[ntp:questions] Purpose of a leap second file?

David E. Ross nobody at nowhere.invalid
Sun Mar 4 07:13:09 UTC 2012

On 3/3/12 1:13 PM, John Hasler wrote:
> David E. Ross writes:
>> Why require a UTC adjustment from TAI?  If the data represent a
>> satellite orbiting the earth and the sub-satellite point on the
>> earth's surface is needed, the TAI is converted to UTC, which is then
>> converted through UT2 to UT1 and possibly to UT0 before obtaining the
>> sidereal time.  Sidereal time then gives the current angle of the
>> earth's rotation for computing longitude.
> Thus UTC is just a kind of timezone-like localization and leap-seconds
> belong in tzdata.

No, it is not a localization.

Not only is the duration of a UTC second equal to the duration of a TAI
second, but a UTC clock also ticks its seconds exactly when a TAI clock
ticks its seconds.  On the other hand, UTC noon is never more than 0.9
seconds different from the mean noon at Greenwich.  Thus, UTC keeps
aligned with the gradually slowing of the earth's rotation while TAI
ignores the earth's rotation.

The UTC alignment requires an occasional extra second (always added at
midnight).  Thus, you might see a UTC clock reporting
That is, the last minute of a day sometimes (but rarely) has 61 seconds.
 For systems and computations that require uniform time-intervals
(delta-t), this is not acceptable.  Those systems and computations rely
on TAI, which is indeed uniform.  A leap-second file allows conversions
between UTC and TAI.


David E. Ross

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