# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re:Moonrise-over-Connecticut
Date: 2021 Nov 5, 13:30 -0700

Hello Antoine,

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

(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
are provided:

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
true north.

Refr Refraction. Atmospheric refraction correction applicable to object, if
observed at sea level at optical wavelengths, under standard atmospheric
conditions.

SD   Semidiameter.  Half the apparent equatorial angular diameter of object.

PA   Parallax in altitude.  Topocentric parallax correction applicable to
object.

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.

Regards

Peter

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