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.


Our offline compatible app CelNav 360 graphically streamlines the celestial fix workflow, turning your device into a practical accessory to a sextant.





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). You do this in the Fix view, where 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, stars or planets, 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 generally 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.

6 - 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.
(
NB the towards/away qualifier which ordinarily completes an intercept specification is omitted from the inspection box as it can be inferred straightforwardly from the plot.)

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