White Nights

We are coming into the season of “white nights” around the summer solstice, when there is no true darkness because the sun is close below the horizon still illuminating the atmosphere even though the sun itself is not visible. In effect, the sun has started rising as soon as it has finished setting. Of course, if you were to travel further north, this phenomenon is clearer and more dramatic until, at the arctic circle, there are times when the sun does not set at all. We are at 51 degrees North here and the Arctic Circle is roughly 66.5 degrees North so we are about 930 miles (1,722km) short of the midnight sun, which is not that far compared with the size of the Earth.

There are degrees of darkness. We tend to call darkness that is not complete “twilight”. Civil twilight is when the sun is 6 degrees (or a little more than the width of a hand held at arms length) below the horizon. The horizon is clearly visible as are objects (such as mountains) even though, on a very clear night, the brightest stars are becoming apparent. Nautical Twilight is when the sun is 12 degrees below the horizon. The horizon is no longer clear although some objects may be discerned. Astronomical twilight is when the sun is more than 18 degrees below the horizon – complete darkness (apart from light pollution, of course)

Around the summer solstice here, we have a few weeks where there is no astronomical darkness. This is well shown on the “skylight clock” on the weather station web site.

Skylight Clock
Skylight Clock

Yellow is daylight with the sun above the horizon, the light blue is civil twilight, (sunset until the sun reaches 6 degrees below the horizon or from that point until sunrise), the mid blue is nautical twilight (sun reaching 12 degrees below the horizon) and the dark blue is astronomical twilight (the sun reaches 18 degrees below the horizon). Black is darkness. The red hand indicates what time it is relative to this. At the time of writing, there is a sliver of black left on the clock, but this will soon disappear until the evenings begin (slowly) to draw out in July.

All this affects nature. Animals (and small children) go to sleep later and get up earlier. Most British animals are crepuscular: being most active through twilight and in the morning and evening. As people tend to stick to the clock, we miss their most active times through this period of the year. perhaps we should get up and go to bed with the sun, but have a sleep in the day at this time of year!