Twilight comes in three stages, each based on the Sun's position below the horizon. [TW Carlson/Wikipedia]
The crescent Moon glides down the western sky as night falls this evening. And if you look carefully, you may see a bright companion to its right — the planet Jupiter. It’s easier to see from the southern part of the country than from the north. They drop from sight by about the time the sky gets fully dark.
Of course, there’s dark and then there’s dark. Twilight doesn’t turn off like a light bulb — it slowly fades away as the Sun drops farther below the horizon.
There are three basic phases of twilight.
First is civil twilight, which ends when the Sun is six degrees below the horizon. It’s not fully dark then — it’s easy to tell the horizon from the sky, and there’s enough light to make out most of the objects around you. But it’s dark enough that you need to turn on your car’s headlights if you’re out and about.
Next is nautical twilight, which ends when the Sun is 12 degrees below the horizon. The “nautical” comes from the fact that navigators at sea couldn’t make out the horizon by the time the Sun had dropped that far, so they couldn’t use it a reference for measuring the altitude of the stars.
Finally, there’s astronomical twilight, which ends when the Sun is 18 degrees below the horizon. By then, the sky is as dark as it’s going to get, so you can see the faintest stars. Today, of course, artificial light sources add their own glow to the night sky — creating a twilight that can last all night long.
Script by Damond Benningfield, Copyright 2014
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