ApolloHoax.net
Apollo Discussions => The Reality of Apollo => Topic started by: ka9q on November 11, 2012, 02:13:02 PM
-
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!
-
I always liked the references to their fallen Apollo 1 comrades: Navi, Dnoces and Regor.
-
Yeah. These references were made by Gus Grissom himself as a joke. The names aren't official, but they are still widely recognized.
I notice he gave the brightest star (+1.7) to Roger Chaffee, the dimmest (+3.14) to Ed White, and the intermediate one (+2.47) to himself.
-
Referring to Gemini 10, Mike Collins (who had to try to do manual "star shooting" with an actual sextant) states that the target stars were "selected for their positions relative to our orbit". It's only speculation, but the same may have been true for the stars selected for Apollo navigation.
Regarding Alpha Centauri, I'll leave it to the experts to crunch the numbers, but is it possible that with a distance of only 4+ light years, parallax becomes a concern over a quarter-million miles?
-
Regarding Alpha Centauri, I'll leave it to the experts to crunch the numbers, but is it possible that with a distance of only 4+ light years, parallax becomes a concern over a quarter-million miles?
No.
Distance to αCen = 2.5 × 1013 miles (adjacent side or hypotenuse)
Distance to Moon = 2.5 x 108 miles (opposite side)
It pretty much doesn't matter which trig function you use in this example, where the adjacent side and the hypotenuse are effectively the same length, and 100,000 times longer than the opposite side
I have used TAN (Opposite over Adjacent)
TAN (2.5x108/2.5x1013 = 0.000573 °
or 2.06 seconds of arc.
That is unmeasurable using a sextant!
The distanc
-
What about the parallax over the course of a year? Wikipedia says it's 747.1 mas (milliarcseconds) so I guess that's also negligible for our purposes.
Angles were displayed on the DSKY to an accuracy of 0.01 degrees. That's 36 arcseconds. Alpha Centauri A and B are about 1.4 seconds apart in right ascension (measured in hr min sec) and about 11.5 seconds apart in declination (measured in deg min sec). 1.4 seconds in RA is 360/24 * 1.4 = 21 arcseconds. The total is about 24 arcseconds of separation, not quite 0.01 degrees but close. But if they had other usable southern stars (like Canopus) maybe they just didn't need Alpha Centauri.
-
Distance to Moon = 2.5 x 108 miles (opposite side)
Or possibly 2.5 x 105 miles
-
Distance to Moon = 2.5 x 108 miles (opposite side)
Or possibly 2.5 x 105 miles
Ooops. Oh well, I was only out by a factor of 1000
so
TAN (2.5x105/2.5x1013) = 5.73x10-7 0.000000573 °
or .002 seconds of arc. (or 2 millisecond of arc).
Alpha Centauri A and B are about 1.4 seconds apart in right ascension (measured in hr min sec) and about 11.5 seconds apart in declination (measured in deg min sec). 1.4 seconds in RA is 360/24 * 1.4 = 21 arcseconds. The total is about 24 arcseconds of separation, not quite 0.01 degrees but close.
...and of course this varies over their orbital period of about 80 years.