A recent observer surprised me the other day when he walked back into the control room before the sun had officially set. I had told him to go out and enjoy the sunset. I do this for all the observers I support. They may be summit novices or Mauna Kea veterans but they don't have the same opportunity as I do to watch sunsets (or sunrises) from 14,000 feet. This time of day often involves taking calibration data but I also want our guests to experience a sight most people will never see in their life and I'm happy to take over the start-of-night work while they enjoy the view.
Of course, what had happened is the sun had "set" behind the clouds but wasn't actually below the true horizon.
Some visitors to the summit are confused by "official" sunset times. Many media publish the times of sunset and sunrise but it's often calculated for sea level. Because we're so high, the sunset actually occurs several minutes after it does at sea level. This isn't too much of a problem for most as they've already arrived and are ready, but conversely sunrise occurs several minutes before those at sea level experience it, so if you're not prepared, you'll miss it!
For example, this evening the sun set in Hilo at 6:59pm but at the summit sunset was at 7:10pm. Sunrise tomorrow morning occurs at 5:47am from Mauna Kea but from Hilo it'll be 5:55am.
This may seem an insignificant difference to many, but knowing the exact time of sunrise and sunset is crucial to us as we rely on those times to start calibrations. A minute or two wrong either way can mean the data are useless, so it's something we need to make sure we get right.