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Comment by drblast | Hacker News Reader

I was in the last class to have the old celestial navigation class. It's not a difficult thing to do. You take a measurement with a sextant then consult a fifty pound book full of tables, get the two closest numbers to what you want and interpolate.

There are various techniques based on time of day and the stars you can see, but if you're not doing it regularly you're going to have to look up the procedure anyway.

And although you might not always be able to rely on GPS, inertial navigation, dead reckoning, and visual navigation using landmarks, celestial navigation has a huge drawback.

It's defeated by cloud cover and fog.

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Not to be overly pedantic, but if you have an accurate timepiece you can get your longitude from averaging sunrise and sunset times. It's not particularly accurate, if you can't see the sun, but could be useful if you where otherwise lost at sea and under total cloud cover for a long period of time.

Do you mean latitude? I remember the days being longer in the summer when I lived at higher latitudes, but I'm having trouble seeing how you could calculate longitude by looking at sunrise/sunset.

Sunrise and sunset together will give you local noon (or local midnight). Local noon plus an accurate timepiece set to a known location will give you a time offset. Knowing your time offset is equivalent to knowing your longitude (which was the whole purpose behind the Harrison chronographs). You won't know it to a tremendous accuracy, since you can't get the exact moment of sunrise or sunset, just a vague idea, your position is likely to change between sunrise and sunset, and dead reckoning on a featureless sea under a featureless sky can only be so accurate. But it's knowing the difference between local time and a reference time that gets you longitude.

No, it would be the longitude. The average of sunrise and sunset will be local noon, and from that you get the longitude. To get the latitude you would need to see where the Sun rises and sets or see the stars, neither of which you could do very accurately under heavy cloud cover.

Thanks stan_rogers and splat, using local noon plus a clock makes total sense after you described how it works. And I just read a fun Wikipedia node on the history of longitude for good measure too.

https://en.wikipedia.org/wiki/History_of_longitude

There is an excellent book about "the Longitude problem" that details John Harrison and the invention of high precision chronometers: http://www.amazon.com/Longitude-Genius-Greatest-Scientific-P... Great short book for any engineer.

https://en.wikipedia.org/wiki/Sunstone_%28medieval%29

Celestial Navigation was a part of USAF navigator training, but was retired about the same time as the USN - and it should remain that way. The USN's reinstatement of CN is a signal that the Navy does not understand it's own technology and threats(cyber attacks). And just common sense says a ship operating off CN will not be able to bring kinetic effects to the fight. The article mentions CN oriented a disabled Apollo 12? Gimme a break, what saved the mission was the quick wit of the engineering staff... and a crew that understood its own technology.

> It's defeated by cloud cover and fog.

Only on Earth we used it as one of the navigation systems to get astronauts on the moon.

I'd rather give each sailor a course in the kind of math and programming needed to create the tables.

The Naval Academy absolutely does train thousands of naval officers in this kind of math and programming. For the most part, they will apply those skills to more difficult problems, but if those tables had to be recreated, you would have no difficulty finding an Academy officer who could do it.

The Academy doesn't train the "sailors", though, just an elite corps of officers, and you don't need to have everyone on board making his own book, just have a few specially trained people (on land, presumably) who could if they needed to. They Academy makes sure many such people exist.

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