The International Date Line

My son asked a random back-of-the-car question the other day, about whether there was a place where the day changed too, not just the time. There is, of course, and it's the International Date Line, more-or-less located at 180° longitude, opposite the globe from the Greenwich meridian. (the picture attached is from Wikipedia)

What I found interesting about the question, though, was how the idea of such a line requires a leap of how we think about time. For most of us, time is local: our makers are sunrise, noon, sunset, midnight. Like the Archimedean, Earth-centered model of the universe, and like earth-navel cosmologies, it makes "us and ours" the center of all things, both in space and time.

20 years ago, I took a course at Carleton College from the late Mike Casper, "Revolutions in Physics," that was mocked by some as "Physics for Poets." Which isn't really fair, although I can see the case that it really was as much a history of science course as anything. The course was divided into three sections: the Archimedean, earth-centered universe, the Newtonian universe, and the Einsteinian universe. The goal of each was to immerse the students in what historically were comprehensive worldviews. And it worked. It was fascinating how useful the oldest model, the one we've mocked as "wrong" since grade school, really is.

It's of course incorrect that the sun goes around the earth, but there's a lot to learn about seasons and the sun's movement in the sky if you think that way. And I became aware that I simply had not paid that much attention to how the sun moves in the sky over the years. For example, that it is always due east or west at 6 o'clock (either one) local time. Or that the angle of the sun's path is constant in the same location, but that the constant-angled path moves up and down vertically with the seasons in relation to the horizon. I dunno, maybe everyone else got that from day one, but it was new to this college student and it was cool, and it depended on thinking locally.

The Copernican/Newtonian model of the universe that shifts this around. Suddenly, we're on a planet, and really there is only one day, and it keeps rotating around the globe—or rather, the globe keeps turning and the day is the glow from the star at the center of our solar system. Instead of the sun as a clock that keeps our time, we are fixed points on a moving sphere, which keeps its time in turning us and everything else in the world.

And it's in this world that International Date Lines become necessary.

Interestingly, it wasn't scientists who first proposed such a line. It was an 11th-century Jewish scholar, who was concerned that all the Jews in the diaspora should observe the same Sabbath, and so proposed a system which kept the same day-observance for all of Asia, and made a break somewhere in the Pacific. The "Circumnavigator's Paradox" was in fact a real paradox, discussed in the late middle ages (see the excellent History of the International Date Line for much of the source material in this post): Apparently it surfaced when Magellan arrived at the Spanish outpost in the Philippines, having come from the east by way of Cape Horn, and disgareed with the Spanish officers there, who had come from the west, via the Cape of Good Hope. Their dates, of course, were off by one.

In a world where one set of grandparents is an hour earlier and another is an hour later, and where we can fly to Europe where it's six hours later, it's commonplace to think about time zones. But it was not always so. It was not until railroads needed to keep precise time in their east-west journeys that the need for standard time became apparent. Before the railroads, punctuality was enforced within local communities. A parishioner coming to church on time, or a worker arriving at the mill, or any citizen keeping any of the other appointment-keeping arrangements we make, either had to judge by the sun, or by the bells of the church tower, or later by the local-time clock or sundial, how close they were going to cut it. And travel, by foot or horse, essentially re-set the clock.

By instituting standard time, we essentially said, railroad time is more important than where the sun stands in our sky.

And much the same thing, on a global level, happens with the International Date Line: we are forced to recognize that this is a round, whole planet, which moves in one time, simultaneously. And for better or worse, this means we depend less on what we see—here and now over our heads—to regulate our lives by.