[ntp:questions] Leap second functional question

David Woolley david at ex.djwhome.demon.co.uk.invalid
Wed Feb 20 08:07:33 UTC 2008

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
> 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.

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