Rene-NASA-Mooned-America

Space Navigation / Chap. 7 p. 61
years whenever I run into something that doesn't make sense I have great problems with it. I
cannot memorize it and I can't work with it.
For their navigation package NASA went to MIT early in the game and asked for a
method that would give the Apollo program the ability to go to the Moon and back. A
couple of professors produced the package on time. In fact it was one of the few times that
anyone met a NASA schedule.
Michael Collins was designated the navigator for Apollo 11. In his book he lists the 37
navigation stars they were to use, plus their corresponding octal numbers which identified
them to the computers. Here's how Michael explains that navigation package.
"The astronaut, peering out through either his telescope or his sextant finds
one of the chosen few, superimposes a + on it, and pushes a button at the instant of
perfect alignment. He then tells the computer which star it was, by numbers. Repeating
this process on a second star allows the computer and the platform to determine which
way the spacecraft is pointing. So we now know which way is up Well, not exactly,
because "up" is a rather fragile concept meaning away from the center of the earth, a
direction opposite the gravity vector used to clutch us tightly by. But suppose we
cannot even see the earth in our window, suppose we are floating free of earth's gravity.
What now, M.I.T. Back to our friendly stars. We simply define a new up-down and
left-right, using the stars in place of earth. All will be well as long as we all play the
game by the same rules, as long as the ground controllers send us instructions using the
same stellar frame of reference. Now we are free of all terrestrial conventions and can
correct our course to and from the Moon by pointing in the proper direction relative to
the stars. 4
Collins seems to be saying that the sextant had a cross hair in its optics. But sextants
don't have cross-hairs. Curiously, I went sniffing through his book and found out that sure
enough he was talking about a sextant. Almost 100 pages later he continues:
"Unlike Gemini, however, Apollo has a fancy computer tied to the optics, and now I
call on it for help; it responds by swinging the sextant around until it points at where it
thinks Menkent is. Aha! There it is, in plain view, and it's a simple task for me to align
the cross-hairs precisely on it and push a button at the instant of alignment. Now I repeat
the process using Nunki, and the computer pats me on the back by flashing the
information that my measurements differ from its stored star angle data by .01 degree. It
displays this information as 00001. In M.I.T.-ese, a perfect reading of 00000 is called
five balls." 5
Aha! There it is, in plain view: he does claim his sextant has a cross-hair, but this cannot
be true! A sextant is an instrument that uses mirrors mounted on a calibrated movable leg.
The essence of the instrument is to superimpose one object over the image of another
thereby measuring the angle between them. On Earth, one of the objects is usually the
horizon but here he is measuring the arc distance between two stars. As I said, there is no
reason to have a cross-hair.
The instrument he probably means is a theodolite. This is a telescope with cross hairs
with accuracy greater than a sextant; although it is extremely hard to believe that a pilot
turned astronaut doesn't know a sextant from a theodolite.
NASA MOONED AMERICA! / Rene