Compasses and Sea Voyage



If you've ever been to sea, you know how hard it is to use a compass to navigate. How can you tell exactly where you're starting from, and exactly how far you've gone?

A big issue is that a compass points at the magnetic north pole - but that spot is not stationary. As the earth spins and moves, the magnetic north pole shifts - at a rate of 5 mph. That causes "bends" in the magnetic field, meaning that a magnet won't necessarily point exactly at that north spot. Depending on where you are in the world, your magnet could be anywhere from -20 to 80 degrees or more off. And that warping will change from day to day.

There is a REALLY cool map of that effect here, that even shows you the changes over time -

USGS.GOV Moving Declination Map

I love this sort of thing. You can imagine why sea captains had such trouble navigating when they were out in the open ocean.

This problem is made even worse in places like the Bermuda Triangle. There are simply spots on earth that have an anomoly. Sometimes it's because the earth's crust is thin and the magnetic field is therefore altered. Sometimes it's because a large ore deposit makes the magnetic field strange. Ships moving through the Bermuda Triangle often find that their compasses do not point towards magnetic north even when taking the current declination into account.

If you're going on a cruise, try taking along a compass, and seeing what it does! The further you travel, the more interesting your notes might be! Even better, if you have a GPS, bring that along, and compare the two!

Cruising Tips and Information Main Page