Adventures In Strange Vocabulary: Nautical Twilight

This one is an important phenomenon to consider when planning aircraft carrier flight operations. Maybe it's not so important anymore in our age of computerized radar navigation and fly-by-wire, but it was very important back when pilots needed to see where they were going. It is also interesting to know what you are looking at when you admire a California sunset.

Whenever the sun is retreating westward over endless stretches of ocean and under clear skies you will notice a lot more light after the sun has sunken below the horizon. This is called, “nautical twilight.” In California you may get the poetic impression that the daylight is loathe to give up its ascendancy over the world. On an aircraft carrier you will also be happy for the extra time that this allows for flight operations.

The mechanics of this phenomenon are simple. The sun is below the horizon, but it is still shining down onto the reflective surface of the ocean. It reflects back into the sky, including the sky where you happen to be at the time. A book that I read recently about carrier operations said that you get nautical twilight until the sun passes about twelve degrees below the horizon. This allows for an additional hour of light both before dawn and after sunset, two additional hours per day at the equator. The length of time shrinks as you move north or south away from the equator. The Navy has charts for those things.

I miss those California sunsets. As the sun goes down, clouds slowly move in closer to the coastline, and the reflecting sunlight, bouncing back up into those clouds, creates great colors in schemes that you do not find in other places. The colors change as you watch, moving from green to purple to orange before your eyes. The low humidity assists in this act of creation, or at least gives in a different character than, let's say, the west coast of Florida. The Pacific Ocean is also a much bigger lens than the Gulf of Mexico.

There is a trade-off to console me in my case. I now live in the tropics, and we are treated on a regular basis to skies that are dramatic and beautiful. Those California sunsets, in Los Angeles anyway, usually came after a sky of solid blue, unadorned by clouds after about ten in the morning. The cloud vistas in the tropics can be vast and impressive. These often culminate in solid gray and biblical rainfall, but that's another story.