lkml.org 
[lkml]   [1998]   [Jun]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: Y2K
Date
linker@nightshade.ml.org wrote:
>I thought the second was the basic metric unit of time and was defined by
>the time it takes light in a vaccume to travel some distance or sumpting..

Originally, the second was defined as 1/86400 of a solar day. When
timepieces became sufficiently accurate for it to matter, the solar day
was measured from noon to noon. This definition of the second (and day)
gives the time standard known as `local time'; this is the time based
purely on the position of the Sun in the sky.

Due to the elliptical orbit of the Earth around the Sun, the length of
the solar day (and therefore the local time second) varies throughout
the year. Consequently, a clock kept strictly to local time would
have to vary its rate over the course of the year in order to stay
accurate. To solve this problem, `mean time' was invented; this is
defined equivalently to local time, but for a hypothetical mean Sun,
which moves through the sky at a completely constant rate, matching
the average rate of the real Sun over the course of a year. Mean time
gives a definition of the second (and day) as an awkward fraction of
the solar year. Note that the time standard GMT, Greenwich Mean Time,
is defined to be mean time at the Royal Greenwich Observatory.

When atomic clocks became available, a third definition of the second
was invented, based on the oscillations of the caesium-133 atom. This
defined the second more precisely than was possible with astronomical
observations, so this is now the definition used for scientific purposes.
A new time standard, based on the SI second and measured with atomic
clocks, was invented, called TAI (International Atomic Time). This was
synchronised with GMT at the beginning of 1958, but the differences in
the two standards means that TAI is now more than 30 seconds ahead of GMT.

UTC was introduced in 1972 as a compromise between GMT and TAI.
It uses the SI second, and is kept precisely synchronised with TAI,
but at all times is some integral number of seconds offset from TAI, to
keep it within 0.9 seconds of GMT. This offset changing is what causes
`leap seconds'.

So the basic problem that POSIX ducked is "should computers keep track of
mean solar time or count in SI seconds?" The committee's answer seems
to have been "both". The lack of leap seconds in the time definition
means that, over long periods of time, they intend to track solar time,
implying a count in solar seconds. But as they refuse to acknowledge
the existence of GMT, they mandate use of SI seconds over short periods
of time, with the effect that a POSIX clock *must*, strictly speaking,
be discontinuous at leap seconds.

To get back to the original question, the part about the speed of light
is actually the other way round -- the metre is defined as the distance
travelled by light in a vacuum in 1/299792458 seconds.

-zefram

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.rutgers.edu

\
 
 \ /
  Last update: 2005-03-22 13:43    [W:0.097 / U:0.152 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site