NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
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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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
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To unsubscribe, send email to NavList-unsubscribe@fer3.com
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