# NavList:

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

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Re: Nautical Almac Question
From: Frank Reed CT
Date: 2006 May 31, 14:25 -0500

"Can anyone tell me why some of the "Eqn. of  Time" entries in the Nautical
Almanac are shaded and some are not? (A good  example is the page for June
12, 13 and 14 where there's an example of  both).  I'm guessing that's
related to the Equation of Time reversing  sign (maybe the shaded entries are
negative?)"

Yes, it's a sign  reversal. Have you ever graphed these numbers directly from
the almanac pages.  Maybe plot the values for the 1st and 15th of each
month...

I think  you'll find that the Explanation section of the commercial edition
is 100%  identical to that in the government-issued almanacs. Go to section 18
of the  Explanation. It's entitled "Sun and Moon" under "Auxiliary and
Planning Data"  (maybe they should include a little link that you can click on in the
main pages  that takes you to correct section of the explanation <g>).
Regarding the  equation of time, the Explanation says, "The sign is positive for
unshaded  values and negative for shaded values". Equivalently, the Sun is
"early" when  the values are unshaded and "late" when shaded. You can confirm this
by looking  at the Sun's GHA at 12h GMT on any date. Let's take February 10. It
varies a bit  from year to year depending on the leap year cycle, but the EqT
for that date is  about 14m 14s. It is shaded in the Nautical Almanac, so the
Sun will be late to  the meridian. When you look at the Sun's GHA at noon, on
the same date, you'll  find that it's 356d 26.5'. So it hasn't yet reached
the Greenwich meridian  --it's "late". How long will it take to get there? Well,
it's going to have to  cover a little more than 3.5 degrees of longitude to
get to GHA=0. At 15 degrees  per hour, that means it will need a little more
than 14 minutes. And that's just  what the equation of time tells us.

-FER
42.0N 87.7W, or 41.4N  72.1W.
www.HistoricalAtlas.com/lunars

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