On 02/02/2011 09:47, Stephen Colebourne wrote:
Statements so far - disgree or add please (in particular something on
UT1/UT/etc as I will only get it wrong...):
General:
- the terms seconds, minutes, hours and days are overloaded, thus
pedantic and explicit terms are used here
SI
- the SI-second is a standardised unit of measurement
- the SI-second is currently defined as a fixed number of transitions
of a caesium atom
- the current definition of the SI-second was ratified in 1967
Agreed. I'd also add
- the SI-second forms the basis for many other fundamental units of measure.
- an SI-based-minute is formed from exactly 60 SI-seconds
- an SI-based-hour is formed from exactly 60 SI-based-minutes and thus
exactly 3600 SI-seconds
- an SI-based-day is formed from exactly 24 SI-based-hours and thus
exactly 86400 SI-seconds
These are true only most of the time. On leap second day, they are all
false and off by 1, typically one low. It all depends on the timescale
that you use, and thus there can't be a universal definition here.
Solar
- a solar-day is a measured period of time
- the length of a solar day in in SI-seconds varies over time
- the length of a solar day in in SI-seconds is on average increasing with time
- a solar day is not a fixed number of SI-seconds
- a solar-hour is the period of 1/24th of a single measured solar-day
- a solar-minute is the period of 1/60th of a solar-hour and thus
1/1440th of a single measured solar-day
- a solar-second is the period of 1/60th of a solar-minute and thus
1/86400th of a single measured solar-day
Except that the last bits are wrong here too. It isn't the measured
solar day, except for UT, but the average of the measured day smoothed
in different ways for UT1 and UT2.
Also, implicit in this definition is the astronomical term solar-day, so
your definition is circular. A solar day is the amount of time it takes
the earth to rotate until the sun is in the same apparent position in
the sky, as opposed to a sidereal day, which is 360 degrees of rotation.
Humanity
- a humanity-day is a non-scientific, commonly used term understood by
6bn humans
- a humanity-day is interpreted in line with the rising and setting of the Sun
The tolerance for a human day is also governed by what the government
tells them, and is a very squishy unit of measure. people routinely
tolerate 23 hour and 25 hour days. These terms should not be confused
with 86400 "seconds", even though they are often about this length.
TAI-2011
- the TAI-2011 time-scale is known as TAI in the year 2011
- the TAI-2011 time-scale is defined as a uniformly increasing count
of SI-seconds from a fixed epoch
UT-2011
???
UT1-2011
???
UTC-2011
- the UTC-2011 time-scale is known as UTC in the year 2011
- the UTC-2011 time-scale is a continuous count of SI-seconds
- the UTC-2011 time-scale defines UTC-2011-days
- a UTC-2011-day is either 86400 SI-seconds or 86401 SI-seconds long
- the additional SI-second in a UTC-2011-day is a leap-second
- the presence or absence of a leap-second is determined up to 6
months in advance
yes. This time Bulletin came out this morning, giving only 5 months of
notice of no leap second.
I'm also not sure that this notation is the best. We're really using
UTC-1972 right now, if we follow the convention of using the year of the
last change. We'd also be on TAI-2008 based on the last time the
averaging algorithm of TAI was changed.
Lets see, how we go on these...
Stephen
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