Sight Reduction Calculator V1.51 – Copyright © 1998-2016 Henning Umland
Geographic position of body:               Assumed position:
Greenwich hour angle  °  Longitude  °  E W
Declination  °  N S Latitude  °  N S
Instrument reading:  °         
Altitude correction parameters:                      Results:
Index error*  Observed altitude  ° 
Height of eye m ft Computed altitude  ° 
Air temperature  C F Azimuth  ° 
Atm. pressure  Intercept  nm 
Horizontal parallax  km 

* The index error is regarded as positive when the angle displayed by the instrument is greater than the actual angle.

This celestial navigation software written in JavaScript performs all calculations necessary to derive a line of position. Enter Greenwich hour angle and declination of the observed body (from the Nautical Almanac or Interactive Computer Ephemeris), longitude and latitude of your assumed position, and the altitude displayed by the instrument (sextant or theodolite). It is also possible to enter a zenith distance (measured by some theodolites). Enter the altitude correction parameters, and hit the [Reduce sight] button. The program does the rest. If the dialog boxes for air temperature or atmospheric pressure are left blank, the program calculates with standard values, 10°C (50°F) and 1010 mbar (29.83 in.Hg). Any other blank dialog box is regarded as zero. Use radio buttons and pulldown menus to to change program settings. Requires a JavaScript compatible browser like Netscape Navigator 4 or Microsoft Internet Explorer 4 (or later versions).

The altitude corrections for the moon include oblateness of the earth. The semidiameter of the moon is internally computed from the horizontal parallax if the latter exceeds 50'. In this case, entering a semidiameter is not required (will be ignored).

The program internally sets dip=0 if any form of an artificial horizon or a zenith distance is chosen. If an external artificial horizon (pan with liquid) is used, the sextant altitude is automatically divided by two after subtracting the index error.


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 ( for more details.

Henning Umland    N 53° 20' 34''   E 9° 52' 00''

Check this web site for updated versions: