Instrument 02
Computes sunrise, solar noon, sunset and civil twilight from solar position equations, then returns the latest departure time that still returns you before the light goes.
Civil twilight gives usable light after sunset. Use sunset itself on technical ground or under heavy cloud.
Out-and-back turnaround at — from the start.
Sunrise — · Solar noon —
Sunset — · Civil dusk —
Sunrise to sunset —
Civil dawn to civil dusk —
Sunrise and sunset use the standard solar position algorithm: solar mean anomaly and true longitude are derived from day of year, then right ascension and declination, then the local hour angle at a specified zenith. Geometric sunrise and sunset use a zenith of 90.833 degrees, which accounts for the angular radius of the solar disc and standard atmospheric refraction at the horizon. Civil twilight uses 96 degrees, placing the sun 6 degrees below the horizon.
Results are converted to your device's local time, so daylight saving is handled automatically. Solar noon is reported separately because it is frequently 30 to 90 minutes from clock noon depending on your position within the time zone and the equation of time.
The arc represents the span from sunrise to your selected finish boundary. When the calculation is for today, the marker shows current position within that span and the amber segment shows light already spent. For other dates the arc shows the shape of the day without a position marker.
Size the buffer to the consequence of being wrong. Thirty minutes on familiar level ground; sixty to ninety on technical terrain, in unfamiliar country, or with a group.