Western Celestial Navigation
Celestial navigation is a rather complex skill, which requires either the ability to do trigonometric function calculations with a calculator easily or the ability to look up a bunch of things in tables accurately. It is now largely an obsolete skill, since satellite navigation with the Global Positioning System is more accurate and easier. Nevertheless, celestial navigation took Western navies and merchant fleets all over the globe for two centuries - the nineteenth, and almost all of the twentieth. There are some websites which can explain this obsolete art in more detail:
http://www.umland.onlinehome.de/page2.htm links you to a page where you can download a "short book" on celestial navigation. I haven't checked out the book to see whether it really is short or not.
While celestial navigation is complex - I've taught a two-credit course on it - you can understand how it basically works by understanding the "overhead star" method. I've introduced this method in celestial navigation courses and actually practiced this method when I sailed off the coast of Maine.
Imagine that you are on a sailboat, lying down on the deck and looking up the mast, which is perfectly vertical. You see a star - let's suppose that it is the bright star Deneb - which is exactly lined up with the mast. This means that Deneb is exactly overhead.
Because the earth's axis does not move very much in space, you can use this observation of Deneb to figure out where you are. Suppose that you draw a long imaginary line from the center of the earth to your position. If you extend this line past you, up the mast of your boat, and thousands of light-years out into space, you will come to Deneb.
When I actually made this sighting, many years ago, I was located in Pulpit Harbor, off the coast of Maine. At the time I was looking at Deneb, this imaginary line extended up from the center of the earth, through my eyeballs, and right smack into the center of Deneb. Since some energetic astronomers have measured the position of every bright star in the sky, this would be a nice way of finding out where you are at sea. Deneb is 44.8 degrees north of the equator, and so was I.
The overhead star method of navigation would be really easy if the earth didn't rotate. Knowing that I was 44.8 degrees north of the equator is only a partial measure of where you are. If the earth didn't spin, I could also determine how far east or west I was. But because the earth does spin, the overhead star method of navigating is only good for determining how far north or south you are. It doesn't give you how far east and west you are.
This problem was quite severe and tragic for a large fleet of English sailing vessels who were approaching the English Channel in the early part of the 18th century. They knew that they were north of France, so they wouldn't sail into France. They knew they were south of Ireland, so they wouldn't sail into Ireland. However, there is a group of islands just west of the English Channel, called the Scilly Isles. These islands are tiny little things, with few if any inhabitants. But because this fleet was farther east than its commander thought it was, it ran into these tiny little islands at night, when they couldn't be seen. Two thousand sailors died.
The death of these sailors and the loss of so many ships was a great concern to the British government. In a way that is so typical of Western science, they focussed on one solution. Find out what time it is, and you can figure out how far east or west your are.
Go back again to my ship located off the coast of Maine, sitting on a spinning earth. As the earth spins, there is only one time that the bright star Deneb will be exactly overhead. Just look at the stars or the sun and you can see this - the stars do the same thing that the sun does, rising in the east and setting in the west. If I had a good watch, I could record the time that Deneb was exactly overhead, and I'd know where I was in the earth's spin cycle.
Back in the eighteenth century, they didn't have good watches, especially ones which could work on a seagoing vessel which pitches and rolls in the steep waves. The British Government was so set on using time to get your east-west position that they offered a prize for anyone who could determine your east-west position, technically called your longitude, at sea. After about 60 years, the prize was won by John Harrison, who built the first accurate marine chronometer. A very nice book by Dava Sobel, Longitude, tells the story. A+E has produced a six-hour television program on the story of Harrison's chronometer; I find the book to be better, but the A+E video is available in the University of Delaware library.
Polynesian wayfinders had no way of building accurate clocks, since their civilization did not have access to the right kind of iron ores. However, they solved the problem of finding their position at sea in a completely different way, by using multiple indicators of the nearness of land. The story of how they do it has only become clear in the past 20 years, and a separate webpage (wayfinders.htm) has some of the details.
Updated by Harry Shipman
May 20, 2001