A star, in particular the Sun, is in the twilight when, by the effect of the rotation of the Earth, it crosses the plane of the horizon and passes from the visible to the non-visible hemisphere. That is, when its height is zero, going from positive to negative. In the case of the Sun, this determines the end of the day. The antonym of sunset is ortho. The sunset is also called dusk.
Green flash, or green ray, is an atmospheric optical phenomenon occurring shortly after sunset or shortly before sunrise, where you can see a green dot, usually for one or two seconds, on the position of the sun. It can also be seen as a green ray coming out of the spot where the sun has set. The green flashes are actually a group of phenomena that arise from different causes, some being more common than others. Green flashes can be observed from any altitude, even from an airplane. They are usually seen when the horizon presents no obstacles, as in the ocean, but they also appear on clouds and on mountains.
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From @oscarps