Light versus darkness

As the sun sinks below the horizon astronomers at Mauna Kea's summit start to work, the tourists leave for lower altitudes and the sky takes on some amazing colours. There comes a time when the light from the sun can no longer illuminate even the highest clouds and it tends to happen very quickly. One moment the cirrus clouds are bright red and the next they're a dull grey.

On occasion I catch the moment when day turns to night but it's not often. In this case you can see the dark clouds above and the brilliantly lit clouds off in the distance. In a few more minutes the distant clouds will be dark as well and shortly after that the sky will be dark. You wouldn't even know there are clouds there.

Well, you would if you're an astronomer working at one of the telescopes. Our data reduction pipeline shows that the stars aren't as bright as they should be, it even tells us by how much, so it's time for a backup plan until the clouds go away.