What is the difference between civil twilight, astronomical twilight, and nautical twilight?
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They go one after the other.
In the morning, first comes astronomical twilight, than after that ends nautical twilight starts, when nautical twilight ends, civil twilight starts and that ends with dawn.
In the evening they go in the opposite order: sunset, civil, nautical and then astronomical twilight.
The exact definitions are not based on time before dawn or after dusk or how bright or dark it is but on the position of the sun.
* 0° to 6° below the horizon is civil twilight
* 6° to 12° below the horizon is nautical twilight
* 12° to 18° below the horizon is astronomical twilight
If you get close enough to the poles you may not ever reach dawn or night and just get stuck in one of the twilight phases before going back the other way.
Because the length of those phases depends on how far north/south and time of the year you are (and even how high up you are), some places just define “civil twilight” to mean c minutes before dawn/after dusk. This makes laws easier to write, but can fail to mesh with reality.
There is an actual definition about how many degrees below the horizon the sun is at each level, but practically: Astronomical is when you can just barely see the sky starting to brighten but it’s still pretty much dark on the ground. Nautical is about an hour before sunrise when you can make out mountains, and other big landmarks. Civil is the about half hour before sunrise when you can operate pretty normally.
Sunset
**Civil twilight** – sun is setting, artificial light is not needed
Sun falls below horizon
**Nautical twilight** – sun has set, but it is possible to navigate by the shape of the horizon
Horizon is no longer visible
**Astronomical twilight** – Bright stars and planets become visible
Sky is fully dark, all stars can be observed
And then in reverse in the morning.
I was a Navigator in the US Navy recently and so I was taught all of this but I did not use it practically.
As we head into night from day—–>
Civil Twilight – The sun has set (aka gone below the horizon) but it is still bright enough to see without artificial light.
Nautical Twilight – The sun has traveled further below the horizon and you can no longer see activities without artificial light. More importantly, their is enough light that you can still make out the horizon, but, with the drop in light you can now see stars and planets. This is important because to use astronomical bodies for navigation you must be able to see the star/planet and the horizon to utilize a sextant.
Astronomical Twilight – (We didn’t use in the Navy Navigation course) Seems to be a special distinction astronomers make between a time when, without light pollution, you can only make out the brighter, more visible stars and planets. Once this period is over, the sun’s light is no longer visible and all of visible astronomical bodies should be observable.
The corresponding angles with respect to the horizon are all on Wikipedia.
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