The international dateline

Someone asked me to write a description of why the international dateline makes sense. I didn’t totally get why it would as a kid, but eventually figured out a way to look at it that made sense to me. Apparently they were in the same situation, and it worked for them as well. Anyhow..
The international dateline is a particular North to South line (slightly zigzagged for bureaucratic reasons) that works like a time zone line, except instead of gaining/losing an hour or two, crossing it gains or loses 24 hours. It’s located mostly around 180 longitude (diametrically opposite 0, i.e. Greenwich, England), location chosen to interfere as little as possible with normal things, which is probably a thinly veiled synonym for “English things” or “European things”.

There is a reason we’re stuck with it, provided we use local time at all. Local times are set so that the sun is approximately as high in the sky as it will go at 12:00 noon. This used to be a lot more useful, but it’s still the norm. Moving with the earths rotation will make that occur sooner (as opposed to just standing there waiting for the earth to move you along), so moving east will put local noon sooner. Going “counter rotation”, i.e. west, will do the opposite. Since pegging “noon local” at exactly when the sun peaks isn’t precise enough to coordinate some things, so rather than define it as noon wherever you are, blocks of area will agree to follow a certain exactly synchronized time. The edges usually follow national or in-country borders of some kind, and usually agree on an even to the hour offset (but not always, 30 minutes off zones exist). Within these time zones, the sun will usually peak less than half an hour before or after noon, sometimes the desire to be “on the same time” as a larger block has larger offsets, but rounding it off a little.

So, lets suppose you stand at a spot, and then travel eastwards, a whole 360/24 degrees along the earth. You’ve moved over the equivalent of one hour, and indeed local time is now one hour forward from when you started. If it was noon when you started (and you didn’t spend any time moving) it’s now 1 pm. Sweet, things are going according to plan. Move another equally-sized chunk, bam, 2 pm. You’re two hours ahead, and all is well. Keeping it up, after 23 times, you move yet another time and.. you’re 24 hours ahead, and back where you started. And it’s true too – you did spin around the earths axis one more time than those around you who just stayed there. But you can’t have your own 24-hours-offset timezone. Well, you can perhaps, but the area you’re in can’t ambiguously be two separate zones at once – if everyone in your local zone is to agree on a time, including date and year, you must somehow come to terms with that you’re not 24 hours ahead – you must at some point *not* go one more hour ahead, but instead 23 hours *back*. It’ll all be the same to you if you’re merely looking at a clock (no date) – you’ll be at (say) 4am, then it’ll be 5am, but it’ll really go 4am, Tue Aug 7 to 5am, Mon Aug 6.

Or, if you don’t feel like overly complicating certain time zone boundaries (they were so nice and understandable after all) you could just designate a specific pseudo time zone line for just that – go east, lose those 24 hours of illogical offset you’ve built up the rest of the lap, go west, add them on. And that’s what they did – thus the international dateline.

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