Thursday, December 9, 2010

Where Are We?

Navigation is one of my favorite things, and I got to do a little bit last night as a passenger on AAL945. From way back in the cabin.

Those of you who have been on international flights in the past few years have seen the moving map displays on the in-flight entertainment (IFE) system, but don't scoff. This was an older 767-300 and rather than individual screens (where each passenger can choose the channel) there were monitors hanging down from the middle of the ceiling. And the passengers all watched the same channel, which featured a movie starring someone not among my favorites.

Those IFE moving maps are pretty funny, too, highlighting either the smallest town in the area or the town with the longest name.

But all I had was the view out the window. I could (barely) see what appeared to be the two pointer stars in the Big Dipper, and they were pointing at the tail of the airplane. Just like they should! It was hard to be sure, because the rest of the Dipper was below the horizon, but the orientation looked about the same as it did at my N42W112 house.

So now I had an idea where the North Star (Polaris) was.

Polaris is not exactly over the North Pole; almanacs have tables showing how to correct for its motion. But its angular distance above the horizon (its altitude) is a pretty good estimate of your latitude.

Then I used another old trick: a fist at arm's length subtends about 9 or 10 degrees. (Try it: start at the horizon and go fist-over-fist to the zenith.) Using that, I estimated that we were about 10 degrees North.

Another key axiom of navigation is that you don't know where you are until you know where you're not. The Marcq Ste.-Hillaire or Sumner Line methods of Celestial Navigation depend on this axiom. It involves a couple of steps:


  • Take a guess where you are; this is the assumed position;

  • Calculate the altitude of a star or planet, based on that guess;

  • Observe the altitude;

  • Using the difference, adjust the assumed position. That's your fix.

    In this case, I assumed that I was at N10W095 at 0700Z. Today I went to an online star chart to look at the night sky from that position at that time. Here's what it showed me.




    And that's what I saw! So, in retrospect, my estimate was pretty good.

    As the night wore on (and it was a long night, over 9 hours) and the Dipper rose, the pointer stars pointed further and further down, until they pointed below the horizon. In other words, we crossed the Equator.

    And then I was stuck: I don't know the Southern hemisphere constellations. Besides, it was time to sleep.



    There are no navigational heroics from row 36, so here's a real story. When Amundsen and Nobile were flying the airship Norge from Spitsbergern to Alaska by way of the North Pole, it was very difficult to fix their position. The navigator, Riiser-Larsen, couldn't take sights from the gondola, so he climbed up to the roof of the ship to take sextant sights while exposed to the 60mph Arctic (relative) wind.

    (This doesn't have much to do with my flight, but I thought it would be fun to mention the Norge in two consecutive posts.)
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    4 Comments:

    At December 9, 2010 at 4:15 PM , Blogger Toriafly said...

    Ok, I have always wanted to know how to do that. You just convinced me that I must learn. Any good book suggestions?

     
    At December 10, 2010 at 5:02 AM , Blogger Frank Van Haste said...

    Jim:

    Re: Riiser-Larsen "up on the roof"...

    I believe he might've been cold but comfortable if he'd been sufficiently aft of the airship's nose for the boundary layer to have developed sufficient thickness. Things are going to be pretty laminar for that size and speed.

    I remember seeing a long time ago (and am now completely unable to find) a photo of (USN?) airship crewmen having a picnic on top of an underway dirigible airship. I bet 400 feet aft of the nose on Akron or Shenandoah you could've easily stood up while underway.

    I wish you a safe journey,

    Frank

     
    At December 10, 2010 at 7:11 AM , Blogger Dr.ATP said...

    Frank,

    Gee, I always pictured him at the bow! But of corse he stayed aft. Nevil Shute in Slide Rule wrote about riding on the roof of the R-101 (or was it R-100?) during test flights.

     
    At December 11, 2010 at 10:13 AM , Blogger Frank Van Haste said...

    Jim:

    I'd forgotten that episode from Slide Rule! It was the R-100. The R-101 was the competing project that crashed, essentially ending British LTA programs.

    Frank

     

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