Archive for May 27th, 2009

Latitude

May 27, 2009

Astrolabe 1Suppose you’re on the open seas, and your GPS falls overboard.  Or maybe it’s 400 years ago.  How can you tell what your latitude and longitude are?

We’ll start with latitude, because it’s easier.  A LOT easier.  If it’s a nice night, and you’re somewhere in the Northern Hemisphere, the easiest way is to take a gander at the North Star using an astrolabe.  Some astrolabes are simple, others are complicated, but they can all be used to find the North Star.

Hey, how about if Godzilla shows us how to use a homemade one!

Astrolabe 1

This model was originally designed by…someone.  I don’t actually know who came up with it.  But all you need is a protractor, a straw, tape, string, a weight, and someone to drill the hole in the protractor so you can attach the string [or you can use a paper protractor].

In theory, the bottom of the straw should be held up to the eye.  Godzilla’s short arms aren’t really conducive to holding things up (he’s had to survive using his brains as much as his brawn), so here’s a drawing of what it would look like:

Astrolabe 2

When you look through the straw at an object like the top of a tree or the North Star (Hmm, why didn’t I draw a star?  Because Godzilla was actually facing tree in our yard at the time, so I got distracted).  Anyway, when you look through the straw, the weight causes the string to fall right at the angle that the object is above the horizontal!

Here’s why:

Astrolabe 3

So sailors just grab their handy dandy astrolabe, measure the distance of the North Star above the horizon, and that’s the latitude!  [I always have to think that one through.  On the equator, the North Star would pretty much be on the horizon, so that's 0° above the horizon, and at the North Pole the North Star would be directly overhead, which is 90° above the horizon.]

Next up:  Longitude (Part I and Part II).


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