POSITION-LINE-NAVIGATION
using SEXTANT and CLOCK    - V.35

    Copyright © 2022 see below
Y M D H M S
Date: Time:
ΔT: Star*:
N/S deg min
DR-Latitude:
E/W deg min
DR-Longitude:  
deg min ±min {0;1}
Sextant:  Ic:  M/S_Rim:
deg_C hPa meter
Temp: Press: Eye:  


RESULTS: Position-Line by Azimuth and Δh

  Altitude calc.   Azimuth Δh' =nm
Sun     
Moon     
Venus     
Mars     
Jupiter     
Saturn     
* Star    
     Sun SD Moon SD Moon HP
    
GMST GAST Eq. of Time
JD JDE Day of Week


DESCRIPTION:

General: This program calculates a position-line (by method Marc St. Hilaire) from the observation of sextant-altitude of a celestial body together with the precise time. A second position line, either from a different celestial body or from the same body some two hours or so later, and its intersection with the first position-line gives the position of the ship. If the ships position has changed between the two measurements, the first position line must be shifted by the same amount on the sea-chart.

Usage: Enter date and time of the sextant-observation of a celestial body. Then enter your assumed geographic position (=Dead Reckoning Position, DR) by latitude and longitude. Finally enter the angle above horizon measured by the sextant, and give values for index-correction, temperature, pressure and height-of-eye above sea-level. Finally enter 0 or 1 for either the lower (=0) or the upper rim (=1) when measured on the sun or moon. (This is irrelevant for planets and stars). At last hit "COMPUTE" to complete the calculation. Since sofar you have not specified which celestial body you have observed, the program offers a comparison of the observed with the calculated altitudes for all bodies from Sun through Saturn and Star. The difference is given in the last column denoted by "Δh [nm]" in arcminutes corresponding to nautical miles. The POSITION-LINE then must be drawn orthogonal to the Azimuth-direction of the observed body, and shifted away from DR-position by an amount ±Δh in parallel towards (+) or away from (-) the oberved celestial body (respectively its image point on the surface of the earth). -
In detail, the program calculates geocentric altitudes and azimuths of bodies for an assumed position, with corrections for atmospheric refraction and parallax, and returns the corrected observed sextant-height and deviation Δh from calculated sextant-height (sight reduction). Azimuths are measured clockwise (0°...360°) from the geographic north direction, as is common in marine navigation. See Henning Umlands web-page for the theory behind. When the difference Δh between observed and calculated altitude is larger than about 3°, a wiggly line "~~~" is displayed, otherwise the Δh-value in arc-minutes, equivalent to nautical miles. The results have been tested against various tutorial tables for astronomical navigation. Experience shows that the uncertainties of the measurement and the atmospheric conditions are much larger than the uncertainties of the geocentric altitudes calculated by this ephemeris-program.

Astronomical Almanac: The ephemeris used is identical to the computer almanac Astronomical-Almanac-106 by the same authors. Its precision is about 1 arcsec up to year 2200, provided an actual value for ΔT is given. (If you set "height-of-eye=1", details of rectascension and declination for Moon will be displayed for comparison with other ephemeris programs).

Results for the ephemeris were compared with Interactive Computer Ephemeris 0.51 (US Naval Observatory), JPL-HORIZONS (NASA Jet Propulsion Laboratory, models DE431and DE441), and Astronomical Almanac 5.6 (Stephen L. Moshier). Note that NASA-JPL uses ΔT=TDB-UT1 for years before 1962, but ΔT=TDB-UTC for later years ! Here TDB is solar-systems Barycentric Dynamical Time, differing from TT only periodically by about 2 milliseconds. The difference UT1-UTC can be obtained from IERS, see above.

   (last update: 24.jan.2023).


Copyright and LICENSE

This program ist based on an earlier Almanac by Henning Umland (https://celnav.de) and on an improved version by Henning Umland and Heiner Müller-Krumbhaar (Astronomical-Almanac-106) The current corrections for measured sextant altitudes are added by the latter author. This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the license or any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License (www.gnu.org/licenses/) for more details.

The authors would appreciate any error reports being sent to info@ocean-navigation.de