Function twilight 

pub fn twilight(
    kind: Twilight,
    night_of: OffsetDateTime,
    site: &Location,
) -> TwilightOutcome

Dusk and dawn instants for the night whose solar lower culmination is nearest night_of, at the darkness level kind.

Matches astroplan’s twilight instants within ±60 s (the Sun’s altitude changes ~1° per 4 minutes at mid-latitudes; the 0.01° solar-position accuracy contributes only a few seconds).

use skymath::{twilight, Location, Twilight, TwilightOutcome};
use time::OffsetDateTime;

let site = Location::parse("+52 05 32", "+004 18 27", 6.0)?;
match twilight(Twilight::Astronomical, OffsetDateTime::now_utc(), &site) {
    TwilightOutcome::Night { dusk, dawn } => println!("dark {dusk} to {dawn}"),
    TwilightOutcome::NeverDark => println!("never astronomically dark tonight"),
    TwilightOutcome::AlwaysDark => println!("dark around the clock"),
}