Re: Calendars (was: Re: leap second)
From: [EMAIL PROTECTED] (Kai Henningsen) Not everyone switched in 1752. This is Pope Gregory's calendar reform, isn't it? I think it goes back a century or more before 1752. Actually, it probably was a bad idea to use leap for both. Leap days are fixed by calendar design. Leap seconds are inserted or deleted (both are possible) after comparing the atomic clocks to astronomical observations, with no predictability at all. Two very different animals. Speaking of predictability, isn't 2000 a leap year? The rule is different for the turn of the century. System time should be counted as the number of seconds _elapsed_ since New Year's day 1970 (what Unix uses) or some other fixed point. These days it's the number of seconds elapsed minus the leap seconds, which is sort of silly. Bruce -- Bruce Perens K6BP [EMAIL PROTECTED] 510-215-3502 Finger [EMAIL PROTECTED] for PGP public key. PGP fingerprint = 88 6A 15 D0 65 D4 A3 A6 1F 89 6A 76 95 24 87 B3 -- TO UNSUBSCRIBE FROM THIS MAILING LIST: e-mail the word unsubscribe to [EMAIL PROTECTED] . Trouble? e-mail to [EMAIL PROTECTED] .
Re: Calendars (was: Re: leap second)
In [EMAIL PROTECTED] [EMAIL PROTECTED] (joost witteveen) writes: Now, we know the length of a year/day better, and only 1 in for of those turn-of-century years are leap years. Maybe that will change again. And about the seconds: we (currently, prossibly always) simply cannot calulate the length of a day accurately enough to know well in advance when to insert them. But I'd say the two animals are at least related, if not mother and daughter. It is my understanding that: Leap days are used to keep the calendar in sync with the season. That is, you don't want winter to be in August (in the northern hemisphere.) Leap seconds are used to keep the time of day in sync with the sun rise. That is, you don't want the sun to be rising at midnight (out side the article circles.) The length of the year is almost constant over time periods of thousands of years. It does vary due to gravitational interactions with other planets, but it only makes a significant difference (10%?) when dealing with time periods of around 100,000 years. These gravitational interactions are predictable, so if you really wanted to, you can calculate the exact length of the year 1million years ago. (It is my understanding that there are people who do this.) The length of the day is not quite as constant. It depends on how quickly the earth rotates which depends on things like how much snow has fallen on mountain peaks and how much water is in man made reservoirs. I kid you not, these things are significant enough to change things on the order of a second or two per year. Neither the weather nor people's water usage/reservoir building is very predictable. This makes predicting leap seconds futile. -wayne -- Wayne Schlitt can not assert the truth of all statements in this article and still be consistent. -- TO UNSUBSCRIBE FROM THIS MAILING LIST: e-mail the word unsubscribe to [EMAIL PROTECTED] . Trouble? e-mail to [EMAIL PROTECTED] .
Re: Calendars (was: Re: leap second)
On Jun 22, Bruce Perens wrote Speaking of predictability, isn't 2000 a leap year? The rule is different for the turn of the century. 2000/02/29 exists. (the rule is : every for years, but not every hundred years, but every 400 years). AFAIK. regards, andreas -- TO UNSUBSCRIBE FROM THIS MAILING LIST: e-mail the word unsubscribe to [EMAIL PROTECTED] . Trouble? e-mail to [EMAIL PROTECTED] .
Calendars (was: Re: leap second)
[EMAIL PROTECTED] (Bruce Perens) wrote on 21.06.97 in [EMAIL PROTECTED]: Someone wrote: This is completely unacceptable. OS time must be predictable. Run cal 9 1752 and tell me that. Consider it done. And now? (Besides, isn't that a bug in cal? Not everyone switched in 1752. In fact, ISTR that most people switched at other dates - some as late as 1918, I think.) A more serious problem is that the current implementation doesn't allow for non-Christian date systems, of which there are several in active use. I'd expect that to be a problem for people in both parts of Jerusalem, for example. Does anybody know enough about those other systems to tell if the general design would at least work - that is, dates are year/month/day tuples? I guess the hour/minute/second convention is pretty much established worldwide by now (does anyone know for sure?). Can someone explain to me exactly what POSIX time is? Posix time includes leap-year-days, but does not include the finer resolution of leap-seconds. 21 leap-seconds (number 22 is coming up) have been added since New Years Day 1970 to keep clock time in synch with astronomical time. Actually, it probably was a bad idea to use leap for both. Leap days are fixed by calendar design. Leap seconds are inserted or deleted (both are possible) after comparing the atomic clocks to astronomical observations, with no predictability at all. Two very different animals. MfG Kai -- TO UNSUBSCRIBE FROM THIS MAILING LIST: e-mail the word unsubscribe to [EMAIL PROTECTED] . Trouble? e-mail to [EMAIL PROTECTED] .
Re: Calendars (was: Re: leap second)
Run cal 9 1752 and tell me that.
[..]
A more serious problem is that the current implementation doesn't allow
for non-Christian date systems, of which there are several in active use.
I'd expect that to be a problem for people in both parts of Jerusalem, for
example.
Does anybody know enough about those other systems to tell if the general
design would at least work - that is, dates are year/month/day tuples?
Well, about the Muslim calander: year/month/day works for representing
dates, the only problem is that officially you can only tell the
dates in the past, not in the future: the beginning of the next month
is signaled by the moon, and although the position of the moon can be
preditect quite acurately nowadays, it that couldn't be done in
Mohammeds time. So, the next month only starts when _people_see_
the new moon -- and that's impossible to predict reliably.
(This is a problem with ramadan (the nineth month): they never
know exactly when it starts/ends).
Posix time includes leap-year-days, but does not include the finer
resolution of leap-seconds. 21 leap-seconds (number 22 is coming up)
have been added since New Years Day 1970 to keep clock time in synch
with astronomical time.
Actually, it probably was a bad idea to use leap for both. Leap days are
fixed by calendar design. Leap seconds are inserted or deleted (both are
possible) after comparing the atomic clocks to astronomical observations,
with no predictability at all. Two very different animals.
well, depends on how you see it. The before 1752, century turns were
still all leap years. Now, we know the length of a year/day better, and
only 1 in for of those turn-of-century years are leap years. Maybe that
will change again. And about the seconds: we (currently, prossibly always)
simply cannot calulate the length of a day accurately enough to know
well in advance when to insert them. But I'd say the two animals are
at least related, if not mother and daughter.
--
joost witteveen, [EMAIL PROTECTED]
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$/=unpack('H*',$_);$_=`echo 16dio\U$kSK$/SM$n\EsN0p[lN*1
lK[d2%Sa2/d0$^Ixp|dc`;s/\W//g;$_=pack('H*',/((..)*)$/)
#what's this? see http://www.dcs.ex.ac.uk/~aba/rsa/
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