Twilight
Twilights are states of the sun defined by the number of degrees below the horizon. Each Twilight has a TwilightInfo.angleRange that defines the range of angles for which the sun would be considered to be in that associated Twilight.
CivilTwilight, NauticalTwilight, AstronomicalTwilight would be Twilights but SolarPhase.Day and SolarPhase.Night are not considered Twilights.
Many Twilights can occur twice during a full solar cycle. For instance, CivilTwilight can occur during both dusk and dawn, SolarPhase.CivilDusk and SolarPhase.CivilDawn respectively. Those are both considered to be CivilTwilight.
Inheritors
Types
Astronomical twilight is observed when the sun is between -12.0 and -18.0 degrees.
Civil twilight is observed when the sun is between 0.0 and -6.0 degrees.
Nautical twilight is observed when the sun is between -6.0 and -12.0 degrees.