A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2015 Aug 17, 13:33 -0700
I think the "overlapping" that's being discussed here is not a matter of a few minutes of overlap but rather an extension from the instant of sunrise/sunset all the way to the dark limit. In other words, does nautical twilight cover the period from altitude 0° to -12°? Or does nautical twilight cover the period when the altitude of the Sun's center is between -6° and -12°? And similarly for astronomical twilight. My sense is that this appears to be a problem (for some people!) because the "bright limit" of the various twilights has never been made explicit and really doesn't need to be. If we only agree on the defined "dark limits" then there are four alternatives: 1) the darker twilight extends from the end of the next-brighter twilight, or 2) each twilight begins when the Sun is on the horizon and extends to its own dark limit, or 3) a "bright limit" is not required and does not exist, or 4) a "bright limit" is meaningful and maybe defined for certain types of users, but not others.
When is civil twilight? For this one, we assume that there is not a lot of ambiguity anymore. Abundant source material states that civil twilight has a bright limit at "sunrise" or "sunset" and a dark limit when the center of the Sun is 6° below the true horizon. The dark limit is actually easy here. But what exactly is sunset? There is a conventional definition used in astronomy (altitude of Sun's center exactly at sea level is -50 minutes of arc). Or should we use the observed disappearance of the Sun? If you have hills to the west that cover the Sun three minutes early for everyone in your town, does that mean that the Sun has set?
When is astronomical twilight? This is the oldest type of twilight that was identified, and its dark limit is agreed to be when the Sun's center is 18° below the true horizon. When is the bright limit? Well, who cares?? Astronomers have no use for a starting time for evening astronomical twilight. For truly dark skies, we need to be between the end of evening astronomical twilight and the beginning of morning astronomical twilight. Astronomers don't need a "bright limit" for astronomical limit, and you won't normally find one defined.
And finally, when is nautical twilight? This was the last form of twilight defined. A navigator in 1915 would have no idea what you were talking about if you asked about the limits of "nautical twilight". There was a generally understood period of time when twiklight star sights could be taken: the bright limit was when the first magnitude stars became visible, and the dark limit was when the horizon was barely visible. This period had no name. The dark limit, introducing the term "nautical twilight" some years later, was defined (by fiat!) to be the time when the Sun's center is 12° below the true horizon. This neatly splits the difference between the other two twilights, and navigators have found that it is "about right" for deciding when the horizon is no longer visible. But it is no law of nature. At the other end, the bright limit could be considered either sunrise/sunset or it could be considered to coincidence with the dark limit for civil twilight. In the latter case, nautical twilight would not overlap civil twilight at all. That's the real question here.
Wikipedia is dependent on sources. It doesn't matter what you "think" should be the case, or what you "believe" to be correct. If there are multiple sources that appear to conflict, then that is what the article should say! Multiple definitions and ambiguous definitions exist even in the sciences. Wikipedia, as an encyclopedia, is not intended to decide between alternative definitions -- it's supposed to present them.
Navigational practice is a different matter. Ask yourself this: when the Sun sets in the evning, and you begin to prep your sextant for twilight star sights, would you say that nautical twilight has begun, but you're waiting for civil twilight to end before you take your sights? Or would you say that you're "waiting for nautical twilight"?