Jack Forster analyzes the Apple Watch Solar Face for Hodinkee and starts talking about twilight:
Twilight, as it turns out, is further divided into three phases: Civil Twilight, Nautical Twilight, and Astronomical Twilight, and it is the phases of twilight, plus sunset, which are indicated by the four dots clustered at sundown.
He continues describing every form of twilight and why it is important to be defined. One example:
Civil Twilight is not only an astronomical event – it is also important in fields as diverse as aviation and law. Here in the United States, the FAA defines night, and the additional regulations pertaining to nighttime aircraft operations, as the period between the end of Civil Twilight and the beginning of morning Civil Twilight.