A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Douglas Denny
Date: 2010 May 4, 04:01 -0700
This business of the US Nautical Office Almanac data on the internet becomes curiouser and curiouser.
I tried George's suggestion to compare the results of ICE at 12-00 noon today (Tues 4th May 2010) at my location (Long West 000-51.2 Lat North 50-49.9) with those of another almanac.
Unfortunately I do not have a current Nautical or Astronomical Almanac in book form - not having bought one for many years as I use computer generated data and an HP 50 calculator I have programmed.
So I tried a comparison with the US Nautical Office results obtainable from the internet US Naval Office that Gary LaPook first mentioned having difficulties with.
The results are _identical_ for todays date/time of 12-00 noon, between ICE and the internet listing from:-
...... which implies it is the same programme using the same delta T of 72.3 s which is what my copy of the 1988 programme ICE uses.
One obtains from ICE and from the US Nautical Offcie internet site:-
Sun: GHA 0-48.4 Dec N16-01.0 Hc 55-11.1 Zn 179.9
If one enters date/time for 5 seconds earlier in an attempt to alter delta T from the ICE value of 72.3 to 67.3 then the results of both ICE and the US Nautical Office are identical again, being:-
Sun: GHA 0-47.1 Dec N. 16-01.0 Hc 55-11.1 Zn 179.9
Looking into the information file for ICE in 'word' format (which I should have done firstly to get a better understanding) it states the following (lower down) which I am going to have to go away and digest.
The question is: why is the current on-line US Almanac Office giving the same results as ICE which has an incorrect delta T? It would appear they are using ICE as the basic core programme still, (produced before 1988).
Any further comments will be appreciated.
If you want ICE I can send it in zipped form it's 965 Kbytes.
4.2.1 Dates and Intervals
The date which you enter in response to a prompt from the Floppy Almanac is
used to specify the epoch for the tabulation. For fixedÄinterval tabulations
this date is used as the beginning date of a running table; you must also
select the tabulation interval and the number of lines of output you wish. The
tabulation interval is the difference in tabular epochs between successive
As in the Astronomical Almanac, the independent argument for most Floppy
Almanac tabulations is dynamical time, specifically, Terrestrial Dynamical Time
(TDT). TDT is effectively equal to TAI (International Atomic Time) + 32.184
seconds, and is the extension of the old Ephemeris Time scale, abandoned in
1984. The more familiar Universal Time (UT), which is tied to the (irregular)
rotation of the Earth, is currently "slow" with respect to TDT and the TDT - UT
difference is now almost one minute. A complete table of past and projected
values of TDT-UT, accurate to a fraction of a second, can be found on page K9
of the Astronomical Almanac.
Therefore, in using the Floppy Almanac, dates and intervals to be entered
should be expressed in Terrestrial Dynamical Time (TDT), except for the
following cases. "F7 Navigation" and "F5 Sidereal Time" require Universal Time
(UT, or, more precisely, UT1), otherwise known as Greenwich Mean Time (GMT).
Within the "F3 Positions" selection, the time scale for heliocentric and
barycentric coordinates is, technically, Barycentric Dynamical Time (TDB),
although TDB never differs from TDT by more than 1.7 milliseconds.
Dates may be entered in either of two forms:
year-month-day-hour-minute-second, or Julian date and fraction. The former is
entered using the format YYMMDD.HHMMSS, for example, 880314.052741 for 1988
March 14, 5:27:41 TDT. Alternatively, a Julian date, including an appropriate
fraction, may be entered, for example, 2447234.72756 which corresponds to the
above date. The Floppy Almanac can distinguish between the two forms since
they require, respectively, six and seven digits to the left of the decimal
Intervals may also be entered in either of two formats:
days-hours-minutes-seconds, or days and fraction. The format of the interval
must correspond to the format of the date. If the date is entered as
year-month-day-hour-minute-second, the interval must be entered as
days-hours-minutes-seconds, using the format DD.HHMMSS. Similarly, if the date
is entered as Julian date and fraction, the interval must be entered in the
form days and fraction. For example, if a tabulation interval of 8.5 minutes
is desired, it must be entered as 0.000830 if the date is entered as
880314.052741 and as 0.005903 if the date is entered as 2447234.72756.
from George and Antoine:-
I invite Douglas Denny to obtain the GHA of the Sun at noon (UT) today from
ICE predictions, by the procedure he proposes, and compare his result with
the corresponding figure taken from a current almanac.
And Antoine wrote:-
Is it too simple to just enter a date/time 5.3 s earlier than the observation? or am I missing something?
Although I am not familiar with ICE, I would guess that if you enter a " date/time 5.3 s earlier than the observation ", then you will compute both RA and Dec for the correct TT (was called TDT then ...) as related to your "target UT". But for this " date/time 5.3 s earlier than the observation " you will get GMST/GAST value for "target UT - 5.3s" rather than for your "target UT".
Since you can not modify delta T, you need to compute RA and D with your method. But it will not give you the correct GHA values, since GAST would not be computed at the time you would need it.
Hope it can help you.
And of course, if I am totally off track, let some charitable soul put me back onto the right track.
Antoine M. "Kermit" Couëtte
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