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.

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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.)