Sky Watch: Sunset, twilight definitions

Sometimes we astronomers toss around terms that we consider to be "obvious" -- and to us, they are -- but people who hear or read our words often wonder just what the heck we're talking about.

Let's define "sunset." Just what exactly does that mean?

define "sunset" and "sunrise" as the time when the upper edge of the sun's disk appears on the unobstructed horizon. An exact time for this is tough to determine because it depends on atmospheric conditions, elevation, the terrain of landscape and more.

And "twilight" refers to the period between sunset and darkness (or darkness and sunrise) where sky brightness decreases (or increases); some outdoor activities can continue without artificial lighting, but only for so long.

"Civil twilight" is said to begin in the morning -- and to end in the evening -- when the center of the sun lies six degrees below the horizon. In the morning before civil twilight and in the evening after civil twilight, artificial illumination is required to carry on outdoor (civil) activities.

"Nautical twilight" is when the center of the sun lies 12 degrees below the horizon. At the beginning or end of nautical twilight, under good atmospheric conditions and without other illumination, outlines of ground objects are distinguishable, but detailed outdoor operations are not possible.

The horizon -- especially at sea -- is indistinct, hence the term "nautical" twilight.stronomical twilight" is when the center of the sun lies 18 degrees below the horizon. Before astronomical twilight and after astronomical twilight, the sun does not contribute to sky illumination; for awhile after the beginning of morning twilight and before the end of evening twilight, sky illumination is so faint that it is almost imperceptible.