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And my best implementation of Meeus has it at 20 Mar 2004 at 6:44:23 UTC,
for both zero degrees of declination and zero hours of right ascension, but
apparently this time is off by 4-5 minutes.

The Astronomical Almanac should be more accurate than the Nautical Almanac,
as the Nautical Almanac does adjust GHA somewhat as you note.

Every year I am frustrated by the lack of agreement on when Spring starts.

Dan

-----Original Message-----
From: Navigation Mailing List
[mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of Fred Hebard
Sent: Sunday, March 21, 2004 12:21 PM
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Subject: Re: equinox


My calculation was based on the Nautical Almanac, which had a 0 degree
declination for the Sun exactly at 6:48:00 UT.  The Sun's GHA is
adjusted by up to 1.5' to reduce the error arising from ignoring the v
correction.  The GHA of the Sun may be in error by up to 0.25'.

MICA reports:
                                      Sun

                         Apparent Topocentric Positions
                           Local Zenith and True North

                  Location:  E 79?51'26",   0?00'00",     0m
                  (Longitude referred to Greenwich meridian)

    Date        Time          Zenith          Azimuth        Distance
         (UT1)               Distance        (E of N)       to Object

              h  m   s        ?  '   "        ?  '   "          AU
2004 Mar 20 06:48:00.2       0 00 02.4     111 21 02.9     0.995960666
2004 Mar 20 06:48:00.3       0 00 01.1     141 03 00.3     0.995960666
2004 Mar 20 06:48:00.4       0 00 01.2     223 27 24.7     0.995960666
2004 Mar 20 06:48:00.5       0 00 02.5     249 45 07.1     0.995960667


The Naval Observatory reports 6:49 for the vernal equinox:
2004                        2004
Perihelion  Jan   4 18    Equinoxes  Mar   20 06 49    Sept  22 16 30
Aphelion    July  5 11    Solstices  June  21 00 57    Dec   21 12 42

Perhaps one of the astronomically literate will explain the discrepancy.

On Mar 21, 2004, at 2:01 PM, Gary Harkins wrote:

> In a message dated 3/21/2004 1:23:55 PM Eastern Standard Time,
> legras@MAD.SCIENTIST.COM writes:
> spring equinox was on earth 20/03/2004 at 06:49 utc according to Jean
> Meeus' sky calender 2004
>
> On which planet does it occur at 6:48 ????
>
> Using the Nautical Almanac and interpolating between .8' South at 0600
> utc and .2' North at 0700 utc the equinox calculates to occur at
> exactly 0648 utc.  Are you saying that Jean Meeus' sky calendar is
> more accurate than the nautical almanac?  Does he carry his
> calculations out to more precision than .1' of declination?  And who
> cares anyway, it's cold as most winter days here regardless of what
> the calendar says.

   

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