A project of the Dark Energy Survey collaboration


Moonrise over Tololo


In the east, peeking through a rare mountaintop tree, the Moon rises toward the Milky Way.  Not too far behind, the Sun‘s rays reach over nearby mountains to at once expose the valley fog and cover the escape of distant stellar brethren into the daylight.

To the south (on the right) sits the Blanco telescope, readying for its daytime rest. Across the ridge live neighboring telescopes at the Cerro Pachon site, including world-class telescopes SOAR and GEMINI. Can you spot them on the distant ridge? (Hint: in the far right of the photo). The white lines on the ground are metal walkways that astronomers use during the night to move from one building to the other in the utter darkness; their reflectivity allows someone walking to find their way even before their eyes have fully acclimated to the darkness.

It had been a quiet night. Well, they’re all quiet nights: the loudest sound by far originates in the slow whir of motors as domes turn toward new expanses of polka-dot sky. Earlier in the evenings, the temperature drop as day changes into night causes the metal in the domes to contract slightly, and rhythmically—ka-chunk, ka-chunk, ka-chunk. It’s a reminder that we brought these machines to an alien environment, an outpost between humans and the heavens. Nestled within the dome, Blanco and the Dark Energy Camera (DECam) toil away, far more impervious to the elements and designed to be compatible with such temperature changes.

This was one of the last nights of my 10-day run at Cerro Tololo Inter-American Observatory. The moon rose earlier on nights past; when that happens, that part of the sky becomes too bright for the highly sensitive DECam. Nevertheless, we observed several patches of sky to extraordinary depth in the hopes of finding old, distant exploding stars—one of the types of objects that will help illuminate dark energy’s impact on the fate of the cosmos.

Where does moonlight originate? Hints: the moon is not a star, and it is very reflective.

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

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