The sky, explained

For the curious: what the watch faces show and how it is computed.

The sun

• Height: angle of the sun above the horizon (0° at the horizon, negative below).
• Azimuth: its direction. It rises due east only at the equinoxes: further north-east in summer, south-east in winter.
• Sunrise, sunset: the edge of the sun touches the horizon. The air bends light, so you see it slightly before it is really up.
• Solar noon: the sun at its highest. Rarely at 12:00 exactly (time zone, daylight saving). Solar midnight: at its lowest.
• Golden hour: sun between 0 and 6° above the horizon, warm low light.

Twilights

After sunset, night comes in steps, depending on how far the sun is below the horizon:

The other way round in the morning. Around the June solstice, in Paris, there is no full night. Beyond the Arctic Circle: the sun never sets (polar day) or never rises (polar night).

The moon

• Phases: a 29.5-day cycle. New moon (on the sun's side, invisible), crescent, first quarter, gibbous, full moon (opposite the sun), then waning. The percentage is the lit part.
• Moonrise: about 50 minutes later each day. The full moon rises as the sun sets.
• Height: the full moon is high in winter, low in summer, the opposite of the sun.
• Southern hemisphere: the phase is seen the other way round.

How it is computed

• On the watch, from the time and your location. Nothing is sent.
• Formulas by astronomer Jean Meeus (Astronomical Algorithms), describing the apparent paths of the sun and moon.
• Once a day (and when your location or time zone changes), then followed every two minutes by interpolation: easy on the battery.
• Checked over 354 days (from Quito to Tromsø) against NASA-JPL and USNO ephemerides:

Near the poles, when the sun skims the horizon, times are less precise.