Why are there two twilights: civil and nautical?
There aren't two twilights. There are three: civil, nautical and astronomical. Some people say there is a fourth - actual twilight - but this is confusing enough as it is so we'll stick to three.
In a nutshell, twilight is the period before sunrise and after sunset when the sun's light is not shining directly on Earth but is diffused by the upper atmosphere, casting various degrees of illumination on us.
Unless you are sailor or astronomer, you really don't have to worry about all this. Come to think of it, you probably really don't have to worry much about civil twilight, but I suppose you might have your reasons for doing so.
Civil twilight is the period before sunrise and after sunset when the sun is 6 degrees below the horizon.
During civil twilight, there is enough illumination to work outside without artificial lighting. It's a good time to do stuff in peace and quiet while there aren't many people around, like sneaking over to your neighbors' driveway to "borrow" their Republic.
Nautical twilight is the period after sunset and before sunrise when the sun is 12 degrees below the horizon.
This is when the horizon at sea can be made out but it is still dark enough to see the major stars. That means sailors can use a sextant to get their morning or evening bearings.
I don't know how many sailors still use sextants, but I suppose there are some.
Astronomical twilight is the period before sunrise and after sunset when the sun is 12 to 18 degrees below the horizon.
The hours between the night and morning twilights are when it is dark enough to do some serious astronomy.
by Clay Thompson, columnist The Arizona Republic Sept. 19, 2010 12:00 AM
This is about types of twilight (not the vampire kind)
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