# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Almanacs & UTC redefinition
From: Paul Hirose
Date: 2011 Dec 03, 19:05 -0800

```In October there was a "Decoupling Civil Timekeeping from Earth
Rotation" colloquium to discuss the UTC time scale and the proposal to
end leap seconds. Celestial navigation may be significantly affected if

Papers from the colloquium proceedings are online:
http://futureofutc.org/preprints/

George Kaplan's paper looks at how a UTC redefinition will affect
almanacs and their users. In theory, there's no effect. "Since almanac
data are generally computed and displayed as a function of either UT1 or
TT, the initial, naive, answer is that nothing changes; UTC is not used
in the computation of the data and UTC is not the independent argument
of any of the tabulations. It has been the user’s responsibility to
convert between UTC (or whatever his external time scale is) and the
time scale(s) used for the almanac data, and we can simply assert that
this responsibility remains unchanged. In fact, the basic conversion
formulas from UTC to UT1 or TT won’t change if leap seconds no longer
occur."

In practice, few navigators have used the formula. "It is a safe bet
that fewer than one astronomer in ten, and a similarly small fraction of
navigators, know the difference between UTC and UT1, or even that there
are two kinds of Universal Time. That’s because, up to this point, the
difference has been bounded at 0.9 s, and for the vast majority of
purposes, we can set UT1=UTC to sufficient accuracy...

"Obviously, this will not be the case if leap seconds end and UT1–UTC
becomes unbounded. At a minimum, we will have to re-label our time
argument to be explicitly “UT1” and make sure that the explanatory text
is very clear about the conversion from UTC, something that most users
have not had to worry about. That is, people who have never heard of the
IERS or the difference between UT1 and UTC will have to come up to speed
on these time conversions. In the U.S. Navy, celestial navigation is
generally performed by enlisted quartermasters, and we expect that some
new training will be required."

On the other hand, he notes that "John Bangert of USNO has suggested
that if leap seconds are dropped from UTC, we could switch our Earth
rotation dependent tabulations [in the Almanac] from UT1 to UTC... That
would undoubtedly be a great convenience for users. In the case of Earth
rotation dependent data, however, the convenience would come at the
price of some degradation in accuracy. It seems likely that the
would not be worse than that from assuming UT1=UTC now, which is
probably a common assumption. Furthermore, if necessary, the error could
be removed by application of corrections based on the measured value of
UT1–UTC for the date on which the data are needed."

The ITU will put the matter to a vote next month.

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I filter out messages with attachments or HTML.

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