use chrono::{DateTime, Utc};

/// Compute Julian Date from a UTC datetime.
pub fn julian_date(dt: DateTime<Utc>) -> f64 {
    let unix_seconds = dt.timestamp() as f64;
    // J2000.0 epoch is 2000-01-01 12:00:00 UTC = Unix 946728000
    // JD of Unix epoch 0 = 2440587.5
    unix_seconds / 86400.0 + 2440587.5
}

/// Compute Local Sidereal Time in degrees [0, 360).
/// Uses IAU formula via Julian Date and observer longitude.
pub fn local_sidereal_time(jd: f64, lon_deg: f64) -> f64 {
    // Days since J2000.0
    let d = jd - 2451545.0;
    // Greenwich Mean Sidereal Time in degrees
    let gmst_deg = 280.46061837 + 360.98564736629 * d;
    let lst = (gmst_deg + lon_deg).rem_euclid(360.0);
    lst
}