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Wikipedia says the international date line is “a cartographic convention, and is not defined by international law.”

The “line” wasn’t a convention at all in the 16th century so how did sailors experience loss of a day?

https://en.m.wikipedia.org/wiki/International_Date_Line#:~:t....

The line is the reason you don’t lose a day now. When you circumnavigate the world westward, the earth has rotated one less time for you than for everyone else, so it appears you have lost a day. By artificially switching the day at the date line you avoid this kind of slippage.
On a ship, you also keep the same time zone as local time for the area you are steaming through. Going consistently westward to get somewhere is a bonus, because every other day you get to sleep in an extra hour.
Oddly enough, on the Queen Mary 2, sailing eastward from NYC to Southampton, they change the time just after lunch. No extra sleep-ins!
Because the count of the days for the people they encountered was derived by traveling in the other direction around the earth.
Obligatory Umberto Eco plug: https://en.wikipedia.org/wiki/The_Island_of_the_Day_Before

Hopefully Eco + the title is enough but if it helps IMO it’s arguably his best work. All of the stepwise logic leading to madness of Name of the Rose and Foucault’s pendulum, a somewhat less slapstick sensibility than either, without descending into dryness as his later work. Super recommended.

A +1 for this recommenation. I particularly enjoyed this for how the sailor explains his situation, using the mental tools avilable to him at that period of time. I always enjoy the how author goes to great lengths to explicate a period mind set (Name of the Rose/Baudolino/Island/Focualts) so it feeds the fabric of the narrative for the reader.
I'm surprised this is still up. The UU has been on a quest of destroying all web pages hosted under uu.nl domain that are not managed by "the cloud", Office365 or blackboard with the argument of "its not gdpr compliant". All the old personal web pages of professors of the CS department (cs.uu.nl) have already been purged in the name of "compliance". Decades of Internet History and science just wiped because of somebody's career fetish. Seems the math department had more backbone than the computer science department.

I wrote a bot that archived it all and it's on my old university laptop still. Lots of gems, blog posts, articles that were all just deleted. I still need to upload it somewhere

(comment deleted)
I wonder how the discovery of the need of this International Date Line influence not just navigation and timekeeping, but also the understanding of experience of the flow of time, did it perhaps influence the Englightment, as it just occured before, or was it the age of discovery of the Americas and the Pacific? Difficult to seperate.

A Romanian girl called Iliana/Ilana from Galati or Brailia I was in love with long ago recommended me Umberto Eco's The Island of the Day before when I met her in Taize. Sadly I lost her contact data and her lastname, forever lost in the day before.

Interestingly, the date line wasn't always where it is now. For example if you look at the tz database it has Asia/Manila with an offset of -15:56 until 1844; that's because the Philippines were actually colonized from Mexico, so they kept the same day count as the Americas. (https://webspace.science.uu.nl/~gent0113/idl/idl_philippines...). But as they came to be more integrated with their geographical neighbors they switched to counting the days like them.

Similarly tz has time zones as far east as +15:13:42 for Metlakatla in far south eastern Alaska, until 1867, since Alaska was settled from Russia. I think these are basically the furthest east and west the date line ever went (although maybe the Russians made it further south and east along the west coast of North America?)

I thought historical data in the tz database was recklessly deleted a few years ago. (To my horror)
It was deleted from the default build of the timezone files but remains in the source tree; patches are accepted.

Choices have to be made for usefulness vs completeness vs size as it's quite likely that at least one and perhaps multiple copies of the database are present on any system out there running code in C that needs to know what time it is in human terms.

But if you're a packrat, you can include the extras in your build of the timezone data by setting PACKRATDATA=backzone during the build.

That's actually a total relieve. I must have misinterpreted this change when it happened.
> the Russians made it further south and east along the west coast of North America

This is why there is a Russian River [1] in northern California. One of its names is "Slavyanka River" which was given by Russians.

[1] https://en.wikipedia.org/wiki/Russian_River_(California)

Russian fur traders made it to what is now called the Russian River in northern California in the early 1800's, founding Fort Ross.
Of course! And it looks like the Spanish weren't north of San Francisco at the time, so the only people in the area using something resembling our current time system would have been Russians coming from across the Pacific.

So the easternmost "time zone" ever is at least +15:47 or so.

And of course Manila is not the westernmost point in the Philippines. Wikipedia has local mean time in the Philippines going from -16:12 to -15:34 (https://en.wikipedia.org/wiki/Philippine_Standard_Time), although that assumes that the current Philippines have the same longitudinal extent as in 1844 - and I can't quickly figure out the history well enough to know if that's true.
> Interestingly, the date line wasn't always where it is now.

Different empires defined their own prime meridians (and thus implicitly their date line 180 degrees away) from the ~16th-19th century. Eventually in the late 1800s a conference was organized to pick one and since Britain was the center of commercial shipping (with all the concomitant infrastructure like Lloyds, project finance, Admiralty law etc) there was little dissent.

I wonder if Pigafetta's account was the inspiration for the ending of Jules Verne's "Around The World In Eighty Days"?
Spoiler alert! Some of us haven't yet finished that book.
:grimace: I tried to be vague enough that anyone who hadn't read it wouldn't put two and two together. Sorry! Unfortunately it was spoiled for me in the foreward of the edition that I read.

HN no longer lets me edit the parent comment.

The 'lost day' occurred because the expedition had been traveling westward, in the same direction as the Sun. As they circled the globe, they were effectively chasing the Sun, which meant that each day was slightly longer than 24 hours. Over the course of their journey, these small increments added up to a full day. Thus, by the time they completed their circumnavigation, they had 'lost' one day compared to those who remained in one location.

I like the concept of chasing the sun. That should be a metaphor.

During the Francoist period, the lyrics to the Spanish national anthem (which now officially has no lyrics) read in part

Gloria a la Patria que supo seguir, / sobre el azul del mar el caminar del sol.

Glory to the Fatherland, who knew how to follow / the path of the sun over the blue of the sea.

I think that it is a metaphor. Pink Floyd mentions chasing the sun in the song Time, and I believe that I've heard other references.
Following the sun is perhaps a more common metaphor than chasing it.
"And you run and you run to catch up with the sun, but it's sinking. Racing around, to come up behind you again. The sun is the same, in a relative way, but you're older. Shorter of breath, and one day closer to death."
Spoiler alerts but this exactly the plot twist at the end of the book "Around the World in 80 Days" by Jules Verne

https://en.wikipedia.org/wiki/Around_the_World_in_Eighty_Day...

The following day Fogg apologises to Aouda for bringing her with him since he now has to live in poverty and cannot support her. Aouda confesses that she loves him and asks him to marry her. As Passepartout notifies a minister, he learns that he is mistaken in the date – it is not 22 December, but instead 21 December. Because the party had travelled eastward, their days were shortened by four minutes for every degree of longitude they crossed; thus, although they had experienced the same amount of time abroad as people had experienced in London, they had seen 80 sunrises and sunsets while London had seen only 79. Passepartout informs Fogg of his mistake and Fogg hurries to the Club just in time to meet his deadline and win the wager. Having spent almost £19,000 of his travel money during the journey, he divides the remainder between Passepartout and Fix and marries Aouda.

It is not only sailors who get confused going around the world. The lightning map on the WeatherBug site and in the WeatherBug iOS/iPadOS app gets confused if you scroll all the way around.

When opened in the US the map is centered on the US. Scroll east to bring Europe and Africa into view and it shows lightning there. Keep scrolling east to see lighting in Asia and beyond.

When you come all the way around and the US comes into view there is no lightning. Keep going east and Europe and Africa and Asia have no lightning.

Go around east a few mores times. Then reverse direction. You've got to go around west the same number of times you went around east to get the lightning back.

For US users who want to check lighting in Europe or Africa this probably won't cause problems. They will most likely scroll east.

But for US users who want to check lighting in say Japan or Australia they will probably scroll west, and there will be no lightning. To see lighting in Japan or Australia US users have to scroll east past Europe.