A project of the Dark Energy Survey collaboration


Fire in the Sky


As day gives way to night, our star plummets into the pacific. We refuel our brains for a night of work and then watch the sun scorch the horizon into darkness. This is our nightly ritual.

After dinner, our crew heads back to the telescope. Some of us take a car up the roads, while others make their way up the winding paths through the clay and dirt. Like clockwork, we pass a family of zorros (“foxes”), who often wait outside the kitchen for tasty scraps. There are more mouths to feed now: this past spring, a new litter of pups appeared. Occasionally, a few viscachas (rabbit-like rodents in the chinchilla family) graze on the rare sprig of fauna in the dry mountaintops and then rest on warm rocks in the fading sunlight.

The Dark Energy Survey (DES) observes during these summer months, and the community has priority access to the instrument during the remainder of the year. DES runs optimally during the dry summer (in the southern hemisphere, lasting from December to February) to avoid atmospheric water absorbing and scattering light from the higher-wavelength portions of the electromagnetic spectrum. We desperately need that light to see older, more distant cosmic structures.

On especially dry evenings, a green flash can be seen in the moments before the last of the sun falls below the horizon. Earth’s prismatic atmosphere scatters the suns rays and splits the light by color. As the sun drops, the spread-out spectrum rolls vertically across our eyes, quickly from red to orange and very very briefly through green.

Celestial objects that DES observes set just like the sun does; between the beginning of night and the time a galaxy has fallen below the horizon there is very little time—from minutes to hours.   If the Universe were a year old, humanity has existed for about 20 seconds.  We have but mere fractions of a moment to take snapshots of these galaxies and stars that have lived for millions and billions of years and that reside millions of light-years away. Utterly ephemeral, the green flash reminds us of the difficulty of our endeavor, of the challenge of catching light from billions of objects so distant in time and space from us.

Written by: Det. B. Nord [FNAL]
Image credit: Det. B. Nord [FNAL]

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