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Category - Light,FAA
Posted - 03/27/2018 11:56am
What is Civil Twilight?
What is Civil Twilight?
It's an interesting question, and a more interesting "time" reference.

The Essence:

The limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished;

(The rest of this is all trying to quantify what that means, because, you know, we're scientific and  litigious, and every once in awhile you have to try to prove what it is, make it more tangible, but it's never the same, so it's hard to prove that it existed, or not, in a particular moment or defined space in time, and it assumes that there also exists an acceptable range of moments and variables which all qualify to fit the definition. 
Recreating it is all but impossible, not to mention that it's subjective to less definitive qualifiers, like vision, perception and definition of terms like, good atmospheric conditions and the Sun's upper edge, which in themselves must be qualified and proven before they can be used in additional calculations. 

The definition goes on to state:

at the beginning of morning civil twilight, or end of evening civil twilight, the horizon is clearly defined and the brightest stars are visible under good atmospheric conditions

The Scientific part:

Each twilight phase is defined by the solar elevation angle, which is the position of the Sun in relation to the horizon. During civil twilight, the geometric center of the Sun's disk is at most 6 degrees below the horizon. In the morning, this twilight phase ends at sunrise; in the evening it begins at sunset. Sunrise and sunset are the moments when the Sun's upper edge touches the horizon. 

The litigious part:

Lawmakers have enshrined the concept of civil twilight. Such statutes typically use a fixed period after sunset or before sunrise (most commonly 20-30 minutes), rather than how many degrees the sun is below the horizon. (since this would be very hard to "prove"). Adding a "range" makes it easier to argue from both sides since not only does there exist an acceptable range, but also acknowledges that there is some flexibility and deviation in the very definition of that range.

In layman's terms:

It seems to me that if it takes the earth 24 hours, or 1440 minutes to rotate 360 degrees, then it's reasonable to assume it takes about 4 minutes to rotate 1 degree (1440/360=4), or, by that math, 24 minutes to rotate 6 degrees (6x4=24), so a general "legal" reference of 20 - 30 minutes (as made above) seems also correct, in a less-definitive sort of way, and while 24 minutes is perhaps more precise, it would rarely, if ever, be exact.

The FAA just refers to it this way

Daylight-only operations, or civil twilight (30 minutes before official sunrise to 30 minutes after official sunset, local time) with appropriate anti-collision lighting. 

I like the FAA. Keep it simple. Find out what time the sun rises and sets in the location you're flying, and you can start flying 30 minutes before it rises, and 30 minutes after it good atmospheric conditions.