I am an Australian merchant naval officer and in the Southern Ocean we see some spectacular sights, but on 22 June 2014 at sunset I saw a phenomenon that I have rarely seen. Rays of light were emanating from the horizon at 180 degrees from the setting sun (see photo). Can anyone explain this quirk of nature?
鈥� The phenomenon seen by your correspondent is not rays of light emanating from the horizon, but the reverse: rays of shadow apparently converging. They are called anticrepuscular rays, and are the result of the setting sun 鈥� behind the observer 鈥� casting shadows from clouds.
We are all familiar with the sight of the sun illuminating broken clouds from behind, resulting in characteristic sunbeams. These sunbeams are parallel and only appear to spread out because of our perspective from the ground. The process is identical to the way parallel railway tracks appear to converge as they recede into the distance.
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Close to sunset, the light rays and cloud shadows will be cast almost parallel to Earth鈥檚 surface, so can continue for many miles. Dust or mist in the air can reveal the path of light and shadow disappearing into the distance behind us, converging on a point exactly opposite the sun. This is the anti-solar point.
At sea level the phenomenon will only be seen at sunrise or sunset. However, if you are high enough, your anti-solar point will be above the visible horizon, giving you longer to experience this effect. So mountaineers and pilots like myself will have far more chance to see them. Mountaineers occasionally have a shadow display of their mountain range cast behind them while the sun is low in the sky.
Martin Powell, Westgate-on-Sea, Kent, UK
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