# NavList:

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

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Re: Transit time at Greenwich, England 4-5 seconds different from my meridian.
From: Trevor Kenchington
Date: 2003 Dec 22, 21:18 +0000

```Jim,

You wrote:

> 1. Why are these estimates of transit time at my meridian different by 4-5
> seconds from the answers I got by the other methods posted at that webpage?

As noted before, the value of the Equation of Time is changing over
time. Meridian passage at the Greenwich meridian is at 11h 55m 41s (give
or take a second) but by the time the Sun crosses the 60 degree meridian
(about four hours later) the Equation of Time will have moved about one
third the way from its 1200 GMT value towards its value at 0000 GMT on
15 September. That is, its value at the Sun's passage across your
meridian is 4 min 22 seconds (to the nearest second or so) and the time
of meridian passage has dropped back to 11h 55m 38s LMT.

That 3 second change should reconcile your various calculations, to  a
maximum of one second difference.

> 2. Why do the daily pages of the Almanac give time of Meridian Passage to
> the nearest minute, rather than the nearest second?  Because it assumes that
> the navigator only needs an estimate of transit time to an accuracy of +/- 1
> minute?

As I suggested and Doug confirmed, those values are only given as an aid
to planning your observations, not for use in precise calculations. If
that seems like overkill, imagine yourself in the position of a harried
Third Mate on a freighter somewhere who finally got his eye to his
sextant as the Sun was heading downwards from its maximum noon altitude.
The Old Man would not be pleased. And a displeased captain is not a
comfortable shipmate to have around. There can be a lot of value in
having the approximate clock time of local noon quickly and readily
available.

Trevor Kenchington

--
R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555

Science Serving the Fisheries

```
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