# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Message:αβγ
Message:abc
 Add Images & Files Posting Code: Name: Email:
Re: It's Moon-landing Monday
Date: 2009 Jul 21, 08:59 -0400

```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

-----Original Message-----
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of frankreed@HistoricalAtlas.com
Sent: Tuesday, July 21, 2009 3:55 AM
To: NavList@fer3.com
Subject: [NavList 9160] Re: It's Moon-landing Monday

"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

"Confidentiality and Privilege Notice
The information transmitted by this electronic mail (and any attachments) is
being sent by or on behalf of Tactronics; it is intended for the exclusive
use of the addressee named above and may constitute information that is
privileged or confidential or otherwise legally exempt from disclosure. If
you are not the addressee or an employee or agent responsible for delivering
this message to same, you are not authorized to retain, read, copy or
disseminate this electronic mail (or any attachments) or any part thereof. If
you have received this electronic mail (and any attachments) in error, please
call us immediately and send written confirmation that same has been deleted

--~--~---------~--~----~------------~-------~--~----~
NavList message boards: www.fer3.com/arc
Or post by email to: NavList@fer3.com
To unsubscribe, email NavList-unsubscribe@fer3.com
-~----------~----~----~----~------~----~------~--~---

```
Browse Files

Drop Files

### Join NavList

 Name: (please, no nicknames or handles) Email:
 Do you want to receive all group messages by email? Yes No
You can also join by posting. Your first on-topic post automatically makes you a member.

### Posting Code

Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
 Email:

### Email Settings

 Posting Code:

### Custom Index

 Subject: Author: Start date: (yyyymm dd) End date: (yyyymm dd)