[LEAPSECS] Consensus building?

Warner Losh imp at bsdimp.com
Wed Feb 2 18:10:51 EST 2011

On 02/02/2011 11:44, Stephen Colebourne wrote:

> On 2 February 2011 18:13, Warner Losh<imp at bsdimp.com> wrote:

>>> - 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.

> You're reading more into the statement than is intended by trying to

> interpret them as a time-scale or clock. I'm defining a unit of

> SI-based-minute that is a multiple of 60 of the unit SI-second. No

> more no less.

But the SI folks don't define minute, hour, day, etc. You are applying
those terms in a way that isn't quite consistent with standards. That's
why I read it one way and you another: there's no standard here. SI
defines what a second is, but does not define how to label them. TAI
and UTC do that.

UT has a different definition of the second based on 1/86400th of an
ephemeris day, which likely is a better term to use in some of my other
objections. However, I'm not an astronomer, so that might be something
that is closely related to the day we are looking for in UT, but still
not quite right.

>>> 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.

> So why not tweak these, or try to build consensus around a different

> set of statements?

I'd suggest pulling in the definition of the different time scales to
tweak them. I'm not an astronomer, or I'd suggest the proper definition
or term here. I'm happy to agree with tweaks that make them more
accurate and less ambiguous.

Warner

>>> 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

>> 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.

> We should use UTC-1972 as a terminology if has broader agreement.

> Stephen

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