At this point in our navigation education, we're able to find our latitude and longitude using the position of the sun. So if we were on a ship in Antarctica, we should be able to figure out where we're located. Next we might be tempted to set a course to travel and use our compass to sail the ship in the correct direction. But we have to be very careful.

When we left Robertson Bay to begin sailing around Cape Adare, the nautical charts showed that we had a clear path through the bay toward the open sea if we sailed at a bearing of 355°. However, let's pretend that the ship had lost electricity, that it was pitch black outside, and that all we could use to navigate was a small, battery powered flashlight to read our instruments and a handheld magnetic compass to direct us. If we had used the compass to direct the ship toward a bearing of 355°, we would have crashed into almost immediately. This may sound weird, but using the compass to direct us to a bearing of 355° (nearly due north) would have actually caused us to travel in a southwest direction and crash into Duke of York Island. Confused yet? Let me explain why this would happen.

Because a compass gives a bearing based on the Earth's magnetic field, and the magnetic poles are located very far away from the geographic poles, there is a difference between magnetic north (the direction that a magnetic compass will tell you is north) and true north. This difference is called the magnetic declination or the magnetic variation. In San Francisco, the magnetic declination is about 13°W (according to the NOAA). This means that a magnetic compass would show north in a direction that is actually 13°W of true north. So if you want to head in a particular bearing, you have to subtract 13° from that true bearing to get the equivalent magnetic bearing.

Quick Reference: To convert a true bearing to a magnetic bearing, we add the magnetic declination, with the convention that east is positive and west is negative. This is what we'll be doing here. But if you ever wanted to convert a magnetic bearing to a true bearing, you subtract the magnetic declination (since you're going in the opposite direction - no pun intended).

So the magnetic declination in San Francisco isn't very much. But suppose you were on a ship in Commonwealth Bay, located in between the Magnetic South Pole and the Adelie Coast of Antarctica (see the map above). A magnetic compass would tell you that south is in the direction of the Magnetic South Pole; however, we know the South Geographic Pole is located almost in the exact opposite direction, toward the interior of Antarctica. So there, the magnetic declination would be almost 180°! Your compass would basically be telling you to go the exact opposite direction you wanted! And if early explorers looking for Antarctica had been caught in a snowstorm and didn't account for magnetic declination, they might have missed the continent completely!

Note that sometimes magnetic declination information is given inside the compass rose, as in the example above. But sometimes it's given in the form of curves of constant magnetic declination (as in the first problem below) or directly in the chart's key for smaller areas (as in the second problem below).

Sample Problems

1. Suppose you're located in Marguerite Bay and you need to sail at a bearing of 320° to exit the bay. Use the information from the first map below, whose magnetic information was published in 2010, to determine the bearing you would need to set on a magnetic compass.

2. Suppose you're located on the west side of Peter I Island and you want to sail at a bearing of 178° to head down the coast of the island. Use the information from the second map below to determine the bearing you would need to set on a magnetic compass.