a Robinson Crusoe dilemma?

September 18, 2005 3:49 AM

curiosity filter: how to tell the weekday on a desert island?

here goes: let's say we somehow have washed up on a desert island and we have no idea what day it is. with no outside communication, is there any way to find this out?

posted by mirileh to science & nature (16 comments total)

Can you watch any plane traffic flying high overhead? Maybe if you keep a close watch and note how many planes fly over each day, you might pick up a pattern...five days might have slightly more planes than the other two days (although airline schedules are pretty constant these days, I'd imagine there would still some differences between weekend and weekday flights). Maybe you could work it out from there.
posted by Jimbob at 4:00 AM on September 18, 2005

No, unless you can figure out the date. Weeks have no astronomical basis.
posted by cillit bang at 4:05 AM on September 18, 2005

Check the position of the moon's cycle (you'll need to do this over at least two nights to check if it's waxing or waning) in comparsion to a) a pocket diary which includes the moon's cycles [if there are any business people among you I think the odds of someone having these are quite high]; or b) any of your observant friends who can remember a recent full moon (or, indeed, any moon). I don't know how common it is for women to know what part of the lunar cycle their menstruation tends to coincide with, but if you can summon the courage this might be one line of enquiry.

It might take you a few days, or until a full moon, to be sure of the present position in the lunar cycle; but you can then at least count the days on from your reference point and arrive at what day of the week it is.

Not ideal, though, as it does rely on certain factors that may not be in your favour.
posted by nthdegx at 4:11 AM on September 18, 2005

Generally speaking, without outside assistance the answer is no. The week (and thereby the fortnight) is the only commonly-used time period not based on the movement of the Earth, moon or stars. See here or here for more details.
posted by alby at 4:58 AM on September 18, 2005

alby, cillit bang, that's what i thought. and it makes it amazing that the whole world is synchronized on the same weekdays.
posted by mirileh at 5:20 AM on September 18, 2005

I don't know how common it is for women to know what part of the lunar cycle their menstruation tends to coincide with, but if you can summon the courage this might be one line of enquiry.

If you're (un)lucky enough to have a nutty new-ager on the island then this might work, except she'll be too busy howling at the moon to help you out. I know of no other type of woman who would believe in such nonsense.
posted by speranza at 5:56 AM on September 18, 2005

How about waiting for an equinox (by timing the amount of sunlight per day or when sunrise or sunset occurs, or if you know which way East is, you can figure out when it is), then if you know the year, you can use the Doomsday algorithm to figure out the day of the week.
posted by i love cheese at 6:06 AM on September 18, 2005

I know of no other type of woman who would believe in such nonsense.

Etymology [of menstruation] and the lunar month. Frequency of menstruation. Lunar cycle.

I'm not sure about "nonsense" exactly; but it does seem I was significantly overestimating the chances of correlation.
posted by nthdegx at 6:08 AM on September 18, 2005

No idea if this has any vallidity:

I read somewhere once that 80% of American women start their cycle on a Tuesday, thanks to birth control pills. I guess you could ask, if you had one on the island.
posted by unrepentanthippie at 6:33 AM on September 18, 2005

Well, if you're by yourself then it may be hard to do. If you're a group of people, odds are someone is going to have a watch (or pda, agenda, phone, etc) on them.
posted by furtive at 6:58 AM on September 18, 2005

perhaps too simplistic of an answer, but how about knowing what day it was before you washed up and just keeping track from there?
posted by whatitis at 7:17 AM on September 18, 2005

and it makes it amazing that the whole world is synchronized on the same weekdays.

Perhaps it is not a good idea, since the whole world isn't.
posted by squink at 8:13 AM on September 18, 2005

squink, Islam also has a 7 day week with Friday as a holiday (which is mentioned in your link btw).
posted by mirileh at 8:32 AM on September 18, 2005

Yeah, I misunderstood the point. Still, there is this:

All historical peoples either invented the week or copied it from others, because a time unit longer than a day but smaller than a month is essential to human affairs.

Some African groups had weeks of four days, possibly in honor of the four seasons. Early Romans had weeks of eight days—around that same time Greeks and Egyptians divided their 30-day months into three 10-day weeks. You can find out how our current week-length came about at Origin of the 7-day week.

Now, at least among major nations of the world, the seven-day week is universal. Attempts to change week-lengths in modern Western societies have been made at least twice but failed.

In 1792 the French Revolutionary Convention enacted a decimal calendar that called for 10-day weeks. It was abandoned after Napoleon came to power.

In the last century the U.S.S.R. tried twice to alter the week, decreeing its length as five days in 1929 and six days in 1932. For whatever reason, by 1940 the U.S.S.R. had restored the seven-day week.

posted by squink at 10:03 AM on September 18, 2005

I don't know how common it is for women to know what part of the lunar cycle their menstruation tends to coincide with, but if you can summon the courage this might be one line of enquiry.

Well for some women who keep track of their cycles they wouldn't have to know what cycle of the moon it corresponded with, you would just have to know that they got their period today and their period was due on the 17th and the 17th is a wednesday. Of course there aren't many women whose cycles wouldn't be disrupted by washing up on a desert island.

There were attempts to create a more "rational" calander (their words) in the 20s and 30s. It was 13 months per year, each month being 4 weeks, and some leap days at the end of the year that belonged to no month and no week.

The movement was strongly opposed by religious groups (mainly Jews and Seventh Day Adventists) who believed that the continuity of weeks had existed since the time of creation and was divinely mandated. In other words, they would have to treat every 7th day as the sabbath even though the rest of the world would no longer recognize this rhythm. Read: This would have cost them money in lost work days and truancy fines if they kept their kids out of school.
posted by duck at 1:46 PM on September 18, 2005

Although unless they were washed up on the island WITH their birth control pills, stopping the pill usually disturbs the cycle as well. Most women are not naturally 28 days to the letter (number?), or for that matter, even consistent from month to month.
posted by artifarce at 10:03 AM on September 20, 2005