Function getRetrogradeShadow

getRetrogradeShadow(
    period: RetrogradePeriod,
): {
    preShadowStartJD: number;
    postShadowEndJD: number;
    shadowZoneStart: number;
    shadowZoneEnd: number;
}
Get the "shadow" period before/after retrograde.
Parameters
- period: RetrogradePeriod
  Retrograde period
Returns {
    preShadowStartJD: number;
    postShadowEndJD: number;
    shadowZoneStart: number;
    shadowZoneEnd: number;
}
Object with pre-shadow and post-shadow JDs
Remarks
The retrograde "shadow" is the zone the planet transits three times:
- Pre-shadow: From when planet first reaches the station-direct degree
- Post-shadow: Until planet passes the station-retrograde degree
- Defined in transits/retrograde-transits.ts:580