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Re: Equation of time question
From: Paul Lecheler
Date: 1995 Aug 29, 08:35 -0500
From: Paul Lecheler
Date: 1995 Aug 29, 08:35 -0500
Peter Smith said...
>It's still useful in planning the day's work. Given EoT and DR longitude,
>one can quickly estimate the time of Local Apparent Noon for taking a
>meridian altitude of the Sun.
>
I think I understand this now. I looked at the NA for 28 Aug 1995 and found
the following:
Meridian Passage: 12:01Z - 12h EoT: 1:20
If I take my Long (96 38.1) and convert to time I get: +06:02:32
Therefore, corrected meridian passage time is:
12:01:00Z Meridian Passage
06:02:32 Arc to time conversion
1:20 12h EoT
----------
18:27:54Z Corrected Meridian Passage time
This doesn't quite match with NAVIG94 or the Celesticomp computer, however.
Any ideas why? I assume becuse the NA does not show the seconds for
meridian passage.
Am I on the right track?
Thanks
Paul Lecheler
plecheler@tad.eds.com
Electronic Data Systems Corporation
Technology Management
(214) 604-0546 work
(214) 604-0280 fax
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To: navigation@ronin.com
From: plecheler@tad.eds.com (Paul Lecheler)
Subject: Re: Equation of time question
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Status: RO
Philip H. Kohl said:
>
>I get (using the table for conversion of arc to time in the Nautical Almanac)
>
> 96d 6h 24m
> 38.1m 2m 32s
> ----------
> 6h 26m 32s
>
Oops, I forgot to add the 6d to my calculations. (Insert red face here.)
>I get
>
> 12h 00m 00sZ
> 6h 26m 32s Arc to time conversion
> 1m 15s 18h EoT (interpolating between the 12h value
> ____________ on the 28th and the 0h value on the 29th)
> 18h 27m 47s GMT Time of Meridian Passage
>
Why use 12h00m00s instead of the meridian passage time here?
>
>You are close. The NA does show seconds for meridian passage.
At the bottom of the page in the Commercial edition it just shows 12:01 for
the meridian passage (as I remember, but then again my memory is what I
forget with). Is there another place to look?
Paul Lecheler
plecheler@tad.eds.com
Electronic Data Systems Corporation
Technology Management
(214) 604-0546 work
(214) 604-0280 fax
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Date: Sun, 09 Mar 1997 14:44:06 -0800
To: navigation@ronin.com
From: Gordon Talge
Subject: Equation of Time
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Status: RO
Thought some of you might be interested in this.
I am reading a book called "Avigation" by Bradley Jones, copyright 1931
on air navigation.=20
At the time it seems that there was not a special almanac for the air and
the nautical almanac was used. For the Sun it listed the Declination and
the Equation of time every 2 hours, with an hourly difference for the day.
The LHA was not used. What was used was called the HA, what we would now
call the meridian angle. ( The same angle t in H.O. 214 )
Example of a calculation:
Requried the HA for 21h 00m 00s G.C.T. ( G.C.T. =3D Greenwich Civil Time=
)
March 9, 1997 We would call this GMT, but
before 1925 GMT at 0h was what
we would call 12 Noon )
Probably to avoid confusion, the
term GMT was not used since 1925
was just 5 years before.
Long 118degs West.
or 7h 52m 00s ( Arc to time conversion )
>From the Almanac ( 1930 style ) Eq.T =3D - 10m 27s =20
Dec =3D - 4 degs 13.3'
=20
For March 9, 1997 21h 00m 00s
G.C.T. 9d 21h 00m 00s
Eq.T -10m 27s
---------------------------
G.A.T. 9d 20h 49m 33s ( G.A.T. is Greenwich Apparent Time )
Long 7 52 00 =20
--------------------------
L.A.T. 9d 12h 57m 33s (L.A.T. is Local Apparent Time)
Since Local Apparent Noon is 12h 00m 33s, the sextant shot was taken=20
57m 33s AFTER the Sun had crossed 118 degs West meridian, hence the=20
H.A. is 14 degs 23.3' ( t =3D 14 degs 23.3' West )
--------------------------------------------------------------------
1997 style calculations:
at 21h 00m 00s March 9, 1997=20
GHA Sun 132 degs 23.3' Dec. S 4 degs 13.3'
GHA 132 degs 23.3'
Long 118 00.0
------------------------ ( Subtract since West Long )
LHA 14 degs 23.3'
Since LHA < 180, LHA =3D HA or t, t is then labeled West.
If LHA were > 180, HA =3D 360 - LHA, t is then labeled East.
=20
In 1930 the GHA for the Sun is NOT listed, only the Eq. of Time. I did
notice that in the 1937 almanac, the GHA, Dec and Eq. of Time were listed
every 2 hours.
I am not sure of the year, but sometime in the 1930s the newer method of
using GHA that we use today came in. Eq. of Time continued to be listed
for those who were familar with the method of using it and then it seems
it was sometime phased out.
=20
,,,=20
(. .)=20
+-----------------------ooO-(_)-Ooo----------------------+
| Gordon Talge WB6YKK e-mail: gtalge@pe.net |
| Department of Mathematics QTH: Loma Linda, CA |=20
| Mt. San Jacinto College Lat. N 34=B0 03.1' |=20
| San Jacinto, CA Long. W 117=B0 15.2' |
+--------------------------------------------------------+
