Does anyone know how the Apollo navigation stars were selected?
I found the list, typed it into a spreadsheet, and sorted by magnitude. As you'd expect, it includes the brightest stars in the sky with one glaring exception: Alpha Centauri/Rigel Kent. Its apparent visual magnitude of -0.27 makes it the fourth brightest star in the night sky. Does anyone know why it was omitted?
I can imagine there were selection considerations other than brightness. The navigation stars needed to be distributed over the celestial sphere so that pairs could be selected that were roughly 90 degrees apart to minimize the effect of any sighting errors on the actual orientation of the inertial platform. This would exclude Betelgeuse, close to four other stars on the list.
There needed to be enough usable stars regardless of where the sun was along the ecliptic (i.e., what time of year it was) so the crews didn't have to point their scopes too close to the sun.
The stars needed to be easily identifiable, maybe by being in or near easily recognized constellations (e.g. Sirius and Rigel) or by their color (e.g., Arcturus).
Alpha Centauri is a binary (or triple) star. Perhaps it was excluded because they're too far apart and A and B look too much alike. Several other multiple stars are are on the list, such as Rigel and Capella (4 stars in two binary groups) but for all I've checked so far the primaries are much brighter than the others in their systems.
The dimmest star on the list is Acamar, theta Eridani, at 3.2. Many brighter magnitude 2 stars are skipped. Hmm.
I got to wondering about this because the crews complained that it was often difficult to see the navigation stars. They could do P52 updates when the computer automatically put the scope close to the selected star and they just had to mark it, but platform alignments from scratch that required them to identify constellations were far more difficult.
If second magnitude stars were difficult to see even through the scope, it's not hard at all to understand why they couldn't see any at all with the naked eye in daytime!
