The descent profile is read from a schematic figure (heights ±0.7 nautical miles).