[ntp:questions] Leap second functional question

Unruh unruh-spam at physics.ubc.ca
Fri Feb 22 10:28:28 UTC 2008

Greg Hennessy <greg.hennessy at localhost.localdomain> writes:

>On 2008-02-22, Unruh <unruh-spam at physics.ubc.ca> wrote:
>>>High precision orbital dynamics, such as the JPL's DE405, are done in 
>>>Barycentric Dynamic Time, which runs at a different rate than TAI, but
>>>is "TAI like" in that it has no leap seconds.
>> It does? The second was defined to make the speed of light a consant. Are
>> you saying they use a system in which the speed of light changes from time
>> to time? That sounds horrible. 

>I am saying that high precision orbital dynamics are done from the
>reference frame of the solar system, which we call the barycentric
>frame. The barycentric time rate is different from that of TAI by a
>factor of about 1e-15 if I remember correctly. 

Ah. OK, There are General relativistic corrections, agreed. 

>> While it may be slowing on average, it is noise and it could well speed up
>> as well.

>Noise implies that the changes are both positive and negative, in
They are
>which case a leap second won't be needed. If it is systematic,
They would still be needed. Just because your computer's drift rate is both
positive and negative does not mean that compensation is not needed. 
They do not necessarily average out on the time scale of years. 

There is a net drift to longer days. but superposed on that is a noise
which even over the time scale ofyears makes a difference. An earthquake in
Java rearranges the moment of inertia of the earth and changes the rotation
rate of the earth, and it can be positive or negative.

>i.e. the changes are more in one direction than another, a leap second
>will be needed.

It is needed even if it equal in both directions over a long time.

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