The swimming Moon

The famously dry Atacama Desert may seem like an odd place for the Moon to take a dip — but in this intriguing image, taken by ESO Photo Ambassador Petr Horálek, it appears to do just that!

Initially high, the crescent Moon slowly descends through the clear Chilean sky, before hitting the thickest parts of the atmosphere right above the horizon.

It was here that a “rare theatre” began, said Horálek. The thin sliver of the Moon was optically distorted into a “weird, snaky shape” as its light passed through layers of different air densities, caused by different pressures, temperatures and humidities. The Moon lost its smooth curves and instead appeared as a rippling and squiggly zig-zag — “as if it were swimming”. The effect of the closely spaced layers in the atmosphere caused different parts of the Moon’s image to be refracted differently as it disappeared below the horizon.

All these pictures were taken at ESO’s La Silla Observatory in Chile, facing the Pacific Ocean.