Paranal does the majority of its work in service mode, where we, the observatory staff, carry out observations that have been requested and designed by other astronomers. This has the advantage that if they need certain weather conditions, then we simply wait until we have conditions as good or better than needed and take the observations then. But for the astronomers to come and carry out their own observations also has its advantages - they can see much better how the observatory works, and make sure the data is exactly as they want it. But though the weather is good here at least 90 per cent of the time, there's still a chance that a visitor will get no data at all. That happened tonight, and it was sadder than usual because it was Christmas night. People had travelled thousands of miles to be away from friends and families, in the hope of better understanding some of the mysteries of the universe in compensation, but it was cloudy for most of the night and we couldn't do any observing at all.