Finding True South

If you want to be precise, and you probably do, borrow a compass. It doesn't have to be a super-accurate one, just a solidly built one that's truly a working compass and not simply a toy. The hardest part of using a compass is making sure it's level. It's difficult to hold a compass level in your hand, so it you have a surface you know is level, use it. The compass's pointer will point to north, so you'll want to align that with the north pointer on your compass. The other pole is south.

So far so good, but you're not quite finished. The compass is pointing to magnetic north, which may or not correspond closely to true north. The difference is caused by a slowly shifting phenomenon known as magnetic declination - the difference between magnetic north and true north. Depending on where you live and when you're measuring, magnetic declination can be significant. If you're in the U.S., you can input your Zip Code and find out the magnetic declination for your residence and this date with the calculator provided by the National Geophysical Data Center. While you're at it, jot down your latitude and longitude. Trust us, in a post-petroleum world, your latitude and longitude will matter, and besides, you'll impress (or frighten) your friends when you know your latitude and longitude from memory. If you're outside the U.S., you can consult the calculator provided to help Muslims find Mecca at Qibla.com.

 If your magnetic declination is less than 5 degrees east or west, as it is for a good bit of the eastern U.S., breathe a sigh of relief and stop reading, because that little bit of inaccuracy really doesn't matter. Remember that a compass has 360 degrees, so 5 degrees isn't enough to worry about.

If your magnetic declination is more than 5 degrees, you'll want to adjust for it. At this point, to keep the concepts manageable, focus exclusively on finding true north. Finding true south will then be a snap. Let's say that you live in Seattle, WA (Zip Code 98109) and that you're measuring on January 20, 2007 (yeah, we know that's in the past, but that's when we wrote this). You enter your Zip Code at the National Geophysical Data Center and find out that your latitude is 47.633875 degrees north and your longitude is 122.347615 west. You click on "Compute Declination," and you learn that your magnetic declination is 17 degrees 30 minutes East. The minutes don't matter, and neither does the rate of change because once you measure this right, the direction won't change, just the compass bearing's declination from it.

Here's the tricky part. Once you find magnetic north, move 17 degrees to the left. That's true north. The opposite of that is True South.

Now let's assume you're at the other side of the U.S. in Acadia National Park (Zip Code 04609) and that you're measuring on the same day, January 20, 2007. You enter 04609 at the NGDC and find out that your latitude is 44.373799 degrees north and your longitude is 68.244791 degrees west. You click on "Compute Declination," and you learn that your magnetic declination is 17 degrees 15 minutes West. Again, neither the minutes nor the rate of change matter to you. Now find magnetic north and move 17 degrees to the right. That's true north. The opposite is True South.

If you want to tell at a glance where the magnetic declination is greatest, you can check the NGDC map of magnetic declination. It tells you quickly, for example, that if you live between Houston and Atlanta in the south or between Omaha and Indianapolis further north, your magnetic declination is less than five degrees and not enough to worry about.

Now having told you all that, let us tell you what we did to find true south. It's foolproof, and it costs little or nothing. You're going to calculate solar noon and see where the sun is at that time.