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>
>
>Geoffrey Kolbe wrote:
> > Well, my Long Term Almanac is nominally good up to the year 2050 and will
> > actually work OK up until 2060. So this is a very real question for me.
> >
> > I am assuming that if leap seconds are abolished, anyone using my tables -
> > which assume GMT, or UT1 as it is called these days - will know the
> > difference between broadcast time and GMT and use this as a correction to
> > broadcast time.
> >

Lu Abel wrote:

>
>On the contrary, your long term almanac is based on the measurement of
>"what time is it" at some point in time (likely when it was published).
>So it does NOT correct for leap-seconds, since there is no way to
>predict when they will be inserted into UTC.   So it is effectively
>already practicing leap-second-less navigation.
>
>I don't know how old your long term almanac is, but if it's old enough
>it would be interesting to compare its data with current Nautical
>Almanac data; I'll bet there might be some small discrepancies in some
>GHA predictions.
>
>Lu


Geoffrey Kolbe responds:

Hello Lu.

Actually, the theory used to generate my tables includes an estimation for
delta T, the difference between ephemeris time and Universal Time. How
delta T will vary in the future is not that well understood, of course,
which means that by 2050 my tables will probably be out by a second or so.
But never-the-less, a knowledge of DUT1, which is the difference between
UT1 (which used to be called GMT and is based on the mean length of the
solar day, which is growing longer) and UTC (broadcast time, derived from
an atomic clock), is necessary to adjust broadcast time for use with my
almanac, which is based on UT1.

At the moment, the use of leap-seconds currently keeps DUT1 to less than
0.9 seconds. So if UTC is treated as being the same as UT1, which is what
most of us do, then a maximum error of about quarter of a minute in
longitude for a position using traditional celestial navigation methods, is
quite tolerable.  But if leap-seconds are abolished, it would only be a few
years before broadcast time would have to be routinely adjusted using DUT1
for use with my tables.

Current and historical values of DUT1 can be obtained from
http://maia.usno.navy.mil/search/search.html

At the moment, the value of DUT1, which is UT1 - UTC, is about -0.38 seconds

Geoffrey Kolbe


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