You are watching a beautiful sunset over the ocean - the bottom edge of the sun kisses the horizon and begins to disappear. At the last moment before it vanishes, the upper tip of the sun turns, for an instant, a brilliant green. You have just seen the remarkable Green Flash.

The Green Flash is an astonishing sight. Most people, including most astronomers have never seen it.

The effect is real, though rare, because conditions have to be exactly right for it to occur. You must have an unobstructed view to the horizon with no mountains, trees or buildings to block the view. There must be no cloud or haze or fog to swallow up the sun before it touches the horizon. A view west towards the ocean is ideal, but even then the Green Flash can be obscured more often than it is visible.

Unmistakable and unforgettable despite its name, there is no flash of light. The flash refers to its brevity. lt is over quick as a flash.

How to see the Green Flash

lf conditions are right, there is a trick to looking for it. First, just before sunset, check out the view. lf you can see the bottom edge of the sun touch the sea, chances are you will see the Green Flash. At that point look away from the sun, so your eyes don't succumb to the glare. Steal momentary glances over your shoulder every few seconds to get a sense of the sun's rate of descent. When about ¾ to 7/8 of the sun has set, look at the sun, as the last bit disappears, you will see it glow green like an emerald.

Why does it happen?

lt is fairly complex phenomenon and is caused by the fact that the different colours of sunlight, the colours that form the rainbow's spectrum, behave differently when travelling through the air. Some of the colours are absorbed, some are scattered. The thick atmosphere in the direction of the horizon bends light at different angles. At the moment the sun sets, the green light is bent so it runs parallel to the surface of the Earth and straight into your eyes. The other colours are either absorbed or bent at an angle so they miss your line of sight. A ship at sea is one of the best places in the world to see the Green Flash. Seeing a Green Flash is sure to bring a moment of pure happiness!