Getting Started with the CelNav 360 app (iOS version)

Celestial navigation exploits the principle that every measurement taken with a sextant converts to a line of position on a chart, the intersections of two or more such lines providing a position fix.

While this principle is elegant and easy to grasp in itself, its associated mathematics, known as sight reduction, is quite involved. In the past celestial navigators have needed to rely variously on the use of almanac publications, numerical tables, pocket calculators, spreadsheets and the like.

With the steps outlined below we aim for a sight reduction workflow that keeps the numerical heavy lifting compactly under the hood as far as possible. 

1 - Set Assumed Position


The app comprises just two views: a planisphere-like Shoot view of the the horizon, vs an initially empty Fix view in which lines of position are plotted on a Lat-Long grid. Use the tab bar at the base of the screen to toggle between the two.


To prepare a position fix you must first roughly set your Assumed Position (AP). In the Fix view it appears as a draggable marker.

2 - Sight Preparation

Switch to the Shoot view, it shows the computed horizon at the given Assumed Position, and should therefore correspond fairly well to the real world horizon you see about you.


Pan the crosshair to select a celestial body you are able to identify in the real world, hence suitable to 'shoot' with your sextant, i.e. measure its altitude above the horizon.  In navigation terminology a sextant measurement is known as a sight.


You can choose from the sun, moon,  planets or 'the 58' almanac-listed navigational stars, subject of course to whether you are shooting during the day or at the twilights.

3 - Enter a Sight


When you hit the shoot button (sextant icon) to confirm a selection, a dialog prompts for your sextant's altitude reading. The dialog also accommodates the horizon dip correction which always complements any sextant measurement. 


The precise shoot time at which the reading is taken is very important. In the app's default realtime mode, shoot time corresponds to aforementioned button hit. To change this click on the time monitor shown in the Shoot view.

4 - Lines of Position


For each sight you enter the app calculates and plots the corresponding line of position (LoP) in the Fix view.


Ideally for a good fix you will aim to shoot multiple sights over a wide spread of compass directions. The Fix view cursor (black pin icon) then lets you probe Lat-Long of the resulting LoP intersection region and hence decide on your fix.

Hovering the cursor over an LoP activates a green inspection box, showing sighted body and shoot time along with a discard option.

5 - Advanced

An extension flap on the inspection box reveals the LoP's  intercept parameters, useful for accurate transfer to a physical chart, running fixes etc. They comprise the  azimuthal bearing Zn of the sighted body with respect to AP, and the perpendicular distance from AP to the LoP, given in nautical miles, NM. 

Click on the detail icon next to the intercept parameters to see the sight reduction housekeeping from which they derive.

6 - Bibliography

A standard reference work for sight reduction and other techniques of celestial navigation is
Bowditch's American Practical Navigator
https://msi.nga.mil/Publications/APN