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Extravoice
2004-Oct-07, 05:50 PM
Maybe this isn't the proper forum, but my question does have the word astronomical in it :wink: ...

Would someone be so kind as to explain the meaning of the following terms:

- Astronomical Twilight
- Civil Twilight
- Nautical Twilight

as they relate to sunset?

Thanks!

aurora
2004-Oct-07, 05:58 PM
They have definitions based on how far the sun is below the horizon.

six degrees for Civil twilight, 12 for nautical, and 18 for astronomical.

You can find the exact text through a google search.

However, in practical terms, I have always thought of it as:

civil: need illumination such as automobiles need to have their headlights on, and only the very brightest celestical objects (such as Venus) would be easily visible

nautical: It would be hard to see an unlit buoy on the water through binoculars from the deck of a boat. The brightest stars are visible, if you have an equatorial mount, it is time to align your scope to Polaris.

astronomical: It's as dark as it is going to get, your telescope should already be collimated and the finder scopes aligned!

edited to note that I found this:
http://www.word-detective.com/howcome/stilllight.html

Kaptain K
2004-Oct-07, 06:14 PM
aurora nailed it!

ngc3314
2004-Oct-07, 09:15 PM
They have definitions based on how far the sun is below the horizon.

six degrees for Civil twilight, 12 for nautical, and 18 for astronomical.

You can find the exact text through a google search.

However, in practical terms, I have always thought of it as:

civil: need illumination such as automobiles need to have their headlights on, and only the very brightest celestical objects (such as Venus) would be easily visible

nautical: It would be hard to see an unlit buoy on the water through binoculars from the deck of a boat. The brightest stars are visible, if you have an equatorial mount, it is time to align your scope to Polaris.

astronomical: It's as dark as it is going to get, your telescope should already be collimated and the finder scopes aligned!



IIRC, the nautical definition more or less corresponds to the darkest it can be and still get a good fix on the horizon (say using a sextant), and astronomical twilight means it's "completely dark" straight overhead (and therefore everywhere in the hemisphere opposite the sun as well). And from a case I once advied an attorney on, I gather civil twilight ends at the point where not having your lights on means the accident was your fault...

aurora
2004-Oct-07, 09:51 PM
IIRC, the nautical definition more or less corresponds to the darkest it can be and still get a good fix on the horizon (say using a sextant

That makes sense. I always thought it was something like that but didn't know for sure. Guess I'm not a sailor.


Nothing happens here.

George
2004-Oct-07, 09:58 PM
IIRC, the nautical definition more or less corresponds to the darkest it can be and still get a good fix on the horizon (say using a sextant

That makes sense. I always thought it was something like that but didn't know for sure. Guess I'm not a sailor.


Nothing happens here.
Sounds familiar....Sierra?


Nautical twilight is defined to begin in the morning, and to end in the evening, when the center of the sun is geometrically 12 degrees below the horizon. At the beginning or end of nautical twilight, under good atmospheric conditions and in the absence of other illumination, general outlines of ground objects may be distinguishable, but detailed outdoor operations are not possible, and the horizon is indistinct.
Their site (http://aa.usno.navy.mil/faq/docs/RST_defs.html)