Sight Reduction Calculator V1.51 – Copyright © 1998-2016 Henning Umland (with tweaks by Bob Goethe in 2016).

To use as a result checker for the Mark 1 Navigator's Slide Rule, leave all of the fields for "Altitude correction parameters" blank. Leave "Sea Horizon" and "Center" as they are.

Fill in the fields for the "GP" and "My Position", plus the "Slide Rule Generated Hc". Then click "Reduce sight".

If you want to see the original, "untweaked" version of Henning's calculator, go to

Geographic position of body:               My position:
Greenwich hour angle  °  Longitude  °  E W
Declination  °  N S Latitude  °  N S
Slide Rule Generated Hc:      °         
Altitude correction parameters:                      Results:
  Slide Rule Hc  ° 
  m ft Computer-Generated Hc  ° 
  C F Computer-Generated Azimuth  ° 
  Intercept  nm 

In addition to the normal "slide rule accuracy" issue, there are some angles that slide rules simply cannot handle. A moment's glance at your slide rule will tell you that if any equation you calculate has a cosine of less than 5°, you can expect your results to have degraded accuracy. This checker lets you run different problems on your own, to let you begin to develop a feel for the kinds of solutions that your slide rule will do well with, and the kinds of solutions that are going to be difficult to manage accurately.



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: