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Replying to @MattTexEckerle

I ended up using some public domain code, a C library called SUNRISET.C by Paul Schlyter. It’s actually period-appropriate for a late-80s wristwatch hack; the code dates from 1989! https://github.com/joeycastillo/Sensor-Watch/blob/main/movement/lib/sunriset/sunriset.c

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