[time-nuts] Smithsonian Time/Nav Exhibit

[Brian Alsop]

I have a WWII vintage octant used by my Dad to do celestial navigation 
when ferrying bombers to England during the war.  It was also used when 
Colonial Airlines flew from NYC to Bermuda in the late 40's.  Still have it.

It really is possible to navigate with the beastie.  The airplane 
turbulence induced jerks were not an obstacle.  If fact, the motions may 
have been less vexing than use of a hand held sextant on a ship.

The octant has a drum and pencil arrangement.  When you take a fix, it 
marks the drum.  You take a series of measurements of the same object, 
write down the start and finish times.  It has a lighted bubble to 
provide the horizon.  Thus one has a horizon day and night.

Insert a new drum and do another object.

Go back to your desk and do the various lines of position, advanced for 
time difference between object fixes. (Generally an eyeball average of 
the various pencil lines and use of the midpoint time would suffice.) 
The plotting of them results in a fix.  The tables used for data 
reduction were much abridged from the normal several volume 
set--reflecting the less precise nature of the measurement technique. It 
contained something like 50 7"x10" pages compared to the normal 5+ 
pounds of books.

Let's face it, if you get a fix within 5-20 miles, that's good enough if 
you're trying to find England.  Finding the coast of North America would 
have been even easier on the return trip.  However, the trip back was 
via boat.

Dead reckoning using speed and direction is tough on the great circle 
route due to crazy magnetic compass deviations.  The celestial nav fixes 
at a minimum flagged that "something was wrong" if the dead reckoning 
and celestial fixes disagreed a lot.

I learned celestial navigation (on land) using this octant before 
getting a sextant.  You can use it on land because it has a built-in 
horizon.

Neat piece of history.

Brian

On 7/1/2013 12:56, Jim Lux wrote:
> I had a chance to go through the Time and Navigation exhibit at the
> National Air and Space Museum last week. From a "time" standpoint,
> there's probably not much there that time-nuts don't know already, but
> it's kind of cool to see cleaned up examples of equipment from days gone
> by. (there's an old cesium beam from NIST on display, and a Symmetricom
> cesium turned on and counting, but also a lot of old GPS stuff... lots
> of Rb and Cs for space)
>
> Quite a lot of the exhibit space was devoted to the problem of air
> navigation, which, now that I've seen the exhibit, I can understand what
> challenge it was.  Over centuries, folks had figured out how to navigate
> on ships and on land, but those are inherently slow moving, so you can
> do things like take multiple sextant sights and reduce them.
>
> But planes move fast, so you don't have as much time to do it. It took
> real guts to be the navigator in the little cockpit out front of the
> plane, taking sights with your body out in the wind.  And the poor
> fellow who was sucked out of a plane when taking sights standing on his
> seat and the astrodome blew out.
>
> It was interesting to see how many different schemes were used for
> (mostly radio based) nav in airplanes over a fairly short time. Low
> Frequency DF, A/N Ranges, VOR, LORAN, etc.  I didn't see Omega.
>
> They have an inertial nav unit there from a sub, but not much
> explanation of how inertial nav works.
>
> They talk about the DSN (and actually have a 4 bay rack of the old
> time/frequency  distribution gear on display), but not much discussion
> on exactly how we do navigation for deep space.
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