How did you get on with the sunset question?
Colregs Rule 20(b) applies for the should
Colregs Rule 20(a) & (c) for when
COLREGS
The international definition of Sunset is when the top of the sun's disc sinks below the horizon in the evening. Sunrise is defined as the moment the top of the sun's disc first breaks the plane of the horizon in the morning. The precise definition states that sunrise or sunset is when an observer at sea level with an unimpeded view of the horizon sees the upper disc of the sun to be tangent to (touching) the horizon. In reality, the sun's disc is actually below the horizon at the calculated time of sunrise or sunset, but the Earth's atmosphere bends the rays allowing us to see the sun earlier in the morning and later in the evening than if it were being viewed through a vacuum.
If you managed to get all that, BZ.