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Marcel Tschudin wrote:
> Could it be that despite
> the high inaccuracy from refraction and extinction, the observation of
> a setting star was easier to do than an observation of a star
> somewhere in the sky?

I'm sure it would be easier than sextant observations -- Collins had
trouble with those and ran out of time.

The stars in the Module VI experiment were selected to provide a good
solution. That probably would have been impossible by observing setting
stars only.

Not that the sextant worked any better, in practice. Module VI was a
failure, and the crew used data from Mission Control to make the
rendezvous. I wonder how the Dick Carley took the news. (He was the one
who gave Collins that talk on the history of navigation).

It must have been difficult to use a sextant through the small window of
a Gemini spacecraft. I think they must have had to change the craft's
orientation to bring the different stars into view.

The Apollo spacecraft used star observations too. To quote Collins:

"The basic idea behind the Apollo guidance and navigation system was
simple enough. It all began with the stars, whose position in inertial
space was known and unchanging. The spacecraft platform, with its three
gyroscopes isolated from spacecraft motions, could then be aligned in
relation to the stars, providing a fixed frame of reference. The study
of the stars themselves was interesting, I thought, one of the
interesting parts of our training, despite its being pure rote... We
learned them in the Morehead Planetarium in Chapel Hill, North Carolina,
and damned carefully at that -- a star sighting using the wrong star
would be embarrassing at best, and could easily be disastrous..."

"The astronaut, peering out through either his telescope or his sextant,
finds one of the chosen few [the computer knew 37 stars], superimposes a
+ on it, and pushes a button at the instant of perfect alignment. He
then tells the computer which star it was, by number. Repeating this
process on a second star allows the computer and the platform to
determine which way the spacecraft is pointing."

Here he is, on the Apollo 11 mission, using the sextant for the first time:

"I unstow and install two eyepieces, one for the sextant and one for the
telescope... I jettison the protective covering over the optics and peer
out through the telescope. What I see is disappointing, for only the
brightest stars are visible through the telescope, and it is difficult
to recognize them when they are not accompanied by the dimmer stars,
which give each constellation its distinctive visual pattern. The
situation is not helped by the fact that I am looking for Menkent and
Nunki, two of the more nondescript Apollo navigation stars... Unlike the
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 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.' I have scored 'four
balls one.'"

After the rocket firing to leave Earth orbit, he makes more observations:

"My next task involves realigning our inertial platform for the second
time, and again, with help from the computer in pointing the sextant, it
goes swiftly and well. Five balls! How about that, sports fans? But it
doesn't pay to be a smartass, and Magellan gets a nasty start when he
moves on to his next task -- measuring the angles between five selected
stars and the earth's horizon. A couple of stars I can see fine, like
Altair, but with them I have difficulty finding that spot on the horizon
which is directly below them, the substellar point as it is called. In
other cases, such as Enif, the star is not bright enough to be readily
seen. Finally, I wade through it all, but the results are not very
accurate and I am discouraged. This exercise is for practice, really, as
we would not have to rely on such measurements unless we lost radio
contact with the ground, but nonetheless it is a shock to find I am
still a lousy navigator, not much improved since Gemini days."

Quotes are from "Carrying the Fire" by Michael Collins (1974).


A 1969 manual on the Apollo spacecraft, including a chapter on its
guidance and navigation system, is online here:

http://history.nasa.gov/afj/aohindex.htm

It says the sextant magnified 28x and had a 1.8° field of view.

The same chapter also has lot of information on the Command Module
Computer, since a large part of its job was guidance and navigation. It
was a 16 bit device with 38,912 words of storage in core memory. If you
watched the movie "Apollo 13" you got a look at the control panel.
Operation was unusual by modern standards: you entered numbers
representing data ("nouns") and the action to take ("verbs"). Most
people would laugh at it today, but the Apollo missions would have been
impossible without this machine.

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