The Friday Fillip: Sunlight Serenade

• Author: Simon Fodden
• Date: January 02, 2015

I’m partial to daylight.

So it’s probably no wonder that around this time of year here in the Northern Hemisphere I become fascinated by the wanderings of the sun. And even though I’m way down south in Canada, as these things go — on a line1 with Rome and northern California — I’m still light deprived, getting barely more than nine hours at the moment out of the available 24. At noon the sun only just crests a four storey building. Bah!

Now I know you can’t push the river (until it freezes) — or Sol for that matter. But I find it helps a bit to be able to know in detail what’s happening, the consolation of a geek, I guess. If you’re at all like that, I’ve got a website for you. It’s TimeAndDate.com.

As the name suggests, there’s way more here than a light-lorn Canuck needs, such as stuff on time zones or weather. But people geek out on those data too, so have at it. For me it’s the information on the length of days at my latitude that I enjoy, the sunrise and sunset times. How much more daylight will I gain each day? Will the gains accelerate? And what are all these variations on twilight?

Let’s have a look at these liminal states first, the time of day (or night?) when driving becomes difficult because you can’t see clearly, when, as the old saying went, you can’t distinguish between a dog and a wolf. Cockshut. According to TimeAndDate there are three gradations of this state, each one gloomier than the one before: civil twilight, nautical twilight, and astronomical twilight. Of course, each happens after sunset — after the disk of the sun has dropped beneath the horizon and what light there is comes to us as illumination reflected down from the upper atmosphere, from where the sun is still visible to some degree. (There is, so I’m told, a parallel set of phenomena in the morning, as sunrise approaches and is accomplished; I wouldn’t know; I’m rarely awake at that time.)

Civil twilight — here Wikipedia comes to our aid — has a technical definition, as do the others, and a more functional definition as well. As you can see from the diagram that will pop up via this link, each depends on the degree to which the centre of the sun lies below the horizon: 6° for civil, 12° for nautical, and 18° for astronomical. In more mundane terms, civil twilight, lasting about twenty minutes after sunset, is when you can still make out most objects and when some laws would have you start putting on your car’s headlights; nautical twilight occurs when the horizon is lost...