small sextant icon Sunsight.xls

This Excel spreadsheet can be used to check your manual sight reduction worked out using the Sun Almanac and Sight Reduction Tables downloaded from this website. I hope you find it fairly intuitive to use. Enter the information required into the unlocked white cells bounded in green. The grey cells return intermediate results, such as GHA, Declination, LHA, Hc and Ho, which you can compare with results obtained from the Sight Reduction Tables. The final result, in blue, is your Azimuth and Intercept.


Recently updated, Sunsight.xls has a simpler, more intuitive, data entry and better error trapping than before. If you downloaded the older version I recommend you upgrade.

In this version you only need to enter whole degrees of Latitude and Longitude for your Assumed Position (AP) although decimal values are accepted. And instead of entering +ve values for North and East and -ve values for South or West you simply enter N,S E or W. Forget to do that and the cell alerts you by going red. The spreadsheet will also calculate a suitable minutes Longitude for your AP.

Sunsight.xls can be used with an artificial horizon. To do so, enter zero (0) for your Height of Eye. This will force the spreadsheet to apply Index Error in the normal manner, halve the resulting angle, and then apply the remaining altitude corrections to obtain your true observed altitude, Ho.
This is great for backyard navigation practice but works just as well in the wilderness. How else do you think Lewis and Clark navigated their way around America?

Remember, when the bottom of the reflected image is touching the top of the image in your artificial horizon it is a Lower Limb measurement and enter 1 in the Limb cell.
If the two images are superimposed it is a centre sight and enter zero (0) in the Limb cell.
Finally, if the top of the reflected image touches the bottom of the artificial horizon image it is an Upper Limb measurement and enter -1.

Due to rounding in Excel and interpolation within the tables by yourself, the final Azimuth and Intercept for Sunsight.xls may be a decimal point or two adrift from results obtained from tables, this is unlikely to significantly affect your navigation.

The spreadsheet is password protected to prevent accidental corruption. If you want a closer look at the formulae, they are contained in the hidden worksheet 'Calcs'. The password is "backbearing", minus the quotes.