“Tom Sawyer Abroad,” a little-know Mark Twain book, describes the adventures of Tom and Huck as they traveled in a swift dirigible. Following is an excerpt (H is Huck, and T is Tom):
H: Tom, didn’t we start east?
H: How fast have we been going?
T: Well, you heard what the professor said when he was raging round. Sometimes, he said, we was making 50 miles an hour, sometimes 90, sometimes a hundred …
H: Well, then, it’s just as I reckoned. The professor lied … Because if we was going so fast, we ought to be past Illinois, oughtnt we?
H: Well, we ain’t … I know by the color. We’re right over Illinois yet. And you can see for yourself that Indiana ain’t in sight.
T: I wonder what’s the matter with you, Huck. You know by the color?
H: Yes, of course I do … Illinois is green, Indiana is pink. You show me any pink down here, if you can. No sir, it’s green …
T: Huck Finn, did you reckon the states was the same color out-of-doors as they are on the map?
H: Tom Sawyer, what’s a map for? Ain’t it to learn you facts? … Well, then, how’s it going to do that if it tells lies? …
T: Shucks, you muggins! It don’t tell lies …
H: All right, then, if it don’t, there ain’t no two states the same color. You git around that if you can, Tom Sawyer.
Maps are intimately tied to the heavens. The first great imaginary lines applied to the earth’s surface were the equator and the tropics. The equator is the horizontal great circle, a great circle being one that describes the circumference of the planet, its plane passing through the center of the earth. The tropic of Cancer, at 2312 degrees north, marks the point at which the sun shines directly overhead on the summer solstice. The tropic of Capricorn marks the southern equivalent. But they are not great circles.
These lines of latitude have their equivalent in longitude, which is measured from zero to 360 starting at Greenwich, England, and going west. With a system of numbers based on 10, it may seem strange that the earth, and thus all circles, are divided into 360 degrees, until the origin of the degree is understood.
When folks spent more time outdoors they were more attuned to the celestial cycles. The movements of the moon, planets, and stars were as familiar to them as the clock is to us. One can imagine a young man pondering these motions whilst tending his goats. After a time, he would have noticed that the period from one full moon to the next is about 30 days (actually about 29 1/2). This became known as the month, and the four phases of the moon became the weeks. Next he noticed that it took 12 months to pass from one vernal equinox to the next. So, 360, derived from 12 months time 30 days, was applied to the circle of the year and by extension to every other circle. A degree is the unit of measure of an angle based on the zero degree line, which on earth is the Prime Meridian at Greenwich, established when England considered itself the center of the universe.
The degree was further divided into 60 minutes and the minute into 60 seconds. The earth turns 15 degrees in one hour, which is why lines of longitude are marked off in 15-degree increments on most globes, and a nautical mile is the length of one minute of longitude at the equator.
These imaginary lines, inspired by the heavens, are the foundation of cartography, surveying and navigation, and most maps are based on these circles and on the movements of the moon and the sun.
You git around that if you can.
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