A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Peter Blaskett
Date: 2021 Nov 5, 13:30 -0700
A simple example using ICE shows that the positions calculated are for the body centre.
Using latitude 0.0 and computing a meridian crossing gives :-
Celestial Navigation Data for 2021 Nov 6 at 11 43 35 UT (GMT)
Delta T = 87.7 seconds
For Assumed Location: Longitude 0 00.0
Latitude 0 00.0
Almanac Data Altitude Corrections
Object GHA Dec Hc Zn Refr SD PA Sum
ø ' ø ' ø ' ø ' ' ' '
SUN 359 59.3 s16 07.3 +73 52.7 180.0 -.3 16.2 .0 15.9
ARIES 221 48.4
Now Dec plus Hc = 90.0 deg , so Hc is for the position of the body centre.
Also, the Documentation with ICE is explicit. Here it is. The paragraph Hc is definite. The last paragraph refers to the corrections.
Extract from explanatory documentation
6.8 Topocentric Altitude and Azimuth
The altitude and azimuth of a celestial body as observed from any geographic
location at any time can be obtained by using the "F7 Navigation" selection.
The celestial objects which can be specified are not limited to those normally
1988 US Naval Observatory Nautical Floppy Almanac Version 2.00.88 Page 15
used for celestial navigation. In fact, if you redefine the catalog in use
(see section 6.4), it is possible to obtain "F7 Navigation" on objects such as
the Ring Nebula or the quasar 3C 273.
For those not familiar with navigational notation, the following definitions
GHA Greenwich hour angle. Apparent instantaneous hour angle of object
measured westward from the plane of the Greenwich meridian.
Dec Declination. Apparent declination of object.
Hc Computed altitude. Instantaneous altitude of center of object, calculated
using its geocentric position, that is, without adjustments for topocentric
parallax or refraction.
Zn Computed azimuth. Instantaneous azimuth of object, measured eastward from
Refr Refraction. Atmospheric refraction correction applicable to object, if
observed at sea level at optical wavelengths, under standard atmospheric
SD Semidiameter. Half the apparent equatorial angular diameter of object.
PA Parallax in altitude. Topocentric parallax correction applicable to
Sum Sum = Refr + SD + PA. If Sum is subtracted from Hc, the apparent altitude
(that comparable to observation) of the lower limb of the object is obtained.
Your posting about using ICE to get answers corrected for the proper Delta-T might be useful for someone.
This, and the awkwardness of the ICE interface, is what urged me to write my own software years ago.
I hope we are all singing the same music now.