[LEAPSECS] civil-solar correlation with TI

M. Warner Losh imp at bsdimp.com
Sun Dec 28 11:42:06 EST 2008

In message: <C43E1C7D-F4ED-43BC-8B3E-3F7E62950B9D at noao.edu>
Rob Seaman <seaman at noao.edu> writes:

: M. Warner Losh wrote:


: > How is the Olson database fundamentally different than the

: > historical data that a future historian would have based on the

: > measurements of the delta between what we call today TAI and UT1

: > times? It is just more data for them to swizzle into their

: > calculations?


: Because a mean solar clock is automatically a stable subdivision of

: the calendar - stable over long periods of time as well as

: geographically. Noon on two different days is separated by an

: integral number of days no matter what period of time separates the

: two dates and how the length of day may have varied in the interim.


: There is no swizzling needed if civil timekeeping remains tied to the

: Sun.

Actually, they do need to do this now for leap seconds. Plus they
need it for sub-second accuracy.

Also, mean solar time doesn't mean that all days are 1.0000000000000d
long. In actuality, there's day to day variations in the length of
the day. Since civil time is tied to the atomic scales, this means
that noon(local time) is almost never going to be an integral number
of days apart due to this variation. It will only be an average.

So this argument is flawed from that perspective.

So if we keep UTC as is, all we're doing is keeping this variation
below a second. Do historians really care if this variation is below
a second, below a minute or below an hour? No evidence has been
presented as to what level historians care about, the number that
care, and why an extra little swizzle at the end would be a burdon.

Finally, civil time today is atomic based. It is based on the second
as defined by the oscillations of the atoms, not as defined by the
rotation of the earth. UTC today includes leap seconds to
periodically resynchronize the time scale to the earth's rotation, but
it is not based on the earth's rotation. We all know a time will come
that this won't be possible.


More information about the LEAPSECS mailing list