The Difference Between CIvil and Nautical Twilight

[Crafty Angler]

Does anybody know - I'm confused

Thanks in advance -

[redcrbbr]

[edit] Civil twilight

Under civil twilight conditions, the horizon is clearly visible, and terrestrial objects easily perceptible, without artificial light.This begins in the morning when the center of the Sun is less than 6° below the horizon (the point of civil dawn), and ends at sunrise. Evening civil twilight begins at sunset and ends when the center of the Sun is more than 6° below the horizon (the point of civil dusk).

The brightest stars appear during civil twilight, as well as planets, such as Venus which is known as the 'morning star' and/or 'evening star'. During this period there is enough light from the Sun that artificial sources of light may not be needed to carry on outdoor activities. This concept is sometimes enshrined in laws, like when drivers of automobiles must turn on their headlights, or if the crime of burglary is to be treated as night-time burglary, which carries stiffer penalties in some jurisdictions. A fixed period of time (most commonly 30 minutes after sunset or before sunrise) is typically used in such statutes, rather than how many degrees the Sun is below the horizon. Civil twilight can also be described as the limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished; 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 in the absence of moonlight


[edit] Nautical twilight
This is defined as the time when the center of the Sun is more than 6° below the horizon but less than 12°.


Twilight in the midwestern US featuring Venus as a brilliant "evening star" and the crescent moonAt this time, sailors can take reliable star sights of well known stars, using a visible horizon for reference. The end of this period in the evening, or its beginning in the morning, is also the time at which traces of illumination near the sunset or sunrise point of the horizon are very difficult if not impossible to discern (this often being referred to as "first light" before civil dawn and "nightfall" after civil dusk). At the beginning of nautical twilight in the morning (nautical dawn), or at the end of nautical twilight in the evening (nautical dusk), 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. Nautical twilight has military considerations as well. The initialisms BMNT (begin morning nautical twilight) and EENT (end evening nautical twilight) are used and considered when planning military operations. A military unit may treat BMNT and EENT with heightened security (i.e. a process called "stand to" in which everyone pulls security). This is partially due to tactics dating back to the French and Indian War, when combatants on both sides would use BMNT and EENT to launch attacks.


[edit] Astronomical twilight
This is defined as the time when the center of the Sun is more than 12° below the horizon but less than 18°.
Most casual observers would consider the entire sky already fully dark even when astronomical twilight is just beginning in the evening or just ending in the morning, and astronomers can easily make observations of point sources such as stars, but faint diffuse objects such as nebulae and galaxies can only be properly observed beyond the limit of astronomical twilight. Theoretically, the dimmest stars ever visible to the naked eye —those of the sixth magnitude— will appear in the evening once the Sun falls more than 18° below the horizon (i.e. when astronomical dusk occurs) and disappear when the Sun moves to within 18° of the horizon in the morning (when astronomical dawn occurs); however, due to light pollution, some localities —generally those in large cities— may never have the opportunity to view even fourth-magnitude stars, irrespective of the presence of any twilight at all[1].

[Crafty Angler]

Excellent

Thanks, redcrbbr - I knew searching The Google would lead to me not getting any work done around the estate here for the next hour...

[Mike P]

From a legal standpoint, civil twilight is a half hour before sunrise or a half hour after sunset. That's when "daytime" starts and ends for pilots. If you're not current for flying at night, those are the times you can legally fly--between morning and evening civil twilight.