```Am 24.07.2012 19:44, schrieb Eric Hodel:
> On Jul 23, 2012, at 11:52 PM, sto.mar / web.de wrote:
>> UTC is based on the International *Atomic* Time (ITA) and therefore on the SI second definition. (A day is not exactly 86400 SI seconds long, so to avoid that mean solar time and UTC differ too much,
>> leap seconds were introduced.)
>>
>> Please change "is based on solar time" to "is based on atomic time".
>
> The UTC second is the same as the SI second and tracks TAI, but is not allowed to drift from UT1 by more than 0.9 seconds.  This makes both statements true, but in different senses. (TAI for second duration and count, UT1 for daily human-scheduled activities)
>
> Since we're talking about leap seconds here, the solar time reference seems most relevant. How about I change "is based on" to "tracks"? The Wikipedia reference should allow users to learn all the details.

From a physicist's point of view, the statement that

"This allows the system to inject leap seconds every now and then to
correct for the fact that UTC tracks [is based on] solar time so
days are not exactly 86400 seconds or 24 hours long"

would simply be plain wrong. If UTC really would track solar time
the days would be *exactly* 86400 seconds long and there would be
no need for leap seconds. (This is the more or less the principle behind
the "Universal Time" UT1.)

UTC and TAI run perfectly in parallel, with the same, well defined
SI second as basis. They only differ by an exactly known amount of
seconds (the accumulated leap seconds).

UT1 does not even have an exactly defined length of a second:
it varies with the irregular rotation speed of the earth.
(Furthermore, the *mean* solar second is longer than the SI second.)
So, there is no exactly predictable relation between UTC and UT1.
The difference between the two times is constantly fluctuating, and
can only be determined in retrospect from astronomical observations.

So, the principles behind UTC and solar time are completely different.
Yes, they *are* related in the sense that the difference is kept below
0.9 seconds, but this influence of solar time on UTC has to be seen
as a correction, not as the basis of it.

Proposal: either (in order of preference)

1) correct to "...is based on *atomic* time..."

2) correct to "is based on *the SI definition of the second and*
days are not exactly *86400 SI seconds* or 24 hours long"

3) correct to "...is *not based* on solar time..."

4) leave out the explanation and link completely

Best regards,
Marcus

--
<https://github.com/stomar/>

```