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Someday, you’re going to be writing a story where you really, honestly need to know the time of sunrise, sunset, or twilight on a particular day in a particular place.

Maybe you’re writing historical fiction, and need to know whether your characters are going to have enough light left in the day to do whatever it is you want them to do.

Maybe you’re writing fantasy or horror, and need to know when your creepy-crawlies can emerge from their coffins or lairs or shadowy pits and go forth to rule the night.

If so, then the U.S. Naval Observatory has a web page for you.

The web page speaks of “civil twilight,” defined as:

the limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished; at the beginning of morning civil twilight, or end of evening civil twilight, the horizon is clearly defined and the brightest stars are visible under good atmospheric conditions in the absence of moonlight or other illumination. In the morning before the beginning of civil twilight and in the evening after the end of civil twilight, artificial illumination is normally required to carry on ordinary outdoor activities.

This is the kind of twilight you’ll probably be concerned with.  There are two other, more specialized, twilights – nautical twilight is when “the illumination level is such that the horizon is still visible even on a Moonless night, allowing mariners to take reliable star sights for navigational purposes” and astronomical twilight is when “the center of the Sun is geometrically 18 degrees below the horizon.”  But unless your protagonist is a navigator or an astronomer – in which case you’ve got more research ahead of you than a quick web page check is going to handle — you probably won’t need either of those.

In the meantime, think good thoughts about the U. S. Navy, figuring these things out so writers don’t have to.