The Beauty of 22 Degrees

The telescopes at ESO’s La Silla Observatory in Chile are on a mission to observe and understand the vast Universe — but there are plenty of interesting natural phenomena far closer to home. One such example is this angelic 22-degree halo. This photo was taken high up in the remote Atacama Desert, but such a sight is visible year-round all over the world.

These haloes are formed when light from the Sun or Moon passes through cirrus clouds high up in the Earth’s atmosphere. The tiny ice crystals that make up these clouds act as miniature prisms, changing the direction of the light that passes through them (something known as refraction). Light rays that do this tend to “bunch up” at the angle that represents the least amount of deviation from their original path. For the particular shape of ice crystal lurking within the cirrus clouds, this minimum deviation angle happens to be around 22 degrees, which is why we see this concentrated halo of light at a distance corresponding to 22 degrees from the moon.

ESO’s 3-6-metre telescope provides an appropriate foreground to the angelic scene — this telescope hosts the HARPS (High Accuracy Radial velocity Planet Searcher) instrument. HARPS was one of two ESO instruments that in 2016 detected the Earth-mass world Proxima b, which orbits around the closest known star to the Sun.