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The setting moon silhouettes 165 mile distant mountains. Photographs by Alain Origne (more images) of Laboratoire Astrophysique Marseille. Image ©Alain Origne, shown with permission.

Atmospheric refraction lifts the sun and moon when near the horizon so that they still appear after they have actually set.

The same refraction makes over the horizon objects visible when back-lit by the sun or moon. Here, Mt Canigou in the Pyrenees is seen from near Marseilles, France 165 miles away across the Mediterranean.

The refraction occurs because air pressure and density decrease with height. The different density layers act somewhat like a huge curved lens. The Sun or Moon's slanting rays are refracted downwards as they pass from lower to higher density. The net result is that ray paths tend to curve around the Earth.

Alain Origne, see previous OpticsPOD, has made an extensive study of the phenomenon and predicts when it will occur. He would like to hear of similar long distance sightings elsewhere in the world.

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