[ntp:questions] Leap second functional question

Richard B. Gilbert rgilbert88 at comcast.net
Wed Feb 20 13:27:48 UTC 2008

David Woolley wrote:
> 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.
> 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 
> to UTC differences can be measured, means that they can need earth 
> rotation time more accurately than UTC.
> 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.
>> Software converts those number of seconds since epoch into a date. They
> 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.

You are asking for the impossible!  Leap seconds keep time in synch with 
the earth's rotation.  The rate of rotation is subject to small 
variations.  This is why leap seconds occur at irregular intervals and 
why it is possible to have a negative leap second, although I don't 
recall that we ever had one.

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