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Brad,

A pair of Lubbers lines inside the spacecraft by the port hole could
be sighted through the horizon glass of a sextant to obtain a relative
bearing to a celestial body. This should help with thruster
orientation.

Greg

On Jul 21, 5:59�am, Brad Morris  wrote:
> Frank, you wrote:
>
> I was thinking of spacecraft orientation. If your rocket isn't pointed in 
the right direction when you fire, even by a few degrees, you will not get 
where you want to go.
>
> That would be an interesting exercise. �The sextant must therefore be 
aligned to the thrust axis of the vehicle. �In other words, holding your 
sextant in your hand and pointing it out the window (with the previously 
agreed exercise on distance, RA and declination of the center of your 
vehicle) does not really tell you how your vehicle is pointed! �Assuming that 
the roll of the vehicle doesn't matter, then the vehicular pitch and yaw will 
not be known UNLESS the sextant is referenced to the vehicle. �I am not sure 
how we do that with our high end marine sextant!
>
> Best Regards
> Brad
>
> -----Original Message-----
> From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of frankr...@HistoricalAtlas.com
> Sent: Tuesday, July 21, 2009 3:55 AM
> To: NavList@fer3.com
> Subject: [NavList 9160] Re: It's Moon-landing Monday
>
> Brad, you wrote:
>
> "The distance (900 miles) is an easy one. �Use your sextant to measure the 
diameter of the moon, and since you know the true diameter, you can calculate 
the distance to the moons center, using simple triangles similar to HP."
>
> Yes, that would work. The Moon would loom awfully large from 900 miles above 
the surface. Could you measure it through the small windows typical on 
spacecraft? You could do something similar with a couple of small, prominent 
craters. Presumably you have detailed lunar topography (it's 2029 after all), 
so you could ask your software to compute the exact angles between some 
craters beneath your trajectory for the correct altitude.
>
> Any other ways to navigate visually??
>
> You added: "As a check, you can also perform the same calculation using the diameter of the earth."
>
> That wouldn't be accurate enough, would it? The Earth would be about two 
degrees across seen from the Moon. Suppose I can measure its angular diameter 
to +/-0.25 minutes of arc. That's about one five-hundredth of the diameter 
and thus corresponds to a similar proportional error in the distance from the 
Earth or roughly 500 miles. That is, if I measure the Earth's apparent 
diameter as two degrees +/-0.25' then the distance away is 240,000 miles 
+/-500 miles (off the top of my head --somebody check my math)
>
> "In terms of the RA and declination, use the earth as a nadir point and 
determine where you are relative to the star patterns shown. �Nominally, in 
celestial navigation, we use the zenith, but because it is easy to look down 
at the earth and see the star patterns, look at your nadir. �Since the star 
patterns will not shift in parallax for an orbit within the earth-moon 
system, this will provide a very reasonable RA and declination."
>
> Indeed. You measure a couple of angles between stars and the Earth at known 
GMT. We could call them "terran distances" instead of "lunar distances". Then 
those together place you on a "ray of position" extending from the center of 
the Earth. I will mention (again --can't resist) that you can do the same 
thing on the surface of the Earth to fix your position by measuring lunar 
distances at known GMT. It's space navigation on the ground.
>
> You concluded:
> "Finally, since we can assume you are "out of earth orbit" for 6 hours, you 
are generally pointed in the correct direction anyway. �As a result, the time 
to fire the rocket is most dependent on the distance and not so much on the 
RA and declination."
>
> I was thinking of spacecraft orientation. If your rocket isn't pointed in 
the right direction when you fire, even by a few degrees, you will not get 
where you want to go. In fact, the only real practical use of star sights on 
the Apollo missions was to "align the platform" of the inertial navigation 
system which amounts to using star sights as a 3d astro-compass. Without an 
INS and with thrusters on manual, there's no way you could depend on the 
pointing direction of a manned spacecraft for more than a few hours. Light 
pressure differences on various parts of the spacecraft, slight outgassing 
from thrusters and other components, and astronauts shifting around inside 
would all change the spacecraft's orientation substantially. The Apollo 
spacecraft had an auto-pilot system tied to the INS which fired thrusters 
automatically to maintain orientation (or planned rolls). You can see a 
dramatization of it trying to do its job after the explosion during the movie 
"Apollo 13".
>
> -FER
>
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