I understand that the altitude of the north star equals your latitude on earth. Therefore, if you were standing at 90ºN Latitude, than you would have to look directly above you (a 90º angle to where you are standing) to see the star.

If you were at 20ºN Latitude, you would have to look north at a 20º angle to the ground you are standing upon to see the star.

If you were at 0ºN Latitude (the equator), the you would have to look at the bottom of the horizon to see the star, and only on a really clear night.

People sailing at night in the northern hemisphere could use the star without any problem to navigate. However, if you were to sail in the southern hemisphere, what star would you be able to use (if any) to navigate at night? Thanks for your responses!

The Southern Cross in the southern hemisphere serves the same purpose as Polaris in the North.

http://mmbc.bc.ca/source/schoolnet/exploration/navigation.html

To see how it works, go to www.astrolabes.org. This is a brief but very informative website.

If you're really ambitious, try reading Geoffry Chaucer's treatise on the astrolabe here.

I couldn't find anything on navigation using the north star, but that doesn't mean it didn't happen. The astrolabe was used to measure the sun's position in the sky both for telling the time of day and the position of the ship.

They're still used on ships today, even though our electronic navigation systems are much better. (On my ship, there was only one officer who knew how to use it. For everyone else, it was show and tell.)

If you think about it, standing at the north pole and looking straight up at a 90 degree angle, your line of sight is parallel to one seen standing at the equator and looking straight at a 0 degree angle. Since the two lines never cross, you really can't see the same object no matter how far away it is. For anything lower than about 30 degrees north, your line of vision would pass through the earth (if it were a perfect sphere, which, out to sea, it very nearly is).

After reading Frankvan's sight, I've been wracking my brains trying to remember what the signal was called that was generated by the pitsword in a ship.

The pitsword is a piece of steel about 8 feet long (looks like a big sword without an edge). It sticks out of the keel from a watertight opening in the bow and just rides through the water. The water's current around the metal induces a magnetic field, and from the field, we can determine the ship's speed and changes in direction more accurately than anything else. The electronic gear attached reads the inputs and sends the signal through the ship to various places, the only input required is an initial position (which comes from the shipboard gps.)

The signal is called "dummy log". I've always wondered why it was called a "log", since it's not a substitute for a ship's written record. Now I know.

Thanks for your responses, frank and mahal!

In early days, sailors used to toss an actual log overboard with a line attached, then pay out the line which was marked at regular intervals. A very precise time was allowed. Then the line was hauled in again, and measured by counting the marks, and the ship's speed was calculated -- and entered in the 'log book'.

Wow! I never knew the origin of the log book! Thanks for sharing!