July 22, 2011--Twilight

I'm a twilight kind of guy.

I like the lingering end of things; and what better manifestation of that, as well as a metaphor for much in life, is the time after sunset when the western sky is still illuminated.

The sun has dipped below the horizon and common sense would suggest all should be dark but yet there are these after-sunset moments when we are drawn to not yet turn on any lights, to linger in contemplation of what is still possible.

Knowing from science why there is this afterglow, of how this residue of former direct sunshine creeps up over that horizon behind which the sun has already settled is neither needed nor satisfying. Simply rest in the wonder and wonder.

But now I know there is more.

Before turning to this page each morning I check the weather forecast on the Weather Underground. I type in the zip code--04554--and via the miracle of the Internet up pops what is to be expected for today and the next ten.

Today I did that with some trepidation since it is predicted to be infernally hot in the northeast, with temperatures as close as Boston predicted to reach 100. And here, without air-conditioning, with only one small fan, anything approaching that would be disabling. But fortunate as we are, perched right by the Gulf of Maine, where the water temperature is still in the low 50s, the Underground assures me that it will get to only 88. With just the mote of a breeze, this is manageable.

Feeling both guilty to be thus fortunate and with some guilt gleeful that today will be endurable here, I scrolled down the page to check the times for sunrise (5:15 AM) and sunset (8:12 PM). Why I do not know but I do. And then right below was information about the twilights.

Yes, twilights plural because there are at least three--Civil, Nautical, and Astronomical

Civil Twilight, I learned, is actually of two types--Morning Civil Twilight, or Civil Dawn, begins when the geometric center of the sun is 6° below the horizon and ends at sunrise. My passion, Evening Civil Twilight, or Civil Dusk, begins at sunset and ends when the geometric center of the sun reaches 6° below the horizon.

Noteworthy, the brightest stars appear during at that time, as well as planets, such as Venus, which is known, of course, because of twilight, as the morning and evening star. During this evocative time outdoor activities do not require artificial illumination. Sitting in rapt contemplation rarely does.

Nautical Twilight, about which I need to pay attention, perched as we are on Johns Bay, is the time when the center of the sun is further below the horizon, between 6° and 12° below, and it ends when navigation just using the sea's horizon as a reference point is no longer reliable.

But during Nautical Twilight, sailors, without GPSs on board, can take reliable sightings of well-known stars, using the remnants of the semi-visible horizon to aid them. The end of this period is also the time at which traces of illumination near the sunset point of the horizon are very difficult, if not impossible, to discern. Most boats should be in port and sailers should join me with a finger or two of bourbon.

Then there is Astronomical Twilight. It is when the center of the sun is still further below the horizon, between 12° and 18°. From the end of Astronomical Twilight the sky (away from urban light pollution) it is dark enough for serious astronomical observations.

Most casual observers--which surely includes me--would consider the entire sky fully dark even when Astronomical Twilight is even just beginning, and though astronomers can make observations of most stars, faint items such as nebulae and galaxies can be properly observed only when Night, with a capital N, falls.

Night, technically, is when the sun . . .

Setting aside the technical definition, for me night--no capital--is a whole other thing. There is sleep. And, perchance, dreaming. And of course more metaphors.