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The Dusk Till Dawn widget calculates dusk and dawn periods of the current day up to a preceicion of 5 minutes (which is enough for all practical purposes). Whatever your current position is you get the time periods for

astronomical dusk/dawn	-18° to -12°
nautical dusk/dawn	-12° to -6°
civil dusk/dawn	-6° to -0.833°

at a glance. For more information on twilight periods see Wikipedia.

Of course this takes not into account possible shadow from obstructions like mountains or houses. If you're interested in shadow calculation have a look at the iPhone app ShadowCalc .

The Dusk Till Dawn widget runs on fenix, fenix-hr, epix and bravo watches. The only thing required is the GPS sensor of your watch turned on and receiving location data:

Acquiring position data may take up to a minute. If no GPS data are found (GPS is either turned off or no satllites found) and the last position was not yet recorded on the current day, you'll see: If up-to-date recorded data are available they will be used. A small GPS indicator shows you whether calculation is based on fresh or recorded data:

Handling of the app is as simple as usual: