Conditions aboard a cruise ship can be almost ideal for seeing the elusive “green flash,” the ray of green light at the end of sunset. Some travelers search in vain their whole lives, although others who know where and how to look see the flash often. Here’s how to bolster your chances: You must be higher than the apparent horizon, which is why an ocean horizon is perfect but an island will block the green flash every time. It also helps to have clear air. A line of clouds at the horizon can make the flash hard to see unless the sun sets below them. Use binoculars or a telephoto lens since the flash is tiny and disappears fast. Finally, don’t stare at the setting sun. The red rays will bleach your retina and the “green” flash will look yellow. Wait until the sun is almost down to start watching.