Casio WSD-F10 User Manual page 64

Before Using Sunrise/sunset
This function uses smartphone location information. Be sure to perform the
following operation: Open "Settings", tap "Location", use the switch to turn
"Location" on, and then choose "High Accuracy" mode.
Nautical Twilight
Twilight is the time when the sky brightens as the sun is at or just below the
horizon (water horizon or visible horizon). There are actually three types of
twilight: nautical twilight, civil twilight, and astronomical twilight. Nautical
twilight is normally defined as occurring is from six to 12 degrees below the
horizon.
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