When is First Light?

Without the moon or the sun, the stars sure are pretty (assuming a clear sky far from the madding crowds and their cities' glow), but they don't provide for one to competently manoeuvre terrain and manipulate objects without the further aid NVGs or without artificial illumination. As a kid, lacking night vision equipment and being loth to spend more money on alkaline cells for my flashlight than was quite necessary, I enjoyed twilight (or moonlit) periods, although I recall that sometimes a full moon would be rather too bright for my liking.

In the army, this passion for the changing of the light is cultivated by the practice of standing to in all-around defence, to adjust to the changing conditions and in anticipation of possible attack (after all, it's the perfect time of day... clear enough for the attackers to move and to see the static encampment, but perhaps too difficult to discern the outlines and the movement of the attackers in the shadows, especially if those encamped are still blinded by cooking fires and flashlights). I am sure many hunters who like to lie (or sit) in wait for the dawn have come to experience similar emotions as have, for example, the ANZACs with their dawn service ritual (or here).

The other day, however, I was occasioned to question just what constitutes first light, quantitatively. This had not been a problem I had researched much before, as I have previously relied on either my direct observation of the local conditions in the preceding days, or the trusted word of a friend or order of a senior NCO, as the case may be. So, I referred to my trusty online almanack wunderground and was bombarded with all kinds of different facts and figures. Should I use civil twilight? nautical? astronomical? I was in a hurry, and needed some sleep, so I decided to set my alarm clock for civil. It was not a big deal if I was wrong. I was. By the time civil twilight rolled around the land was bathed in light, and some further googling was clearly in order.

First light, as I am accustomed to understanding it, is apparently defined as one half hour before the beginning of nautical twilight. i.e. first light = BMNT - 30'' (and last light = EENT + 30''. I think these are US Army acronyms, although maybe they're NATO wide. Either way, I hadn't heard of them before. More references can be found here, at globalsecurity.org. Moreover, a good definition of just what these various types of twilight actually refer to is to be found here, on the Naval Observatory site.