If you pass through the northern New Hampshire town of Stewartstown and Clarksville, you may notice signs informing you that you are standing on the 45th parallel, the halfway point between the North Pole and the equator.
Such signs appear in quite a few places along the 45th parallel, but some are farther north and some farther south than others.
The 45th parallel, in other words, is a fuzzy line, and in this week’s Granite Geek column for the Concord Monitor, David Brooks explains why the line is so difficult to nail down—and why so many people insist upon trying.
He spoke with NHPR’s Peter Biello about his column.
There’s some disagreement about where the 45th parallel is. What’s the basis of this disagreement?
Disagreement is maybe not the right word. It depends on how you define [the 45th parallel]. To us laymen, the 45th parallel sounds like a very precise thing, but it actually depends on how you define it.
The problem, basically, is that the earth is not a sphere. It’s not a ball. It’s actually slightly squashed… It’s more like a ball that’s slightly deflated and you’re sitting on it. It gets pushed out more around the equator and flattened at the poles.
As a result, if, from the equator to the North Pole, you’re going to divide it up into 90 equal degrees, if you do that [using] angular degrees from the center of the earth, when they hit the surface of the earth they’re going to be slightly different distances apart because you’re not going out to a ball. As I say in my column, if this is confusing you, pick up an egg and try doing it on an egg and you might figure it out.
So, the question is, how are you going to define these 90 equal divisions between the equator and the pole? Generally it’s done from the center of the earth, but that’s not entirely precise. It depends on what sort of model you use for what the earth looks like.
This gets particularly complicated because not only are we not a sphere, we’re not even a regular spheroid. Earth is actually lumpy in ways above and beyond mountains and valleys that complicate it further.
So basically, there are a number of different models you can use to estimate what the earth’s surface looks like, and you use those to decide where the lines of latitude are. Whichever model you use, the location of the lines of latitude will change. They [the models] are called datum.
Perhaps the more important question, David, is who cares? What’s the significance of the 45th parallel?
It’s a goofy, kind of pointless fun thing. Now, having said that, surveyors care if certain things are defined from points of latitude. There have probably been wars fought over this sort of thing, defining where your border is between states or countries.
The advent of GPS satellites, that has changed things yet again and required yet another definition, because there’s no single way to estimate the entire surface of the earth, so you kind of fudge it. Depending on what fudge factor you use—you can actually see this in your GPS unit, if it’s sophisticated enough for you to go into the settings and change the datum. You will see that your latitude will change slightly as if you moved.
So, it’s a little bit arbitrary, which is kind of the point of it despite these lovely signs…It’s not that exact.
The sign marking the 45th parallel in Clarksville has been moved. Why was it moved?
Actually, it wasn’t initially placed in the exact spot the surveyor said it should be, because that was such a dangerous bend in the road that they were afraid, if they had a sign there, people would stop to look at it and get killed. So initially it was a couple hundred yards off, then the land was sold, and the sign got moved and/or stolen, and finally replaced and moved to another property another couple hundred yards away.
So it’s been shifted a bit, so they have long joked that it’s a fuzzy line, and they didn’t realize that geographically as well as historically, it is a fuzzy line.
