One of the fun things about cruising in the tropics is the daily sundown ritual. Every evening, as sundown nears, we relax in the cockpit (perhaps with beverage in hand), and watch the sun disappear behind the horizon.
An important part of this is watching for the elusive "Green Flash." If conditions are just right, the very last bit of the sun on the horizon turns a brilliant green, for less than a second. It requires a clear view of the sea on the horizon, and an absolutely clear sky. This makes it quite rare. Despite watching virtually every day, we never saw the green flash in 2011, and so far only once this year. The rarity is part of the fun.
We have never heard a really good explanation for the physics behind this phenomenon. There is an old joke about floating (green) beer bottles, but it probably has more to do with the sun shining through the sea water.