# [ntp:questions] Leap second functional question

Unruh unruh-spam at physics.ubc.ca
Wed Feb 20 00:33:26 UTC 2008

```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.

>UT1 is more closely based on earth rotation time.  Leap seconds are
>introduced when the discrepancy between UTC and UT1 is getting too high
>(somewhere between half and one second).  Various time signals include
>this difference to a tenth of a second, or maybe better.

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.

>The UT1 second count is always close to the UTC second count, and so UTC
>  second counts are more useful for astronomers than TAI ones, except if
>you are using an epoch close to a leap second and measuring times close
>on the other side.

>I doubt that a year actually exists in TAI.  When people talk about
>using TAI in computers, they are talking about the internal seconds
>count, not any external representation of it.

Yes. But then your computer's itnernal time IS the number of seconds since
epoch. (1970 for Unix) But it is not really the number of seconds. It is
the number of UTC seconds. which differs from the actual number of seconds.

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.

>>
>>
>>
>>> N

```