Saturday, 3 November 2012

Three types of twilight

Whilst the poles experience great extremes of light and darkness for extended periods of time, those who overwinter there in locations such as the Amundsen-Scott Polar station are probably more aware than most of three different classifications of twilight. For periods before the sun finally rises (which is around the 23rd September at the Amundsen-Scott station) the darkness is lessened by prolonged periods of these twilights. Just before sunrise, the brightest of the three is civil twilight.

Civil Twilight occurs when the sun is between 0 and 6 degrees below the horizon, immediately after the sun has set at dusk, further classified as either civil dawn and civil dusk depending upon whether the sun is setting or rising. It tends to encompass the limits where objects can still be clearly distinguished without extra illumination. The brightest stars are now visible.

One phase darker is Nautical Twilight when the sun is 6-12 degrees below the horizon. Navigation at sea by the brightest of stars and the viewable presence of a horizon is possible during the period of nautical twilight. At it’s end (or beginning in the morning) large objects may be discerned, but without detail.

When the sun’s centre lies 12-18 degrees below the horizon, we are in the period of Astronomical Twilight. It is named thus because, under clear conditions certain sky objects, such as nebulae and galaxies, are still not properly visible. To the casual observer however, astronomical twilight often appears as night proper, and this is obviously more marked in towns and cities where light pollution affects the level of ambient light in the sky where such features may never be visible anyway. Other factors, such as the presence of a full moon, may also make discerning astronomical twilight difficult.
The first stirrings of civil dawn from the summit of Ben Hope, with a bright moon in the South East.
The duration of twilight is affected by our latitude, and at the poles, twilight can last for weeks. It’s no surprise therefore that an awareness of these intermediate points of clarity and obscurity is far greater for the inhabitants of those areas. Of course, the vast majority of us experience twilight as a daily occurrence. It’s a time of day I’ve always liked. I feel fortunate to live in a Northern region where our twilights are prolonged. The twenty-minute change from day to night (or vice versa) at the equator seems to me too abrupt for the sensualities of fading or growing light to be savoured.

The beginnings of civil dawn on the summit of Ben Hope. The moon was obscured by cloud for much of the night, although the skies to the North were clear.
I enjoy in particular the way in which our surroundings, away from the artificial lights of towns and cities, take on a kind of granularity, the way the light seems to hover in front of and on top of things as the light begins to fade at dusk. Greens intensify for a short time, and a certain pleasure can be had from not reverting to other forms of illumination, taking the capabilities of twilight to it’s limit.

2am. Jim escaping from the wind in the summit shelter cairn. He and the rocks are illuminated by the twilight rather than the moonlight.
Civil dawn, 2am, looking North East.The lights in the distance are those of the small village of Tongue.
When Jim and I climbed Ben Hope, Scotland’s most northerly Munro (mountain over 3000ft) close to the summer solstice a few years ago in the hope of experiencing a Scottish midnight sun by virtue of the combination of latitude and altitude, the sun did in fact dip down below the horizon. Setting off at 12.05am from our tent, just after the sun set, for the couple of hours it took us to climb the hill, we had no need for extra illumination, being fairly sure of my feet and the terrain, as I had climbed it once before in torrential rain and within thick cloud. For the entirety of the walk and for a chilly hour sat at the summit cairn waiting for the sun to raise above the horizon, we passed through elongated phases of what I now know were the periods of civil and nautical twilight. We can’t have entered astronomical twilight as I was always able to discern at least some detail in my surroundings. Jim found a scrap of paper in his pocket, a receipt, which became a marker and measure of his capacity to see in this twilight since he was able, at all times, to read the characters on that tiny slip of paper. In a way, there is a place and time in the UK where nightfall never really comes.  

Looking North.
We waited on the summit for a long while, but with a low band of cloud on the horizon, a visible sunrise seemed never to come. Growing increasingly cold, we decided to head back down the leeward side of the hill (leeward, sun-wise) watching as colour slowly penetrated the ground below. That sense of the granularity that I describe which veils objects infused the surface of the boulder field near the top and in our shelter for the night at the cairn. Once civil dawn was nearing its end, the colour of the short, wind stunted grasses was at it’s most intense and varied. In fact, in the full light of day, a more bland descriptiveness takes over and objects are perhaps too easily seen, and the lyricism of the indistinct is lost.

Note* most of these images were made within a relatively short time of one another at around 2am. I only made photographs during the hour spent on the summit, so the period of nautical twilight was not documented.