In Search of Pure Dark Skies

Watch in 1080p! From ESOCast with the famous Dr. J. In the pursuit of pristine skies, the European Southern Observatory operates its telescopes in the remote and arid landscape of the Atacama Desert in Chile.

A top-class site for astronomical observations must meet several criteria. To begin with, of course, you need a sky that is free of clouds pretty much all year round. But in addition to that, you also need excellent atmospheric conditions, as well as very dry air with as little water vapor content as possible. And this is exactly the kind of environment that you find in the Atacama Desert in Chile.

The Chilean Coast Range. Here, the cold offshore Humboldt current creates a coastal inversion layer of cool air, which prevents rain clouds from developing. Often, a layer of fog is created, which rapidly disperses in the foothills above the desert. A view from the Paranal Observatory towards the Pacific Ocean clearly shows the top of the cloud layer.

Chilean coastal range Coastal clouds gathering at the foothills. In addition to the coastal inversion layer, a region of high pressure in the south-eastern Pacific Ocean creates circulating winds, forming an anticyclone, which helps to keep the climate of the Atacama dry.

The Andes lie to the east, acting as a natural barrier for clouds coming from this direction – so all the possible paths for moisture to reach the Atacama Desert are literally blocked. This results in extremely dry air and clear blue skies. Ideal conditions for astronomical observations.

But we’re not done yet with our checklist of ideal observing conditions. In addition to cloudless and dry skies, astronomers need dark sites and unpolluted air in order to make the best observations. In most places, the world at night is far from being a dark place and the light pollution caused by modern civilization can easily be spotted. However, light pollution hinders astronomical observations, as it brightens the night sky and makes faint celestial objects