[ntp:questions] Leap second functional question

Unruh unruh-spam at physics.ubc.ca
Wed Feb 20 17:30:52 UTC 2008

David Woolley <david at ex.djwhome.demon.co.uk.invalid> writes:

>Unruh wrote:
>> David Woolley <david at ex.djwhome.demon.co.uk.invalid> writes:
>>> Unruh wrote:
>>>> there were actually  one more second there, but UTC does not care.
>>>> Astronomers do not use UTC.
>>> Astronomers use UT1 or higher.  These DO have variable length seconds, 
>>> which I think was the original cause of confusion; I think they mixed up 
>>> UT1 with UTC.
>> Well, no. Astronomers use TAI since long baseline interferometery relies on
>> accurate time synchronization. The deep space network uses TAI since the
>> speed of light must not change from year to year. 

>Astronomers need both earth rotation time and TAI, although I would 
>argue that long baseline interferometry only has a weak case for TAI 
>over UTC as the distinction only causes a problem for a fraction of a 
>second every few years.

Yes, of course -- because UTC and TAi time differences only differ at those
times. And at those times they use TAI. So they use TAI, since that is what
they use at the only times when the difference is important. 

>Optical astronomers need some form of earth rotation time if they are 
>working with right ascensions to better than about 1 part in 5,000. 
>Planetary astronomers may also need TAI, because orbital dynamics will 
>be closer to that than any other available standard.  The fact that UT1 

No orbital dynamics obeys Newton's ( or Einstein's) law of gravitation only
in TAI not in UTC ot UT1. While earth rotation was adequate for many many
centuries as a clock, it is not any longer. IF you are doing something
where you do not give a damn about how many seconds there are in a year, of
course, use UTC. If what you want is to know when Gamma draconis is
overhead in London, please use UT1, not UTC, since that IS determined by
the earth's rotation. If you want to know when the double Binary Plusar's B
plusar is next going to blink, you had better use TAI, not UT1 or UTC.

>to UTC differences can be measured, means that they can need earth 
>rotation time more accurately than UTC.

Yes, they can. And if I want to know when the next tube from Golder's Green
goes, none of those time standards is of any use whatsoever.

>Long baseline interferometry also needs both, although it only needs TAI 
>over the time it takes light to cross the array, so using UTC only 
>causes problem at the edges of leap seconds.
>> Yes, UTC is an attempt to base the time on the earth's rotation. But the
>> price is that the time differences are not accurate. The number of seconds
>> from time A to B under UTC is not proportional to the number of oscillation
>> of a cesium atom. It varies. 

>It's a compromise, in which the discrepancies are infrequent and only an 
>exact number of seconds.  It's a good compromise for commercial and 
>legal use, where one almost certainly really wants earth rotation time.

Agreed. for commercial and legal use it is fine. For accurate timing work
it is terrible. It is not good for earth rotation (UT1 is better) and it is
not good for accurate timing (TAI is better).


>> Software converts those number of seconds since epoch into a date. They
>> could easily do that with TAI as with UTC except that they would need a
>> table of leap seconds. But then they need a table of time zones anyway, so
>> what's another table.

>Please supply me with the leap second table for the next 100 years.  I 
>know that legislatures mess around with timezone tables, but they at 
>least tend to do so in units of at least half an hour and the general 
>public are used to such effects. (For commercial and legal purposes, 
>even UTC is often too abstract to be useful for time periods of more 
>than a few days.) Generally the POSIX v "True" time debate is one that 
>recurs but is never resolved.

Of course I cannot. Leap seconds are to take into account the random
variations in the earth's rotation to one part in about 10^8. 

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